TERMS AND DEFINITIONS
NOTES: terms in definitions that are set in bold italics are also defined in this glossary.

absolute temperature

Temperature value relative to absolute zero.

absolute vorticity

The sum of planetary vorticity and relative vorticity. See vorticity for a fuller, mathematical explanation of how planetary and relative vorticity relate to each other.

absolute zero

The theoretical temperature at which molecular motion vanishes and a body would have no heat energy; the zero point of the Kelvin and Rankine temperature scales. Absolute zero may be interpreted as the temperature at which the volume of a perfect gas vanishes or, more generally, as the temperature of the cold source that would render a Carnot cycle 100 percent efficient. The value of absolute zero is not estimated to be -273.15° Celsius, -459.67° Fahrenheit, 0° Kelvin, and 0° Rankine.

absorption

The process by which radiant energy is absorbed and converted into other forms of energy. A substance that absorbs energy may also be a medium of refraction, diffraction, or scattering; these processes, however, involve no energy retention or transformation and are to be clearly differentiated from absorption.

absorption coefficient

1. A measure of the amount of normally incident radiant energy absorbed through a unit distance or by a unit mass of absorbing medium. Compare transmission coefficient.

2. A measure of the amount of radiant energy, incident normal to a planar surface, that is absorbed per unit distance or unit mass of a substance.

absorption cross-section

1. A measurement of an atom or molecule's ability to absorb light at a specified wavelength measured in square cm/particle.

2. In radar, the ratio of the amount of power removed from a beam of absorption of radio energy by a target to the power in the beam incident upon the target Compare scattering cross section. See also cross section.

absorptivity

The capacity of a material to absorb incident radiant energy, measured as the absorptance of a specimen of the material thick enough to be completely opaque and having an optically smooth surface.

action spectrum

A measure of the relative effectiveness of radiation in generating a certain biological response over a range of wavelengths; this response might be erythema (sunburn), changes in plant growth, or changes in molecular DNA. The action spectrum for DNA respresents the probability of DNA damage by UV radiation at various wavelengths. Such DNA damage can lead to skin cancer.

adiabatic

Without gain or loss of heat.

adiabatic heating or cooling

See adiabatic process.

adiabatic lapse rate

A special process lapse rate of temperature defined as the rate of decrease of temperature with height of a parcel of dry air lifted adiabatically through an atmosphere in hydrostatic equilibrium. This lapse rate is given by g/cpd, where g is the acceleration due to gravity and cpd is the specific heat of dry air at constant pressure; it is numerically equal to 9.767°C per kilometer or about 5.4°F per 1000 feet. Compare to lapse rate.

adiabatic process

A thermodynamic change of state of a system such that no heat or mass is transferred across the boundaries of the system. In an adiabatic process, expansion always results in cooling, and compression in warming.

advection

1. The process of transport of an atmospheric property solely by the mass motion of the atmosphere; also, the rate of change of the value of the advected property at a given point.

2. Regarding the general distinction (in meteorology) between advection and convection, the former describes the predominantly horizontal, large-scale motions of the atmosphere whereas convection describes the predominantly vertical, locally induced motions.

3. To transport or carry. In air quality, the rate at which particulate matter is transported.

aerosols

Particles, other than water or ice, suspended in the atmosphere ranging in radius from one-hundredth to one-ten-millionth of a centimeter -- or 10² to 10^-3 microns (m). Aerosols are important as nuclei for the condensation of water droplets and ice crystals, and as participants in various atmospheric chemical reactions. Perhaps most significantly, they absorb solar radiation, then emit and scatter it, influencing the radiation budget of the Earth-atmosphere system, which in turn influences the climate on Earth's surface. Aerosols from volcanic eruptions can lead to a cooling at the surface, which may delay greenhouse warming for a few years following a major eruption.

albedo

The ratio of the amount of electromagnetic radiation reflected by a body to the amount incident upon it, often expressed as a percentage; e.g., the albedo of Earth is 34%. The concept is identical with reflectance; however, albedo is more commonly used in astronomy and meteorology and reflectance in physics. Albedo is sometimes used to mean the flux of the reflected radiation; e.g., the Earth albedo is 0.64 calorie per square centimeter. This usage should be discouraged. The albedo is to be distinguished from the spectral reflectance, which refers to one specific wavelength (monochromatic radiation). Usage varies so mewhat with regard to the exact wavelength interval implied in albedo figures; sometimes just the visible portion of the spectrum is considered, sometimes the totality of wavelengths in the solar spectrum.

anthropogenic

Refers to something originating from humans and to the impact of human activities on nature.

anticyclone

An atmospheric high-pressure closed circulation with clockwise rotation in the Northern Hemisphere, counterclockwise in the Southern Hemisphere, and undefined at the Equator.

aphelion

That point in a solar orbit which is most distant from the Sun. The point nearest the Sun is called perihelion.

atmospheric effects

The net effect of the atmosphere varies with differences in path length and with the magnitude of the energy signal being sensed, the atmospheric conditions present, and the wavelengths involved.

atmospheric gravity wave

See gravity wave.

atmospheric window

The spectral region between 8.5 and 11.0 microns where the atmosphere is essentially transparent to longwave radiation.

attenuation

1. Reduction in intensity.

2. The decrease in the magnitude of current, voltage, or power of a signal in transmission between points. Attenuation may be expressed in decibels, and can be caused by interferences such as rain, clouds, or radio frequency signals.

attenuation coefficient

A measure of the space rate of attenuation of any transmitted electromagnetic radiation. The attenuation coefficient is defined by

      dI = -a Io dx

               or

      I = Io exp(-ax)

where I is the flux density at the selected point in space; Io is the flux density at the source; x is the distance from the source; and a is the attenuation coefficient. In general, the attenuation coefficient is specified only when the attenuation is known to be caused by both absorption and scattering, or when it is impossible to determine which is the cause. See absorption coefficient and scattering coefficient.

aurora

The sporadic radiant emission from the upper atmosphere over middle and high latitudes. It is believed to be due primarily to the emission from nitrogen -- atomic N I and N II, molecular, N2, and ionic N2+; atomic oxygen (O I and O II); atomic sodium (Na I); the hydroxyl radial (OH); and hydrogen. According to various theories, auroras seem definitely to be related to magnetic storms and the influx of charged particles from the Sun. The exact details of the nature of the mechanisms involved are still being investigated, but release of trapped particles from the Van Allen belt apparently plays an important part. The aurora is most intense at times of magnetic storms (when it is also observed farthest equatorward), and shows a periodicity related to the Sun's 27-day rotation period and the 11-year sunspot cycle. The distribution with height shows a pronounced maximum near 100 km. The lower limit is probably near 80 km. The aurora can often be clearly seen, and it assumes a variety of shapes and colors that are characteristic patterns of auroral emission.

autoregressive model

A statistical representation for data in which an observation depends on the previous observation. Autoregressive models are typically used in time series analysis. The data (X) are represented as X(t) = a*X(t-1) + b*X(t-2) + ... + Z. If all coefficients are zero except for the first (a), then the equation X(t) = a*X(t-1) + Z is called a Markov process. Z is a random, normally distributed function.

backscatter ultraviolet (BUV) technique

One of several remote sensing techniques used for measuring atmospheric trace gases by satellite. Measurements are made of solar ultraviolet (UV) light entering the atmosphere (the irradiance) at a particular wavelength and of the solar UV that is either reflected from the surface or scattered back from the atmosphere (the radiance) at the same wavelength. By looking directly down at the atmosphere in a viewing geometry called nadir viewing, the satellite is able to get a good horizontal resolution, which is highly advantageous. The total area of the field of view seen by the instrument is called the footprint. The point on Earth directly beneath the satellite is called the subsatellite point. The BUV technique's main disadvantage is that the effects of increased multiple scattering and reduced sensitivity to the shape of the profile lead to poor vertical resolution in the region below roughly 30 km. For ozone measurements, this is below the ozone maximum in the stratosphere. See BUV ozone profiling technique. Example of instruments employing BUV technique include the SBUV and TOMS. Compare to limb emission technique and occultation technique.

bar

A unit of pressure equal to 106 dyne per square centimeter (106 barye), 1000 millibars, 29.53 inches of mercury. See torr. Some writers have used bar as equivalent to barye (1 dyne per square centimeter).

baroclinic

Of, pertaining to, or characterized by baroclinity.

baroclinic model

A model of atmospheric circulation in which constant-pressure surfaces do not have to coincide with constant-density surfaces. By contrast, in a barotropic model, constant-pressure surfaces are made to coincide with constant-density surfaces.

baroclinity

The state of stratification in a fluid in which surfaces of constant pressure (isobaric) intersect surfaces of constant density (isosteric).

biogeochemical cycle

Movements through the Earth system of key chemical constituents essential to life, such as carbon, nitrogen, oxygen, and phosphorus.

blackbody

1. An ideal emitter that radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature. A blackbody also absorbs all the radiant energy in the near visible spectrum incident upon it. No actual substance behaves as a true blackbody, although platinum black and other soots rather closely approximate this ideal. However, one does speak of a blackbody with respect to a particular wavelength interval. This concept is fundamental to all the radiation laws, and is to be compared with the similarly idealized concepts of the whitebody and the graybody. In accordance with Kirchhoff's law, a blackbody not only absorbs all wavelengths but emits at all wavelengths and does so with maximum possible intensity for any given temperature.

2. A laboratory device that simulates the characteristics of a blackbody. See blackbody radiator.

blackbody radiation

1. The electromagnetic radiation emitted by an ideal blackbody; it is the theoretical maximum amount of radiant energy of all wavelengths that can be emitted by a body at a given temperature. The spectral distribution of blackbody radiation is described by Planck law and the related radiation laws. If a tiny opening is made into an otherwise completely enclosed space (hohlraum), the radiation passing out through this hole when the walls of the enclosure have come to thermal equilibrium at some temperature will closely approximate ideal blackbody radiation for that temperature.

2. Any physical body absorbs and emits electromagnetic radiation when its temperature is above absolute zero. Planck's law determines the radiant flux of a body at a specific wavelength. In atmospheric chemistry, the calculation involving Earth's blackbody radiation shows that Earth's surface temperature would be below the freezing point of water if it did not have an atmosphere that absorbed some of the outgoing radiation.

blackbody radiator

A hypothetical, ideal radiator that totally absorbs and reemits all energy incident upon it. Actual objects only approach this ideal.

Boltzmann's constant (1.38 x 10²³ J/K )

The ratio of the universal gas constant to Avogadro number; equal to 1.38054 x 10^-16 erg/degrees K. Sometimes called gas constant per molecule, Boltzmann universal conversion factor.

bootstrap technique

1. Referring to a self-generating or self-sustaining process; specifically, the operation of liquid propellant rocket engines in which, during mainstage operation, the gas generator is fed by the main propellants pumped by the turbopump, and the turbopump in turn is driven by hot gases from the gas generator system. Such a system must be started in its operation by outside power or propellants. When its operation is no longer dependent on outside power or propellants the system is said to be in bootstrap operation.

2. In computer operations, the coded instructions at the beginning of an input tape which together with manually inserted instructions initiate a routine.

Bouguer-Lambert-Beer Law

A relationship describing the rate of decrease of flux density of a plane-parallel beam of monochromatic radiation as it penetrates a medium that both scatters and absorbs at that wavelength. This law was first established experimentally by Bouguer in 1760, Beer applied it to transmission of light through a turbid liquid, and the law was rediscovered by Lambert.

Brewer-Dobson circulation

The simple circulation model suggested by Brewer (1949) and Dobson (1956) consists of three basic parts. The first part is rising tropical motion from the troposphere into the stratosphere. The second part is poleward transport in the stratosphere. The third part is descending motion in both the stratospheric middle and polar latitudes, though there are important differences. The middle latitude descending air is transported back into the troposphere, while the polar latitude descending air is transported into the polar lower stratosphere where it accumulates. This model explains why tropical air is lower in ozone than polar air, even though the source region of ozone is in the tropics. The mechanism behind the Brewer-Dobson circulation is both complex and interesting. At first glance, we might expect that the circulation results from solar heating in the tropics and cooling in the polar region, causing a large equator to pole (meridional) overturning of air as warm (tropical) air rises and cold (polar) air sinks. While this heating and cooling does indeed occur, and while such a meridional overturning exists in the form of the Hadley Cell, it is not the specific reason for the existence of the Brewer-Dobson circulation. Rather, the Brewer-Dobson circulation results from wave motions in the extratropical stratosphere. Planetary waves are occasionally diverted from their equatorward propagation and propagate vertically into the polar stratosphere where they deposit their easterly momentum. This results in a deceleration of the wintertime westerly jet stream (see polar night jet). The polar night jet slows and can even be displaced, which has the effect of displacing the polar vortex region. It produces the phenomenon of stratospheric sudden warming as warmer middle latitude and even tropical air intrudes into the geographic polar region. This result is a situation that is thermodynamically imbalanced. Wintertime radiational cooling in the polar stratosphere quickly begins. This air cooling is accompanied by sinking motions, since colder air is more dense and sinks. It is this sinking motion that establishes the meridional overturning from equator to pole in the winter hemisphere. That is, the sinking air in the polar region must be balanced by a poleward flow of air into this region. By mass continuity requirements this air must come from the tropics. Our Brewer-Dobson circulation cell is thus established as tropical air moving poleward to replace the sinking air at the poles is itself replaced by rising air in the tropics.

BUV ozone profiling technique

Method of measuring total ozone employing the backscatter ultraviolet (BUV) technique. For determining total ozone, two pairs of measurements are made. One measurement of incoming UV light (irradiance) and backscattered UV light (radiance) is made at a wavelength that is strongly absorbed by ozone. The other measurement of incoming UV irradiance and backscattered UV radiance is made at a wavelength that is weakly absorbed by ozone. The measurements of incoming UV irradiance and backscattered UV radiance at the weakly absorbing wavelength are the control case. They tell us how much backscattered UV light we would expect to measure if there were no change caused by ozone absorption. At the other wavelength, the UV light is continuously being absorbed as it passes through the atmosphere by the amount of ozone along the light path. The differences in the pair measurements at the two wavelengths are used to infer how much ozone is present in the atmosphere.

catalyst

A substance, usually present in small amounts, that causes chemical reactions without itself being consumed by those reactions.

catalysis

The causing or accelerating of a chemical change by the addition of a catalyst.

Celsius temperature scale

Same as centigrade temperature scale -- a temperature scale with the ice point at 0° and the boiling point of water at 100°. Now called Celsius temperature scale. Conversion to the Fahrenheit temperature scale is according to the formula °C = 5/9 (°F -32).

Chapman reactions

The set of reactions between molecular and atomic oxygen and solar ultraviolet radiation hypothesized by Champman that are responsible for the creation and destruction of stratospheric ozone. These reactions are given in Lecture 5, Chapter 2.

chlorofluorocarbon (CFC)

A stable chemical compound used in refrigerants, solvents, and (in the past in the U.S.) aerosols that release chlorine (important) and fluorine (less important) into the upper atmosphere. In the stratosphere, CFCs are photolyzed by incoming solar UV to form carbon dioxide, CO2, hydrogen fluoride, HF, and ultimately (after multiple UV absorption events) chlorine radicals. These chlorine species are crucial in the destruction of the ozone layer over Antarctica and probably elsewhere.

chromosphere

A thin layer of relatively transparent gases above the photosphere of the Sun.

collar

A region of higher ozone concentrations that surrounds the Antarctic polar vortex region of lower ozone concentrations. The Brewer-Dobson circulation causes ozone rich air to accumulate in the high southern polar latitudes, while the extremely cold conditions inside the Antarctic vortex leads to the photochemical processes that cause ozone loss. As a result, there is a region of high ozone concentrations surrounding a region of low ozone concentrations centered on the South Pole. This is referred to as the "collar region."

conduction

The transfer of energy within and through a conductor by means of internal particle or molecular activity and without any net external motion. Conduction is to be distinguished from convection (of heat) and radiation (of all electromagnetic energy).

confidence interval

1. An interval around a sample mean that is likely to contain the population mean.

2. In statistics, a range of values believed to include, with a preassigned degree of confidence, the true characteristic of the lot or universe a given percentage of the time. For example: 95% confidence limits for a sample of 10 with a ratio of successes to total number tested of 0.9 (9 successes and 1 failure) would be 0.54 to 1.0. That is, even with an observed success ratio of 0.9 (90 percent) the best that can be said is that the true ratio lies between 0.54 (54 percent) and 1.0 (100 percent) an estimated 95 percent of the time.

continuity equation

A mathematical equation that states that, in a continuous fluid or gaseous medium, the mass of fluid material passing into a given volume must be equal to that coming out unless a density change has occurred in the volume.

continuous spectrum

1. A spectrum in which wavelengths, wave numbers, and frequencies are represented by the continuum of real numbers or a portion thereof rather than by a discrete sequence of numbers.

2. For electromagnetic radiation, a spectrum that exhibits no detailed structure and represents a gradual variation of intensity with wavelength from one end to the other, as the spectrum from an incandescent solid. Also called continuum, continuum radiation.

3. For particles, a spectrum that exhibits a continuous variation of the momentum or energy.

continuum radiation

See continuous spectrum.

Coriolis force

An apparent force arising from the fact that Earth turns on its axis. It is an apparent force that makes sense only because Earth is a noninertial frame of reference. Earth's spinning creates a constant centrifugal acceleration in which objects appear to curve because Earth is spherical, with different points on the surface spinning at different speeds.. If, instead of being a spherical, rotating planet, Earth were flat, there would be no Coriolis force because all points would spin at the same speed. The magnitude of the Coriolis force acting on an object depends on the Coriolis parameter.

Coriolis parameter

The magnitude of acceleration of an object (including a parcel of air) caused by Earth's rotation. It defines a quantity known as planetary vorticity, the spin imparted to objects on Earth, including molecules of air, because of the rotation of the planet itself. The Coriolis parameter is given by f = 2 x x sine where denotes Earth's rotation rate and is the latitude. Note that f varies as the sine of the latitude, so f = 0 at the Equator, since = 0. It increases with increasing latitude, reaching a maximum of f = 2 at the North Pole, since = 90°, and f = -2 at the South Pole, since = -90°. The degree of the acceleration is related to the velocity of the object. For an object moving with both zonal and meridional velocity components, denoted u and v, the acceleration in the zonal direction is given by -fv, while the acceleration in the meridional direction is given by fu. The Coriolis parameter (i.e., planetary vorticity) is used in the definition of absolute vorticity. For full explanation, see vorticity.

cosmic rays (also called cosmic radiation)

The aggregate of extremely high energy subatomic particles that travel the solar system and bombard Earth from all directions. On colliding with atmospheric particles they produce many different kinds of lower energy secondary cosmic radiation. Cosmic rays thought to originate outside the solar system are called galactic cosmic rays. Those thought to originate in the Sun are called solar cosmic rays. In Earth's atmosphere, the maximum flux of cosmic rays is at an altitude of 20 km. Below this the absorption of the atmosphere reduces the flux, though the rays are still readily detectable at sea level. Intensity of cosmic ray showers has also been observed to vary with latitude, being more intense at the poles.

cross-section

A measure of the effectiveness of a particular process expressed either as an area (geometric cross-section), which would produce the observed result, or as a ratio. See absorption cross-section, scattering cross-section.

cyclone

A large-scale, atmospheric wind-and-pressure system characterized by low pressure at its center and by circular wind motion, counterclockwise in the northern hemisphere, clockwise in the southern hemisphere. When viewed from above, a cyclone has the same sense of rotation as that of Earth's rotation when viewed from above the north or south pole.

cyclonic

Having a sense of rotation about the local vertical the same as that of Earth's rotation; that is, as viewed from above, counterclockwise in the northern hemisphere, clockwise in the southern hemisphere, undefined at the equator.

denitrification

In stratospheric photochemistry it is the process of removing reactive nitrogen (in the form of nitric acid, HNO3) from the stratosphere via particle sedimentation that occurs as polar stratospheric clouds slowly settle out of the stratosphere during the polar night. More generally, denitrification is the process of removing HNO3 via particle sedimentation, a step in the nitrogen cycle that involves reducing nitrates into nitrite, nitrous oxide, ammonia, or elemental nitrogen. It is carried out by certain forms of denitrifying bacteria in the soil and serves as an important part of the breakdown of dead organisms. It is responsible for the loss of much of the soil's natural and synthetic fertilizers. This process is favored most in warm, anaerobic conditions.

denoxification

The process of sequestering reactive nitrogen in the form of nitric acid (HNO3). The reaction is a heterogeneous one, meaning it occurs on the surface of polar stratospheric cloud (PSC) particles. The sequestered nitrogen can then be removed from the stratosphere by sedimentation in the denitrification process.

diabatic

Involving a thermodynamic change of state of a system in which there is a transfer of heat energy across system boundaries.

diabatic heating or cooling

See diabatic process.

diabatic process

A process in a thermodynamic system in which there is a transfer of heat across the boundaries of the system. The term diabatic process is preferred to the term nonadiabatic process.

dissociation

The separation of a complex molecule into constituents by collision with a second body, or by absorption of a photon. The product of dissociation of a molecule is two ions, one positively charged and one negatively charged.

Dobson instrument

See Dobson spectrophotometer.

Dobson spectrophotometer

A photoelectric spectrophotometer that is used to determine ozone content of the atmosphere. It compares solar energy at two wavelengths in the absorption band of ozone by permitting the two radiations to fall alternately onto a photocell. The stronger radiation is then attenuated by an optical wedge until the photometer's photoelectric system indicates equality of incident radiation. The ratio of radiation intensity is obtained by this process, and the ozone content of the atmosphere is computed from the ratio. See also photometer and spectrophotometer.

Dobson units (DU)

The standard way to express ozone amounts in the atmosphere. One DU is 2.7 x 10¹⁶ ozone molecules per square centimeter. One Dobson unit refers to a layer of ozone that would be 0.001 cm thick under conditions of standard temperature (0°C) and pressure (the average pressure at the surface of Earth). For example, 300 Dobson units of ozone brought down to the surface at 0°C would occupy a layer only 0.3 cm thick in a column. Dobson was a researcher at Oxford University who, in the 1920s, built the first instrument (now called the Dobson meter) to measure total ozone from the ground.

dry adiabat

A line of constant potential temperature on a thermodynamic diagram. In terms of pressure p, and specific volume v, the equation for a dry adiabat may be written

pvcp /cv = Constant

where cp and cv are the specific heats of dry air at constant pressure and volume, respectively. Meteorologically the dry adiabat is intended to represent the lifting of dry air in a dry adiabatic process. Since this is also an isentropic process, a dry adiabat is an isentrope.

dry adiabatic lapse rate

A special process lapse rate of temperature, defined as the rate of decrease of temperature with height of a parcel of dry air lifted adiabatically through an atmosphere in hydrostatic equilibrium. This lapse rate is g/cpd, where g is the acceleration of gravity and cpd is the specific heat of dry air at constant pressure; numerically equal to 9.767° C per km or about 5.4° F per thousand feet.

dynamical theory

One of the three theories initially proposed for Antarctic stratospheric ozone loss. The dynamical theory proposed that the Antarctic circulation had changed. It had long been recognized that the dominant circulation of the lower stratosphere in winter involves the poleward and downward motion of ozone rich air from the middle and upper stratosphere of the tropics where ozone is photochemically created all year long. This is the Brewer-Dobson circulation, which results in a poleward and downward motion of ozone rich air that leads to a buildup of ozone in the mid to high latitudes during winter. The dynamical theory proposed that this normal pattern was changing and that ozone poor air from the troposphere was being transported into the lower stratosphere instead of the ozone rich air. Observations showed that the dynamical theory for ozone loss was incorrect. Instead, the heterogeneous chemistry theory was shown to be correct. Compare also nitrogen oxide theory.

dynamical wave activity

Refers generally to any wave activity in the atmosphere that arises from momentum imbalances; that is, imbalances in the momentum equation terms, such as pressure gradient force and Coriolis force. See wave. See also momentum balance.

dyne

That unbalanced force that acting for 1 second on a body of 1 gram mass produces a velocity change of 1 centimeter per second. The dyne is the unit of force in the CGS system. Used in the definition of torr.

Earth-Sun distance

See mean Earth-Sun distance.

easterly

Any winds with components from the east, usually applied to broad currents or patterns of persistent easterly winds, the “easterly belts,” such as the equatorial easterlies, the tropical easterlies (see also trade winds), and the polar easterlies.

Ekman layer

One of the main layers of the troposphere, it is the layer of transition between the surface boundary layer, where shearing stress is constant, and the free atmosphere, where the atmosphere is treated as an ideal fluid in approximate geostrophic balance. Also called spiral layer. Together with the surface boundary layer, it makes up the planetary boundary layer.

El Niño events

An irregular variation of ocean current that from January to March flows off the west coast of South America carrying warm, low salinity, nutrient poor water to the south. It does not usually extend farther than a few degrees south of the equator, but occasionally it does penetrate beyond 12° S displacing the relatively cold Peru Current. The effects of this phenomenon are generally short-lived, and fishing is only slightly disrupted. Occasionally (in 1891, 1925, 1941, 1957-58, 1965, 1972-73, 1976, and 1982-83), the effects are major and prolonged. Under these conditions, sea surface temperatures rise along the coast of Peru and in the equatorial eastern Pacific Ocean and may remain high for more than a year, having disastrous effects on marine life and fishing. Excessive rainfall and flooding occur in the normally dry coastal area of western tropical South America during these events. Some oceanographers and meteorologists consider only the major, prolonged events as El Niño phenomena rather than the annually occurring weaker and short-lived ones. The name was originally applied to the latter events because of their occurrence at Christmas time.

electromagnetic radiation

Energy propagated through space or through material media in the form of an advancing disturbance in electric and magnetic fields existing in space or in the media. The term radiation alone is used commonly for this type of energy, although it actually has a broader meaning. Also called electromagnetic energy or simply radiation. See electromagnetic spectrum.

electromagnetic spectrum

The ordered array of known electromagnetic radiations, extending from the shortest cosmic rays, through gamma rays, X-rays, ultraviolet radiation, visible radiation, infrared radiation, and including microwave and all other wavelengths of radio energy. See absorption spectrum. The division of this continuum of wavelengths (or frequencies) into a number of named subportions is rather arbitrary and, with one or two exceptions, the boundaries of the several subportions are only vaguely defined. Nevertheless, to each of the commonly identified subportions there correspond characteristic types of physical systems capable of emitting radiation of those wavelengths. Thus gamma rays are emitted from the nuclei of atoms as they undergo any of several types of nuclear rearrangements; visible light is emitted, for the most part, by atoms whose planetary electrons are undergoing transitions to lower energy states; infrared radiations are associated with characteristic molecular vibrations and rotations; and radio waves, broadly speaking, are emitted by virtue of the accelerations of free electrons (the moving electrons in a radio antenna wire).

electromagnetic waves

A wave produced by oscillation of an electric charge. See electromagnetic radiation.

elliptic

An object having the shape of an ellipse -- a squashed or elongated circle. The Sun follows an elliptic path across the sky.

elliptical

Pertaining to or having the form of an ellipse.

emission spectrum

The array of wavelengths and relative intensities of electromagnetic radiation emitted by a given radiator. Each radiating substance has a unique, characteristic emission spectrum, just as every medium of transmission has its individual absorption spectrum.

emissivity

A property of a material, measured as the emittance of a specimen of the material that is thick enough to be completely opaque and has an optically smooth surface.

ENSO (El Niño-Southern Oscillation)

Interacting parts of a single global system of climate fluctuations, ENSO is the most prominent known source of interannual variability in weather and climate around the world, though not all areas are affected. The Southern Oscillation (SO) is a global scale seesaw in atmospheric pressure between Indonesia and North Australia and the southeast Pacific. In major warm events El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the SO pattern. Many of the countries most affected by ENSO events are developing countries that depend on their agricultural and fishery sectors as a major source of food supply, employment, and foreign exchange. New capabilities to predict the onset of ENSO events can have a global impact. While ENSO is a natural part of Earth's climate system, whether its intensity or frequency may change as a result of global warming is an important concern. See also El Niño events and Southern Oscillation.

entropy

1. A measure of the extent to which the energy of a system is unavailable. A mathematically defined thermodynamic function of state, the increase in which gives a measure of the energy of a system that has ceased to be available for work during a certain process

      ds = (du + pdv)/T >= dq/T

where s is specific entropy; u is specific internal energy; p is pressure; v is specific volume; T is Kelvin temperature; and q is heat per unit mass. For reversible processes,

      ds = dq/T

In terms of potential temperature,

       ds = cp (d/)

where represents potential temperature and cp is the specific heat at constant pressure. See third law of thermodynamics. In an adiabatic process, the entropy increases if the process is irreversible and remains unchanged if the process is reversible. Thus, since all natural processes are irreversible, it is said that in an isolated system the entropy is always increasing as the system tends toward equilibrium, a statement that may be considered a form of the second law of thermodynamics.

equatorial wave

One of several important wave motions that occur in the tropical atmosphere (and tropical oceans). These include the high-frequency inertiogravity waves that can propagate either eastward or westward and the low-frequency Rossby waves that propagate westward. Bridging the gap between these two is the mixed Rossby-gravity wave that propagates eastward at high frequencies like the inertiogravity waves and westward at low frequencies like the Rossby wave. The change in sign of the Coriolis parameter at the Equator plays a key role in these waves. A final, uniquely equatorial wave type is the Kelvin wave.

ERBE (Earth Radiation Budget Experiment)

An experiment to obtain data to study the average radiation budget of Earth and determine the energy transport gradient from the equator to the poles. Three satellites were flown in different orbits to obtain the data: the Earth Radiation Budget Satellite (ERBS) launched in October 1984, NOAA-9 launched in December 1984, and NOAA-10 launched in September 1986.

erythema

A reddening of the skin as a result of capillary dilation. Several forms of erythema can be caused by undue exposure of the human body to weather elements. The most common is sunburn.

extinction coefficient

1. In meteorology, a measure of the space rate of diminution, or extinction, of any transmitted light; thus, the attenuation coefficient applied to visible radiation. The extinction coefficient is identified as

      dI = -Idx

            or

      I = I0 exp(-x)

where I is the illuminance (luminous flux density) at the selected point in space, I0 is the illuminance at the light source; and x is the distance from the source. When so used, the extinction coefficient equals the sum of the medium's absorption coefficient and scattering coefficient, each computed as a weighted average over all wavelengths in the visible spectrum. As long as scattering effects are primary, as in the lower atmosphere, the value of the extinction coefficient is a function of the particle size of atmospheric suspensoids. It varies in order of magnitude from 10 per kilometer with very low visibility to 0.01 per kilometer in very clear air.

2. A measure of the rate of the reduction of transmitted light through a substance.

extinction cross-section

See scattering cross-section.

Extreme Ultraviolet Radiation (EUV)

Solar radiation with wavelength less than 240 nanometers. Such energetic radiation is found mainly in the upper stratosphere.

Faculae

Large patches of bright material forming a veined network in the vicinity of sunspots. They appear to be more permanent than sunspots and are probably caused by elevated clouds of luminous gas. Compare to flocculi, prominence, and solar flare.

Fahrenheit temperature scale

The temperature scale designed by German scientist Gabriel Fahrenheit in 1709 based on water freezing at 32°F and water boiling at 212°F under standard atmospheric pressure. Compare with Celsius temperature scale. Conversion with the Celsius (centigrade) scale is given by the formula F=(9/5)C + 32.

First Law of Thermodynamics

A statement of the conservation of energy for thermodynamic systems (not necessarily in equilibrium). The fundamental form requires that the heat absorbed by the system serve either to raise the internal energy of the system or work on the environment:

      dq = du + dw

where dq is the heat added per unit mass; du is the increment of specific internal energy; and dw is the specific work done by the system on the environment. Although dq and dw are not perfect differentials, their difference, du, is always a perfect differential. Example of the application of this equation: in an adiabatic free expansion of gas into a vacuum, all three terms are zero. For reversible processes the mechanical work is equal to the expansion against the pressure forces, i.e.,

      dw = pdv

where p is the pressure and v is the specific volume. For a perfect gas, the internal energy change is proportional to the temperature change,

      du = cvdT

where cv is the specific heat at constant volume and T is the Kelvin temperature. Therefore, the form of the first law usually used in meteorological applications is

      dq = cvdT + pdv

Use of the equation of state yields an alternative form,

      dq = cpdT - dp

where cp is the specific heat at constant pressure. For open systems the variation of total rather than specific quantities is important:

      dQ = dU + pdV - hdm

where Q is the total heat; U is the total internal energy; V is the volume; m is the mass of the system; and h is the specific enthalpy. If a system contains the possibility of nonmechanical work, such as work done against an electric field, this work must be included in the first law. See Second Law of Thermodynamics and Third Law of Thermodynamics.

flocculi

Patches of relatively dense, dark, or bright clouds in the Sun's atmosphere. They appear in photographs taken with the spectroheliograph. The emission spectra usually studied are those of calcium and hydrogen; e.g., bright calcium flocculi, and dark or bright hydrogen flocculi.

fractionation

The act or process of separating (a mixture) into its ingredients or into portions having different properties, as by distillation or crystallization.

free atmosphere

That portion of Earth's atmosphere above the planetary boundary layer in which the effect of Earth's surface friction on the air motion is negligible, and in which the air is usually treated (dynamically) as an ideal fluid. The base of the free atmosphere is usually taken as the geostrophic wind level. Also called free air.

frequency

Number of cycles and parts of cycles completed per second. F=1/T, where T is the length of one cycle in seconds. In the International System, the cycle per second is called the Hertz.

fumarole

A hole in a volcanic area from which hot smoke and gases rise.

gamma ray

A quantum of electromagnetic radiation emitted by a nucleus, each such photon being emitted as the result of a quantum transition between two energy levels of the nucleus. Gamma rays have energies usually between 10 thousand and 10 million electron volts with correspondingly short wavelengths and high frequencies. Also called gamma radiation. X-rays occur in the same energy range as gamma rays but are of nonnuclear origin. In atmospheric electricity gamma rays, along with alpha and beta particles, are of some importance in contributing to atmospheric ionization. Gamma ray photons have much greater penetration ranges than do alpha and beta particles, often amounting to distances of the order of a hundred meters in air at sea level. These high-energy photons may initiate their ionizing action by ejecting photoelectrons from neutral atoms or molecules of the air, by ejecting electrons by the Compton effect, or (for gamma photons with energies above a few million electron volts) by pair production in which an electron and a positron are created.

gas-phase photochemical production

The production of ozone via the Chapman reactions where molecular oxygen (O2) is broken apart by the Sun's radiation, and the individual oxygen (O) atoms react with O2 to form ozone.

geopotential

The potential energy of a unit mass relative to sea level; numerically equal to the work done in lifting the mass from sea level to the height at which the mass is located

geopotential height

The height of a given point in the atmosphere in units proportional to the potential energy of unit mass (geopotential) at this height, relative to sea level. In the CGS system, the relation between the geopotential height H and the geometric height Z is where g is the acceleration of gravity, so that the two heights are numerically interchangeable for most meteorological purposes. Also, 1 geopotential meter is equal to 0.98 dynamic meter. At present, by convention of the World Meteorological Organization, the geopotential height unit is used for all aerological reports.

geostrophic balance

The balance of the horizontal pressure gradient and the acceleration provided by the spinning of Earth; i.e., the Coriolis force. The horizontal pressure gradient arises because of differential solar heating that occurs at different latitudes. The Coriolis force arises because of Earth's (hence the atmosphere's) spinning on its axis. See pressure gradient force and Coriolis force. See also thermal wind.

geostrophic wind

The horizontal wind velocity for which the Coriolis force exactly balances the horizontal pressure gradient force.

Goddard radiation model

A two-dimensional (2D) or zonal mean stratospheric ozone photochemical model devised by scientists at NASA's Goddard Space Flight Center in Greenbelt, MD. The Goddard model, like all 2D models, takes a zonal mean of some atmospheric variable, such as temperature, ozone concentration, wind speed, et al., by averaging all values of that variable around a circle of constant latitude. For many stratospheric trace gases, their zonal (east-west) variations in concentrations are smaller than their vertical or meridional (north-south) variation. Owing to this fact, the zonal mean value of the constituent is a good approximation to its actual value at a particular longitude, though significant departures from the zonal mean field in certain variables exist. Therefore, it is reasonable to develop a model that neglects zonal (longitudinal) variability and only calculates constituent concentrations as a function of latitude and altitude. The 2D model incorporates both a radiation component and a dynamics component into the overall algorithm. The radiation component attempts to represent accurately the effects of solar radiation and the resulting photochemical processes in the atmosphere. The dynamics component attempts to represent accurately horizontal and vertical motions in the atmosphere. 2D models like the Goddard radiation model are used to understand the meridional and vertical structure of atmospheric trace species, assess the impact of natural and anthropogenic (manmade) perturbations to the stratosphere, understand the factors influencing past trends in ozone and predict future changes, and determine how sensitive (how well) our predictions of changes in various trace gas concentrations over time are to the assumptions we make in the model chemistry. The Goddard radiation model has been used extensively to estimate the changes over time in concentrations of the ozone destroying chlorofluorocarbons (CFCs), and to compare the results to measurements made by satellite instruments. See Lecture 12, Section 3.2 for further details.

gravity waves

Waves whose restoring mechanism is their buoyancy are called gravity waves. These waves can be understood in terms of static stability. Start with an atmosphere in which potential temperature increases with altitude, such as occurs in the stratosphere. If an air parcel is suddenly displaced to a lower altitude it will be warmer than its surroundings and will begin to rise. Like the mass on an oscillating spring or pendulum, as it rises, it picks up momentum, passes through its equilibrium position, then finds itself surrounded by warmer air. Being cooler than its surroundings, it begins to sink back toward equilibrium, but having too much momentum it falls below the equilibrium level to a place where it is again warmer than its surroundings. This oscillation about an equilibrium point occurs because of the stability parameter. The oscillation occurs with a set frequency. The period or time required for one oscillation for these vertical displacements depends on the stability, but it is typically about 7 minutes. The restoring mechanism in the vertical is the stable stability (stratification) of the wave medium. Such stratification is associated with hydrostatic balance. The term gravity is used because gravity is involved in hydrostatic balance. Gravity waves are sometimes referred to as internal gravity waves because oscillations occur inside boundaries; i.e., on sharp density changes. An example are gravity waves in water as they travel along a density discontinuity, either within the water or on its surface. In the atmosphere, where density discontinuities are much smaller, gravity ways can travel vertically from the troposphere into the stratosphere and even the mesosphere. In the atmosphere, such a wave is sometimes called an atmospheric gravity wave.

greenhouse effect

1. The phenomenon in which outgoing infrared radiation that would normally exit from a planet's atmosphere but instead is trapped or reflected because of the presence of the atmosphere and its components is called the greenhouse effect. It has been calculated that this effect is necessary to maintain Earth's climate and surface temperature and, more importantly, the liquid state of water in the majority of Earth's biosphere. However, the best scientific estimates to date suggest that increasing amounts of greenhouse gases are resulting in higher temperatures worldwide. This could result in melting icecaps that would raise sea level and cause devastating floods in coastal areas, more extremes in rainfall and intensity, and the distribution of species in the biosphere.

2. A popular term used to describe the roles of water vapor, carbon dioxide, and other trace gases in keeping Earth's surface warmer than it would otherwise be. These "radiatively active" gases are relatively transparent to incoming shortwave radiation but are relatively opaque to outgoing longwave radiation. The latter radiation, which would otherwise escape to space, is trapped by these gases within the lower levels of the atmosphere. The subsequent reradiation of some of the energy back to the surface maintains surface temperatures higher than they would be if the gases were absent. There is concern that increasing concentrations of greenhouse gases (including carbon dioxide, methane, and manmade chlorofluorocarbons) may enhance the greenhouse effect and cause global warming.

3. The heating effect exerted by the atmosphere on Earth by virtue of the fact that the atmosphere (mainly its water vapor) absorbs and remits infrared radiation. In detail, the shorter wavelengths of insolation are transmitted rather freely through the atmosphere to be absorbed at Earth's surface. Earth then reemits this as long-wave (infrared) terrestrial radiation, a portion of which is absorbed by the atmosphere and again emitted. Some of this is emitted downward back to Earth's surface (counterradiation). The mean surface temperature for the entire world, 14° C, is almost 40° C higher than the mean temperature required for radiative equilibrium of a blackbody at Earth's mean distance from the Sun. In understanding the concept of the greenhouse effect, it is essential to note that the important additional warming is due to the counterradiation from the atmosphere. The glass panes of a greenhouse function in this manner, hence the name.

greenhouse gases

Those atmospheric components that absorb strongly in the infrared region of the spectrum. Infrared radiation is reflected and emitted by Earth's surface as heat and causes a fairly large warming effect when trapped by these gases in the atmosphere. In order of abundance and importance as greenhouse gases are water vapor, carbon dioxide, ozone, nitrous oxide, methane, and chlorofluorocarbons (CFCs). Absorption by water vapor, the most common greenhouse gas, explains why many humid or cloudy days feel much hotter than dry, clear days of the same air temperature. Because of these gases, only about 5% of the radiation escapes from the atmosphere while more than 90% is radiated back to Earth.

ground state

The state of least energy in a physical system.

Hadley cell

A direct thermally driven and zonally symmetric large-scale atmospheric circulation first proposed by George Hadley in 1735 as an explanation for the trade winds. It carries momentum, sensible heat, and potential heat from the tropics to the midlatitudes (30°). The poleward transport aloft is complemented by subsidence in the subtropical high pressure ridge and a surface return flow. The variability of this cell and the Walker cell is hypothesized to be a major factor in short-term climatic change.

halocarbon

A compound containing carbon and at least one halogen.

halogen

Any one of the following elements: astatine, bromine, chlorine, fluorine of iodine.

halon

Compounds containing bromine that are used as fire extinguishing agents.

hard solar UV

Radiation of high penetrating power; that is, radiation of high frequency and short wavelength. Relates to hard radiation. A 10-centimeter thickness of lead is usually used as the criterion upon which the relative penetrating power of various types of radiation is based. Hard radiation will penetrate such a shield; soft radiation will not.

harmonics

1. An integral multiple or submultiple of a given frequency; a sinusoidal component of a periodic wave.

2. A signal having a frequency that is a harmonic (sense 1) of the fundamental frequency.

heat energy

Energy associated with the random motion of atoms and molecules in a substance; in general, this energy can be measured by temperature. Heat energy exists as either sensible heat or latent heat.

heating rates

The increase in temperature of an air parcel with time caused by a physical process, such as sensible or latent heat flux or radiative heating. It can also be due to a dynamical process such as advection. See sensible heat flux, latent heat flux, radiative heating, and advection.

Heisenberg Uncertainty Principle

Werner Heisenberg's principle whereby it is not possible to know simultaneously with full certainty both the position and velocity (or more precisely, the momentum) of a subatomic particle such as an electron. In his 1927 paper Heisenberg stated, "The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa." It is also called the "principle of indeterminacy."

heterogeneous chemical reactions

The sets of chemical reactions that occur on the ice crystal (solid) surfaces of polar stratospheric clouds. The cloud particles provide a solid surface that allow certain types of reactions to occur that would otherwise not occur.

heterogeneous chemistry theory

One of the three proposed theories of Antarctic stratospheric ozone loss. It postulated that reactions were occurring on the surfaces of tiny cloud particles that form in the extremely cold conditions of the Antarctic winter stratosphere. The theory suggested that the "surfaces" provided by the cloud particles known as polar stratospheric clouds (PSCs) were altering the polar stratospheric chemistry. The compounds formed by the reactions on these PSCs were allowing nonreactive compounds containing chlorine to become reactive compounds. These reactive chlorine compounds were then able to catalytically destroy ozone at an extremely rapid rate. In addition to chlorine compounds, bromine compounds were also theorized to be participating in catalytic destruction of ozone. The heterogeneous chemistry theory ultimately proved to be the correct one to explain the large, seasonal losses observed in stratospheric ozone over Antarctica since the early 1980s. The source of the chlorine is now known to be principally from manmade chlorofluorocarbons (CFCs). Compare dynamical theory and nitrogen oxide theory.

heterogeneous processes

See heterogeneous chemical reactions.

homogeneous chemical reactions

The set of chemical reactions that occur in the gas phase only.

homogeneous processes

See homogeneous chemical reactions.

hPa

This is the abbreviation for hecto-Pascal: 1hPa=100 Pascals. A Pascal is a unit of atmospheric pressure or force exerted per unit area by the air. Atmospheric pressure can be thought of as the weight per unit area of the column of atmosphere above a given height. At the surface, this column includes all of the gases in the atmosphere, while at 10 km it includes only those gases above 10 km. The atmospheric pressure at Earth's surface is 101,325 Pa, while at 30 km, the pressure falls to about 1,000 Pa. Atmospheric scientists frequently use the unit hPa (for hectoPascal, which is 100 Pa) for atmospheric pressure. In these units, the surface pressure is 1013.25 hPa. A third unit for pressure used by atmospheric scientists and meteorologists is the millibar (mb). One mb equals one hPa, so average sea level pressure in millibars is 1013.25 mb. See bar and millibar.

hydrocarbons

Chemicals containing only carbon and hydrogen. These are of prime economic importance because they encompass the constituents of the major fossil fuels, petroleum and natural gas, as well as plastics, waxes, and oils. In urban pollution, these components, along with NOx and sunlight, contribute to the formation of tropospheric ozone.

hydrochlorofluorocarbon (HCFC)

The chemical species slated to replace CFCs in the near future in most Western nations. When the normal chlorofluorocarbons reach the stratosphere, they destroy the natural ozone that acts as our umbrella to shield Earth from ultraviolet radiation. With one or more hydrogen-carbon bonds, HCFCs are still useful as replacements for CFCs in most applications; however, they are now much more unstable and subject to hydroxyl radical and ozone attack early in their gas phase career in the atmosphere. Therefore their atmospheric lifetime is, hopefully, much shorter than the CFCs' lifetime, and they will have a lower chance of reaching the stratosphere where their chlorine can be released by destructive photolysis and thereby enter the catalytic ozone destruction cycle.

hydrofluorocarbon (HFC)

Compounds containing hydrogen, fluorine, and carbon. Unlike CFCs, they do not contain chlorine.

hydrostatic equilibrium

1. The state of a fluid whose surfaces of constant pressure and constant mass (or density) coincide and are horizontal throughout. Complete balance exists between the force of gravity and the pressure force.

2. Of a rotating body, a state in which the body maintains, or returns to, the figure generated by this rotation in spite of small disturbances.

hydroxyl radical (OH)

A radical consisting of one hydrogen atom and one oxygen atom that does not normally exist in a stable form; this radical readily reacts with methane and carbon monoxide. The source of the hydroxyl radical in the atmosphere occurs primarily through the photolysis of hydrogen peroxide, heat, and light and the attack on water of an excited oxygen radical (created by the photolysis of ozone).

incident radiation

Radiation falling on a surface. See also irradiance.

inertiogravity waves

Gravity waves with sufficiently long periods will begin to feel Earth's rotation so that the restoring force becomes a combination of rotation and buoyancy. Air or water experiencing buoyancy oscillations will also experience a Coriolis deflection. For instance, zonally propagating waves oscillating in the vertical will feel a Coriolis force that imparts a meridional velocity component. Waves that have both Rossby (see Rossby wave) and inertiogravity characteristics are called Rossby-gravity waves. The restoring force is thus the gradient of potential vorticity, the stability parameter, and the Coriolis parameter.

infrared cooling

The loss of energy to space by an object via infrared or thermal radiation. Objects warmer than their surroundings emit more energetic radiation than those that are cooler than their surroundings, so they cool faster. Therefore, in the absence of an external heat source, all objects in a confined space will eventually reach the same temperature. That is, they will reach thermal equilibrium. Also called longwave cooling. See also radiative cooling.

infrared radiation

Electromagnetic radiation lying in the wavelength interval from 0.7 µ to 1000 µ. Its lower limit is bounded by visible radiation, and its upper limit by microwave radiation. Most energy emitted by Earth and its atmosphere is at infrared wavelength. Infrared radiation is generated almost entirely by large-scale intramolecular processes. The triatomic gases, such as water vapor, carbon dioxide, and ozone, absorb infrared radiation and play important roles in propagating infrared radiation in the atmosphere. Abbreviated IR; also called "longwave radiation."

ionization

The process by which neutral atoms or groups of atoms become electrically charged, either positively or negatively, by the loss or gain of electrons; or the state of a substance whose atoms or groups of atoms have become thus charged.

ionosphere

The atmospheric shell characterized by a high ion density. Its base is at about 70 or 80 kilometers and it extends to an indefinite height. The ionosphere is classically subdivided into layers. Except for the D-layer, each layer is supposedly characterized by a regular maximum of electron density. The D-layer, starting at about 70 to 80 kilometers and merging with the bottom of the E-layer, exists only in daytime and isn't strictly a layer at all since it doesn't exhibit a peak of electron or ion density. The lowest clearly defined layer is the E-layer, occurring between 100 and 120 kilometers. The F1 and F2-layers occur in the general region between 150 and 300 kilometers, the F2-layer being always present and having the higher electron density. The existence of a G-layer has been suggested but is questionable. The portions of the ionosphere in which these layers tend to form are known as ionospheric regions, as in D-region, E-region, F-region, G-region. Sudden increases in ionization are referred to as sporadic, as in "sporadic E" or "sporadic D." The assumption that the ionosphere is stratified in the vertical into discrete layers is currently under serious question. Some evidence supports a belief that ion clouds are the basic elements of the ionosphere. Other investigations appear to reveal the ionosphere as a generally ionized region characterized by more or less random fluctuations of electron density.

ions

1. A charged atom or molecularly bound group of atoms; sometimes also a free electron or other charged subatomic particle. An ion pair consists of a positive ion and a negative ion (usually an electron) having charges of the same magnitude and formed from a neutral atom or molecule by the action of radiation. In spectroscopy, the degree of ionization of an atom is indicated by a Roman numeral following the symbol for the element. A nonionized atom is indicated by the Roman numeral I, a singly ionized atom (one that has lost one electron) is indicated by II, and so on. Thus Fe IX indicates the spectrum of an iron atom that has lost eight electrons.

2. In atmospheric electricity, any of several types of electrically charged submicroscopic particles normally found in the atmosphere. Atmospheric ions are of two principle types, small and large, although a class of intermediate ions has occasionally been reported. The ionization process that forms small ions depends on two distinct agencies, cosmic rays and radioactive emanations. Each of these consists of very energetic particles that ionize neutral air molecules by knocking out one or more planetary electrons. The resulting free electron and positively charged molecule (or atom) very quickly attach themselves to one or, at most, a small number of neutral air molecules, thereby forming new small ions. In the presence of Aitken nuclei, some of the small ions will in turn attach themselves to these nuclei, thereby creating new large ions. The two main classes of ions differ widely in mobility. Only the highly mobile small ions contribute significantly to the electrical conductivity of the air under most conditions. The intermediate ions and large ions are important in certain space charge effects but are too sluggish to contribute much to conductivity. The processes of formation of the ions are offset by certain processes of destruction of the ions.

3. In chemistry, atoms or specific groupings of atoms that have gained or lost one or more electrons, as the chloride ion or ammonium ion. Such ions exist in aqueous solutions and in certain crystal structures.

irradiance

1. The total radiant flux received on a unit area of a given real or imaginary surface. Also called the radiant flux density. Also called irradiancy when applied to a receiver.

2. Radiant flux density; i.e., radiant flux per unit area. Sometimes also called radiant emittance when applied to a source.

isentrope

A line of equal or constant entropy. In meteorology, it may be considered an isopleth (or constant line) of potential temperature, i.e., the same as a dry adiabat. See also tropopause folding.

isentropic

Of equal or constant entropy with respect to either space or time.

isentropic surface

A surface of constant entropy; in meteorology, a surface of constant potential temperature.

jet stream

1. A strong band of wind or winds in the upper troposphere or in the stratosphere, moving in a general direction from west to east and often reaching velocities of hundreds of miles an hour.

2. Rivers of high-speed air in the atmosphere. Jet streams form along the boundaries of global air masses where there is a significant difference in atmospheric temperature. The jet streams may be several hundred miles across and 1-2 miles deep at an altitude of 8-12 miles. They generally move west to east, and are strongest in winter with core wind speeds as high as 250 mph. Changes in the jet stream indicate changes in the motion of the atmosphere and weather.

joule

1. A unit of energy or work in the MKS system; the work done when the point of application of 1 Newton is displaced a distance of 1 meter in the direction of the force. 1 joule = 10,000,000 ergs = 1 watt second.

2. A unit of energy. One joule is equal to the work done when a current of 1 ampere is passed through a resistance of 1 ohm for 1 second. One joule = 10⁷ ergs = 9.48 x 10^-4 BTUs. A 100-watt light bulb uses 100 joules every second. Measuring joules allows the comparison of energy needs, capacities, and efficiencies. For example, all of the world's humanity used 31.5 x 10¹⁸ joules of electrical, mechanical, fossil fuel, and heat energy in 1990.

Junge layer

A layer of sulfuric acid droplets high in the stratosphere that arises from volcanic sulfur dioxide emissions.

Kelvin temperature scale

An absolute temperature scale independent of the thermometric properties of the working substance. On this scale, the difference between two temperatures, T1 and T2, is proportional to the heat converted into mechanical work by a Carnot engine operating between the isotherms and adiabats through T1 and T2. Also called absolute temperature scale, thermodynamic temperature scale. For convenience the Kelvin is identified with the Celsius degree. The ice point in the Kelvin scale is 273.15° K. The triple point of water, the fundamental reference point, is 273.16° K. See absolute zero.

Kelvin wave

A unique type of equatorial wave (see above) that propagates eastward like a pure gravity wave (i.e., a gravity wave whose restoring mechanism is buoyancy only). It has no meridional velocity and its zonal velocity is in geostrophic balance with the latitudinal pressure gradient. The Kelvin wave in the ocean travels along the region of tightest temperature, hence pressure and density gradient called the thermocline. The warming and cooling of the waters of the eastern Pacific in the ENSO phenomenon actually occurs as these Kelvin waves propagate along the thermocline, raising and lowering its height. Moving along the equator where the Coriolis parameter changes sign, these waves are referred to as equatorially trapped Kelvin waves. When it hits the boundary of the ocean -- the South American continent -- it reflects in two directions: a Kelvin wave propagates north and south along the coast while a Rossby wave propagates backward along the equator into the Pacific.

kinetic energy

The energy a body possesses as a consequence of its motion, defined as one-half the product of its mass m and the square of its speed v, 1/2 mv. The kinetic energy per unit volume of a fluid parcel is the 1/2 p v2, where p is the density and v the speed of the parcel. See potential energy.

Kirchhoff's Law

The radiation law that states that at a given temperature the ratio of the emissivity to the absorptivity for a given wavelength is the same for all bodies and is equal to the emissivity of an ideal blackbody at that temperature and wavelength. Loosely put, this important law asserts that good absorbers of a given wavelength are also good emitters of that wavelength. It is essential to note that the Kirchhoff law relates absorption and emission at the same wavelength and at the same temperature. Also called Kirchhoff radiation law.

lamina

A thin plate, sheet, or layer.

lapse rate

1. The decrease of an atmospheric variable with height, the variable being temperature unless otherwise specified. The term applies ambiguously to the environmental lapse rate and the process lapse rate, and the meaning must often be ascertained from the context.

2. The rapidity with which temperature decreases with altitude. The normal lapse rate is defined 3.6 F per 1000 feet change in altitude. The dry adiabatic lapse rate is about 5.5 F per 1000 feet, and the wet adiabatic lapse rate varies between 2-5 F per 1000 feet.

3. The theoretical rate of decrease of temperature with increasing height in the atmosphere. If heat is neither gained nor lost from the air parcel under consideration, then the lapse rate is said to be adiabatic.

latent heat

1. Energy transferred from Earth's surface to the atmosphere through the evaporation and condensation processes.

2. If a change of state occurs from gas to liquid or liquid to solid, internal energy in the form of heat is released. If a change of state occurs from solid to liquid or liquid to gas, heat is required. Different compounds absorb and release different amounts of latent heat. Water takes nearly 600 kilocalories for each kilogram of water condensed.

latent heat exchange

See latent heat flux.

latent heat flux

The rate of transfer of latent heat across a unit area. That is, the rate of energy transfer due to evaporation and condensation processes. Water evaporating off the surface of the ocean into the air is an example of latent heat flux.

lidar (light detection and ranging)

A technique for active remote sensing in which a light source is used to probe the atmosphere. Laser light fired at the atmosphere is reflected back by the atmospheric molecules to a detector and the attenuation (reduction) of this light provides information on atmospheric particles and molecules. Changes in the returned wavelengths can provide information about atmospheric motion. The primary advantage of this technique is its ability to obtain high vertical resolution data at different altitudes. This is important for studying how various trace gases (see Chapter 2) are transported by the wind. Lidar is also used to measure the cloud altitude. This information is important for pilots and for meteorological observations. See also ozone profile.

limb

The edge of the apparent disk of a celestial body, as of the Sun.

limb emission technique

This is one of several remote sensing techniques for measuring atmospheric trace gases by satellite. Also called limb sounding technique. Instruments based upon the limb emission technique infer trace gas amounts (such as ozone) from measurements of longwave radiation (infrared or microwave) thermally emitted in the atmosphere along the line of sight of the instrument. The altitude to which the instrument can see is called the tangent altitude. In theory, the instrument could look all the way to the surface, but below a certain altitude (under 10 km), clouds interfere with the emitted longwave radiation. This radiation emission occurs along the geometric path between the tangent altitude and the satellite instrument. This horizontal path is quite long compared to the tangent altitude. Compare to backscatter ultraviolet (BUV) technique and occultation technique.

limb sounding technique

See limb emission technique.

linear regression model

See simple regression model.

longwave cooling

See infrared cooling.

longwave radiation

See infrared radiation.

Lyman-alpha emission band

The radiation emitted by hydrogen at 1216 Angstrom, first observed in the solar spectrum by rocketborne spectrographs. Lyman-alpha radiation is very important in the heating of the upper atmosphere, thus it affects other atmospheric phenomena. Also called Lyman-alpha radiation.

Lyman-alpha line

See Lyman-alpha emission band.

mass continuity

A term based on conservation of mass principle and its mathematical form, the continuity equation. In meteorology, it refers to the requirement that air moving horizontally or vertically out of one region must be balanced by air moving into the region. Circulation cells, such as the Brewer-Dobson and Hadley cells are driven by mass continuity (conservation of mass) requirements.

Maunder minimum

The period from 1654 to 1714 when it was believed that there were no sunspots. It is now thought that there were some sunspots during that time but fewer than those counted after 1800. Sunspots occur in 11-year periods. They also occur in a 90-year cycle called the Gleissberg cycle. The Maunder minimum may form part of an as-yet-unknown larger cycle of sunspot activity whose periodicity is longer than the historical record.

mb

Abbreviation for millibars: see millibar.

mean Earth-Sun distance

The mean distance of Earth from the Sun, it defines one astronomical unit (AU) and is 149,597,870 km long. Intersteller distances are frequently given in terms of AU. Because Earth's orbit around the Sun is slightly elliptical, the actual distance at any given time of year will be slightly more or less than 1 AU. Geometrically speaking, the mean Earth-Sun distance is the length of the semimajor axis of Earth's orbit about the Sun.

mechanical damping

The suppression of oscillations or wave disturbances.

meridian

A north-south reference line, particularly a great circle through a planet's geographical poles. A terrestrial meridian is a line on Earth's surface that connects points having the same astronomical longitude -- also called astronomical meridian. A geodetic meridian is a line connecting points of equal geodetic longitude. Geodetic and sometimes astronomical meridians are also called geographic meridians. Geodetic meridians are shown on charts. The prime meridian passes through longitude 0°. A fictitious meridian is one of a series of great circles or lines used in place of a meridian for certain purposes. A transverse or inverse meridian is a great circle perpendicular to a transverse equator. An oblique meridian is a great circle perpendicular to an oblique equator. Any meridian used as a reference for reckoning time is called a time meridian. The meridian through any particular place or observer, serving as the reference for local time, is called local meridian, in contrast with the Greenwich meridian, the reference for Greenwich time. A celestial sphere, through the celestial poles and the zenith.

meridional

Referring to a meridian.

mesopause

1. The upper limit and the coldest portion of the mesosphere. The transition zone between the mesosphere and the thermosphere.

2. The top of the mesosphere around an altitude of 85 km where temperatures reach their lowest in the entire atmosphere.

mesosphere

The atmospheric shell in which temperature generally decreases with heights that extend from the stratopause at about 50-55 km to the mesopause at about 80-85 km. Compare to statosphere and troposphere.

Mie scattering

1. Developed by Gustav Mie in 1908, this is a complete mathematical-physical theory of the scattering of electromagnetic radiation by spherical particles. In contrast to Rayleigh scattering, the Mie theory embraces all possible ratios of diameter to wavelength. See size parameter. The Mie theory is very important in meteorological optics, where diameter-wavelength ratios of the order of unity and larger are characteristic of many problems regarding haze and cloud scattering. Scattering of radar energy by raindrops constitutes another significant application of the Mie theory. Compare to Rayleigh Scattering.

2. Processes by which particles of similar size and electrical characteristics separate or disperse different wavelengths (colors) of light. Since the Sun's visible spectrum contains a mixture of red, orange, yellow, green, blue, indigo, and violet colors, these wavelengths are differentially scattered by particles as they travel through the atmosphere.

millibar

A unit of pressure equal to 1000 dynes per square centimeter, or 1/1000 of a bar. The millibar is used as a unit of measure of atmospheric pressure, a standard atmosphere being equal to 1,013.25 millibars or 29.92 inches of mercury. Note that one mb equals one hPa. Compare to nanobar.

mixing ratio

1. In a system of moist air, the dimensionless ratio of the mass of water vapor to the mass of dry air. For many purposes, the mixing ratio may be approximated by the specific humidity. In terms of the pressure p and vapor pressure e, the mixing ratio w is

      w = (0.6222 e) / (p - e)

2. The fixed proportions in which two or more substances may become combined, such as the amount of nitrogen in the air compared to the rest of the air. Atmospheric scientists routinely discuss the gas phase concentrations of trace components in mixing ratios expressed in ratios of VOLUMES, as in "the present tropospheric mixing ratio for methane is 1.7 ppmv."

momentum

Quantity of motion. Linear momentum is the quantity obtained by multiplying the mass of a body by its linear speed. Angular momentum is the quantity obtained by multiplying the moment of inertia of a body by its angular speed. The momentum of a system of particles is given by the sum of the momentums of the individual particles that make up the system or by the product of the total mass of the system and the velocity of the center of gravity of the system. The momentum of a continuous medium is given by the integral of the velocity over the mass of the medium or by the product of the total mass of the medium and the velocity of the center of gravity of the medium.

momentum balance

Refers to the equilibrium condition among the various momentum equation terms, such as pressure gradient force and Coriolis force. Momentum imbalances lead to dynamical wave activity.

monochromatic irradiance

The irradiance from a single wavelength or narrow band of wavelengths of electromagnetic radiation.

monochromatic radiance

The radiance from a single wavelength or narrow band of wavelengths of electromagnetic radiation.

Montreal Protocol

An international agreement to prevent the use of substances that are harmful to ozone, such as chlorofluorocarbons and halons, in order to protect the ozone layer on a global level. It was agreed upon in 1987 and has been amended repeatedly since that time.

multiple regression model

A statistical regression model that contains more than one independent variable and takes the more general form

      y = m1x1 + m2x2 + m3x3 +...+ mnxn + b

where n is the total number of proxy terms in our model and b is the value of y when [x1, x2, x3, ..., xn] = 0. See regression analysis for explanation of proxy terms. Also called a multiple linear regression model. Compare to simple regression model.

N7

An abbreviation that refers to the Nimbus-7 satellite.

N7 TOMS

See both Nimbus-7 and TOMS.

nacreous clouds

Clouds that occur in the stratosphere at altitudes above 20 km and are usually iridescent and luminous in color. They may also be called mother-of-pearl clouds. Nacreous cloud formation occurs during winter conditions at higher latitudes. Nacreous clouds and polar stratospheric clouds are the two main types of stratospheric clouds.

nanobar

A unit of pressure corresponding to 1-billionth of a bar or 1-millionth of a millibar. Such a small unit is used for the partial pressure of trace gases like ozone in the atmosphere.

NAT

An abbreviation of nitric acid trihydrates, believed to be one of the components of Type Ia polar stratospheric clouds.

nb

Abbreviation for nanobar.

NCAR MASP

Abbreviation for the National Center for Atmospheric Research's Multiple-Angle Aerosol Spectrometer Probe. The instrument determines the size and concentration of particles from about 0.2 to 20 microns in diameter and the index of refraction for selected sizes.

NH

Abbreviation for northern hemisphere.

Nimbus-7

The last in a series of Nimbus satellites. See Nimbus Satellite Program. It carried the TOMS (Total Ozone Mapping Spectrometer) instrument and the SBUV instrument. Nimbus-7 is sometimes abbreviated as N7. Instruments aboard the Nimbus-7 satellite are sometimes denoted with the N7 prefix; e.g., N7 TOMS.

Nimbus Satellite Program

A NASA program to develop observation systems meeting the research and development requirements of atmospheric and Earth scientists. The Nimbus satellites, first launched in 1964, carried various instruments: microwave radiometers, atmospheric sounders, ozone mappers, the Coastal Zone Color Scanner, infrared radiometers, etc. Nimbus-7, the last in the series, provided significant global data on sea-ice coverage, atmospheric temperature, atmospheric chemistry including ozone distribution, Earth's radiation budget, and sea-surface temperature. See TOMS and SBUV.

nitrification

The conversion of nitrogen organic compounds to inorganic compounds of nitrogen. This conversion is accomplished, in the main, by bacteria. The conversion of NH4+ (ammonium ions) to NO3- (nitrate ions) with N2O (nitrous oxide) released as a byproduct is an example of a nitrification process by bacterial organisms in the soil. See nitrogen fixation.

nitrogen fixation

1. The process of converting atmospheric N2 to biologically available nitrogen. Biological fixation occurs when bacteria such as cyanobacteria (blue-green algae) convert (i.e., fix) nitrogen from N2 to NH4+ (ammonium ions), which makes it available for use by numerous bacterial organisms as an energy source. Fixed nitrogen taken up by plants is incorporated into plant tissues as amino acids. When the plant is harvested, dies, or drops its leaves, the fixed nitrogen is recycled back into the soil.

2. Conversion by plants of atmospheric nitrogen (N2) into a usable form (nitrates) by certain soil bacteria in their nodules.

nitrogen oxide theory

One of the three early theories proposed for Antarctic stratospheric ozone loss. Callis and Natarajan (1986) proposed that large amounts of NOx compounds were being produced as a result of the peak in sunspot activity in 1979. Sunspots follow an 11-year cycle and the peak period is called a solar maximum. This NOx would be photochemically produced as a result of increased energetic UV light in the middle to upper stratosphere of the tropics and transported into the polar lower stratosphere by the Brewer-Dobson circulation. The loss process would occur catalytically as

      NO+O3 --> NO2 +O2

       NO2 +O --> NO+O2

         --------------------

           O3 +O --> 2 O2

Satellite and airplane measurements did not bear out the nitrogen oxide theory. Instead, the heterogeneous chemistry theory proved correct. Compare also dynamical theory.

noctilucent clouds

1. Relatively unusual wavy, thin, bluish-white clouds that form at altitudes of about 80 to 90 km.

2. Clouds of unknown composition that occur at great heights, 75 to 90 km. They resemble thin cirrus, but usually with a bluish or silverish color, although sometimes orange to red, standing out against a dark night sky. Sometimes called luminous clouds. These clouds have been seen rarely, and then only during twilight, especially with the Sun between 5° and 13° below the horizon. They have been observed only during summer months in both hemispheres (between latitudes 50° to 75° N and 40° to 60° S), and only in some parts of these latitude belts.

noise

A statistical term referring to a completely random time series. Instrument measurement error is a typical example of noise. Compare uncorrelated noise. See also white noise and red noise.

normal temperature and pressure

A temperature of 0°C and a pressure of 1 atmosphere (760 torrs).

NOZE I and II

Acronym for National Ozone Expedition I and II. These were two ground based field campaigns to the Antarctic to study the ozone hole phenomenon. NOZE I took place in 1986 and involved balloon-borne instruments launched from McMurdo Base, Antarctica. NOZE II took place in 1987 and again used balloon-borne instruments to study stratospheric ozone chemistry.

nuclear test

Refers to the extensive atmospheric testing of nuclear bombs that was conducted in the 1950s and 1960s.

number density

The number of particles in a given space; for molecules, it is the number of molecules in a given space or volume. Mathematically, it is represented as

      n = Nv/V

where n is the number density, Nv is the number of molecules in a given volume of space, and V is that volume. This quantity is frequently plotted for profiles of atmospheric constituents like ozone.

occultation technique

One of several remote sensing techniques for measuring atmospheric trace gases by satellite. Occultation instruments measure solar, lunar, and even stellar radiation directly though the limb of the atmosphere during satellite Sun, Moon, and star rise and set events (depending on which celestial radiator is being used by the satellite instrument). By measuring the amount of absorption of radiation through the atmosphere at different wavelengths (e.g., UV, visible, infrared), occultation instruments can infer the vertical profiles of various trace constituents, including ozone. This technique offers improved vertical resolution with solar occultation instruments offering vertical resolutions of 1-2 km. Disadvantage is the limited spatial coverage: measurements can only be made at sunrise and sunset events for the solar occultation instruments, so many orbits are required in order to get global coverage. The solar occultation technique refers to instruments that use only solar radiation for the occultation technique. Improved designs of occultation instruments allow for use of the Moon and even the stars as the occulting light sources, which will expand the spatial coverage. Such instruments will fly aboard future satellite missions. SAGE is an example of an instrument employing an occultation technique. Compare to backscatter ultraviolet technique and limb emission technique.

oxidation

1. The act or process of oxidizing.

2. The state or result of being oxidized. See oxidize.

oxidize

1. To combine with oxygen.

2. To dehydrogenate especially by the action of oxygen.

3. To change (a compound) by increasing the proportion of the electronegative part or change (an element or ion) from a lower to a higher positive valence: remove one or more electrons from (an atom, ion, or molecule).

ozone

An almost colorless, gaseous form of oxygen with an odor similar to weak chlorine. A relatively unstable compound of three atoms of oxygen, ozone constitutes -- on average --- less than 1 part per million (ppm) of the gases in the atmosphere (peak ozone concentration in the stratosphere can get as high as 10 ppm), yet ozone in the stratosphere absorbs nearly all of the biologically damaging solar ultraviolet radiation before it reaches Earth's surface where it can cause skin cancer, cataracts, and immune deficiencies, and can harm crops and aquatic ecosystems. See ozone layer. Ozone is produced naturally in the middle and upper stratosphere through dissociation of molecular oxygen by sunlight. In the absence of chemical species produced by human activity, a number of competing chemical reactions among naturally occurring species -- primarily atomic oxygen, molecular oxygen, and oxides of hydrogen and nitrogen -- maintains the proper ozone balance. In the present day stratosphere this natural balance has been altered, particularly by the introduction of manmade chlorofluorocarbons. If the ozone decreases, the ultraviolet radiation at Earth's surface will increase. See greenhouse gases. Tropospheric ozone is a byproduct of the photochemical (light induced) processes associated with air pollution. See photochemical smog. Ozone in the troposphere can damage plants and humans.

ozone layer

1. The layer of ozone that begins approximately 15 km above Earth and thins to an almost negligible amount at about 50 km. It shields Earth from harmful ultraviolet radiation from the Sun. The highest natural concentration of ozone (approximately 10 parts per million by volume) occurs in the stratosphere at approximately 25 km above Earth. The stratospheric ozone concentration changes throughout the year as stratospheric circulation changes with the seasons. Natural events such as volcanoes and solar flares can produce changes in ozone concentration, but manmade changes are of the greatest concern.

2. More generally, the ozone found throughout the stratosphere.

ozone profile

The amounts of ozone at different levels in the atmosphere respresented in a plot of altitude versus ozone amount (measured typically in number density or partial pressure). Profile measurements are made with ozonesondes, lidar, and profiling satellite instruments such as SAGE. Depending on the units of measurement used, the profiles will appear somewhat different. Other atmospheric quantities, such as temperature and moisture, are also measured at different levels in the atmosphere. These too can be represented as profiles.

ozonesondes

Balloon-borne instruments used to determine ozone profile measurements.

partial pressure

1. The pressure exerted by a designated component or components of a gaseous mixture. This may be separately measured in some cases by suitable selection of gases, traps, or analytical trains. When the percentage composition of the mixture is known, the partial pressure may be calculated from the total pressure by Dalton law of partial pressures.

2. The fraction of atmospheric pressure at a given altitude for which a designated component or components of a gaseous mixture is responsible. In the case of ozone, it is the fraction of atmospheric pressure at a given altitude for which ozone is responsible, and it is measured in nanobars.

perihelion

That point in a solar orbit nearest the Sun. That orbital point farthest from the Sun is called aphelion. The term perihelion should not be confused with parhelion, a form of halo.

period

1. The interval needed to complete a cycle.

2. = orbital period.

3. Specifically, the interval between passages at a fixed point of a given phase of a simple harmonic wave; the reciprocal of frequency.

peroxyacetylnitrates (PANS)

A class of chemical substances found as a pollutant in the troposphere, formed by photolysis from natural and manufactured organic chemicals. These chemicals act as irritants and mutagens in mammals and are toxic to many plants.

phase

1. Of a periodic quantity, for a particular value of the independent variable, the fractional part of a period through which the independent variable has advanced, measured from an arbitrary reference. The arbitrary reference is generally so chosen that the fraction is less than unity. In case of a simple harmonic quantity, the reference is often taken as the last previous passage through zero from the negative to positive direction. Thus, if two wave crest 1/4 cycle apart, they are said to be 90° apart in phase, or 90° out of phase. The moon is said to be at first quarter when it has completed 1/4 of its cycle from new moon.

2. The stage of aggregation of a substance; e.g., solid, liquid, or gas.

3. The extent to which the disk of the moon or the planet, as seen from Earth, is illuminated or not illuminated by the Sun.

4. In astronomy = configuration.

photochemical reaction

A chemical reaction that involves either the absorption or emission of radiation.

photochemical smog

Natural and artificially emitted hydrocarbons in the presence of oxides of nitrogen undergo photochemical reactions that produce a cloud of toxic chemicals including ozone and a variety of caustic agents. This process is powered by sunlight and some of the products, such as ozone, reach a peak soon after photon flux from the sun reaches a maximum, around midday. The thermal inversions often associated with some cities can lead to a dangerous buildup of smog in urban areas. Human deaths have been attributed to photochemical smog since the Industrial Revolution in cities such as London and New York.

photochemistry

See photochemical reaction.

photodissociation

Dissociation (splitting) of a molecule by absorption of a photon. The resulting components may be ionized in the process (photoionization).

photoelectric effect

The emission of an electron from a surface as the surface absorbs a photon of electromagnetic radiation. Electrons so emitted are termed photoelectrons. The effectiveness of the process depends on the surface metal concerned and the wavelength of the radiant energy to which it is expressed. Cesium, for example, will emit electrons when exposed to visible radiation. The energy of the electron produced is equal to the energy of the incident photon minus the amount of work needed to raise the electron to a sufficient energy level to free it from the surface. The resulting energy of the electron, therefore, is proportional to the frequency (i.e., inversely proportional to the wavelength) of the incident radiation.

photoionization

1. The removal of an orbital electron from an atom or molecule by the absorption of a photon; that is, ionization by the photoelectric effect.

2. The ionizationof an atom or molecule by the collision of a high-energy photon with the particle.

photoionization threshold

The frequency at which photoionization first begins.

photolysis

The destruction of a molecule by electromagnetic radiation, which provides the energy required for a constituent atom to break the chemical bonds between it and the other atoms that make up the molecule.

photometer

An instrument for measuring the intensity of light or the relative intensity of a pair of lights. Also called illuminometer. If the instrument is designed to measure the intensity of light as a function of wavelength, it is called a spectrophotometer. Photometers may be divided into two classes: photoelectric photometers, in which a photoelectric cell is used to compare electrically the intensity of an unknown light with that of a standard light, and visual photometers, in which the human eye is the sensor.

photon

1. A particle that has no rest mass or electrical charge and whose energy is determined by its electromagnetic wavelength. A photon is considered to be an amount of electromagnetic radiation energy that is proportional to the frequency of that radiation. It may also be considered as a particle of light with energy, h, where h is Planck's constant and is the frequency of light.

2. According to the quantum theory of radiation, the elementary quantity, or quantum, of radiant energy. It is regarded as a discrete quantity having a momentum equal to h/c, where h is Planck constant, is the frequency of the radiation, and c is the speed of light in a vacuum. The photon is never at rest, has no electric charge or magnetic moment, but does have a spin moment. The energy of a photon (the unit quantum of energy) is equal to h.

photosphere

The intensely bright portion of the sun visible to the unaided eye.

Planck's constant

A constant equal to 6.6256 X 10^-27 erg second. It scales the energy of electromagnetic radiation of frequency so that the radiation appears only in quanta nh, n being an integer.

Planck's Law

This is a derived formula, from the German physicist Max Planck that portrays the amount of radiation emitted by a blackbody as theoretically determined by its temperature. It is an equation that produces a curve, termed Planck's blackbody radiation curve that illustrates that the warmer a body is, the greater is its blackbody emission at each wavelength and the shorter is the wavelength of which emissions peak.

plane-polarized waves

A type of wave, such as an electromagnetic wave, where wave fronts are everywhere parallel planes normal to the direction of propagation.

planetary albedo

The fraction of incident solar radiation that is reflected by a planet and returned to space. The planetary albedo of the Earth-atmosphere system is approximately 30%, most of which is due to backscatter from clouds in the atmosphere.

planetary boundary layer

1. The transition region between the turbulent surface layer and the normally nonturbulent free atmosphere. This region is about 1 km thick and is characterized by a well-developed mixing generated by frictional drag as the air masses move over Earth's surface. This layer contains approximately 10% of the mass of the atmosphere. Also called the "atmospheric boundary layer" or "frictional layer."

2. That layer of the atmosphere from a planet's surface to the geostrophic wind level including, therefore, the surface boundary layer and the Ekman layer. Above this layer lies the free atmosphere. Also called friction layer, atmospheric boundary layer.

planetary vorticity

The vorticity imparted to objects on Earth because of Earth's rotation upon its axis. The magnitude of the spin depends on the latitude of the object. See Coriolis parameter. Compare to relative vorticity and absolute vorticity. See also vorticity.

planetary wave

A type of atmospheric Rossby wave with a wavelength upward of 10,000 km. These waves are mostly generated by large-scale surface topography like the Rocky Mountains and the Himalaya-Tibet complex (orographically forced) or by land-sea boundaries. Such orographically forced planetary waves do not propagate horizontally but instead are stationary. The fact that they are stationary is related to the fact that the topographical forcing occurs at the same locations. Planetary waves often propagate upward from the troposphere into the stratosphere.

POAM II and III satellites

Acronym for the Polar Ozone and Aerosol Measurement instrument series. POAM II and its successor POAM III are solar occultation devices (see solar occultation techniques) that are designed to measure the vertical distribution and overall stratospheric abundances of ozone, water vapor, nitrogen dioxide, and various aerosols. The POAM II instrument was launched aboard the French Space Agency's Satellite Pour l'Observation de la Terre-3 (SPOT-3) satellite in September 1993. It remained operational only until November 1996, but it has since been replaced by the POAM III instrument, which was launched aboard the SPOT-4 satellite in March 1998.

polar jet

Not to be confused with the stratospheric polar night jet, this is one of two upper tropospheric wind speed maxima. Also called the polar front jet, this wind speed maxima follows the westerlies at middle latitudes. Regions of greatest wind speed are referred to as jet cores. Polar jet cores tend to migrate eastward and become involved in middle latitude weather disturbances. Compare to subtropical jet, the other tropospheric wind speed maxima.

polar night

The time of the year in the polar latitudes during which there is no sunlight for up to 6 months.

polar night jet

The jet stream that sets up in the lower to middle stratosphere at the edge or "terminator" of the polar night owing to the strong temperature gradient that sets up between the polar vortex region and the middle latitudes. The lack of solar radiation inside the polar vortex (i.e., in the region of polar night) causes temperatures to fall to extremely low values, typically in the range of 200K (-100° F) in the northern hemisphere and 190K (-117° F) in the southern hemisphere. Owing to thermal wind balance, a strong westerly jet stream sets up. This jet stream is called the polar night jet. Upon return of sunlight in the spring to the polar region, temperatures quickly rise and the polar night jet disappears along with the polar vortex region it encompasses.

polar night region

The region where polar night occurs: see polar night.

polar stratospheric clouds (PSCs)

1. High level clouds that form in the extremely cold and dry conditions that exist in the polar night regions of the stratosphere. Two main categories of PSCs: Type I and Type II. While the composition of the Type II PSC is known to be water ice, the composition of the Type I PSC is rather poorly understood. It was once thought that Type I PSCs were formed as nitric acid trihydrates (NAT). The equilibrium temperature of NAT (i.e., temperature at which the surrounding air is just saturated with respect to NAT ice crystals) was thought to be consistent with the formation temperatures of Type I PSCs. Recent aircraft observations using extremely precise temperature measurements have shown that the formation temperature of a Type I PSC is inconsistent with the NAT equilibrium temperature. Current thinking divides Type I PSCs into Type Ia and Type Ib, where Type Ia includes NAT compounds and Type Ib are swollen sulfate aerosols. Type I PSC cloud particles are also thought to consist of liquid ternary solutions including mixtures of nitric acid (HNO3), water vapor (H2O), and sulfuric acid (H2SO4). Droplets grow as the temperature decreases, achieving sizes providing sufficient surface areas to enable heterogeneous chemical reactions.

2. Clouds that form in the polar vortex regions during winter and early spring under extremely cold conditions, whereby nitric acid, water vapor and other trace chemicals condense to form liquid or solid particles depending on the concentrations of trace gases (among a few other factors). PSCs provide a solid surface on which chlorine containing reservoir compounds can gather, and on this surface the chemical reactions involved in the depletion of ozone are greatly increased.

polar vortex

1. Extremely cold atmospheric region poleward of the polar night jet that develops during the winter season (aka polar cyclone, polar low, circumpolar whirl). The large-scale cyclonic circulation in the middle and upper troposphere centered generally in the polar regions. Specifically, the vortex has two centers in the mean, one near Baffin Island and another over northeast Siberia. The associated cyclonic wind system comprises the Westerlies of middle latitudes.

2. A phenomenon that occurs during the polar winter in which stratospheric air moves in a circular motion, with an area of relatively still air in its center. The temperature in the vortex is approximately -130° F (-80° C), which assists in the formation of polar stratospheric clouds. Though usually more prolonged and colder over Antarctica, the Arctic polar vortex does form to a degree, and when the temperatures there are coldest and the vortex persists, Arctic stratospheric ozone destruction on Arctic polar stratospheric clouds has also been observed.

polar vortex region

The region where the polar vortex exists: see polar vortex.

potential energy

Energy possessed by a body by virtue of its position in a gravity field in contrast with kinetic energy, that is possessed by virtue of its motion.

potential temperature

1. The temperature a parcel of dry air would have if brought adiabatically from its initial state to the (arbitrarily selected) standard pressure of 1000 millibars.

2. The temperature an air parcel would have if expanded or compressed adiabatically (i.e., without any heat being added or taken away) from its existing pressure to a reference pressure, usually taken to be 1000 hPa or 1000 mb. It is usually denoted and may be calculated from the formula

       = T (Po/P) exp^(R/cp)

where P and T are the parcel's pressure and temperature, Po is the reference pressure, R is the gas constant for air, and cp is the specific heat capacity at constant pressure for air. The First Law of Thermodynamics, when formulated using q, the specific heat, rather than T, states that the time change is proportional to the diabatic heating.

potential vorticity

1. A combination of absolute vorticity and the gradient of potential temperature into a scalar quantity (i.e., a quantity that has a magnitude but no direction) that is conserved under frictionless, adiabatic conditions. Because it contains both dynamic (vorticity) and thermodynamic (potential temperature) properties, the statement of its conservation is quite general. Potential vorticity increases when the static stability (given by the vertical gradient of potential temperature) increases, as is the case when you go from the troposphere into the stratosphere. Under appropriate conditions, wind and temperature fields can be derived from potential vorticity. See stability parameter for discussion of static stability. Potential vorticity is approximately conserved following the motion of an air parcel. This conservation property, along with the natural pole-equator gradient, makes potential vorticity an ideal quantity to use as a tracer of atmospheric motion.

2. The product of the absolute vorticity and the static stability, conservative in adiabatic flow, given by the expression, (/)(/p), where is the absolute vorticity of a fluid parcel, is the potential temperature, and p is the pressure. Also known as absolute potential vorticity.

For an individual fluid column bounded by fixed isentropic surfaces, the potential vorticity may be written /p, where p is the increment of pressure between the isentropic surfaces. In this form the potential vorticity has been used to trace the movements of fluid particles on isentropic charts.

pressure gradient force

The force that causes acceleration from higher to lower pressure in a fluid such as the air. It arises because of a pressure gradient or change between higher and lower pressure. As an analogy, consider the pressure gradient that exists at the nozzle of a vacuum cleaner when it is turned on. A motor inside creates a region of relatively low pressure contrasted with the region of higher pressure outside. The dust and air molecules outside the vacuum, being at a higher pressure, feel a force pulling toward the low pressure inside the vacuum. This is a pressure gradient force. In the atmosphere, regions of high and low pressure form ultimately as a result of unequal heating. In moving from a high pressure region to a low pressure region, the motion is said to be along the pressure gradient. The pressure gradient force itself points in the direction of the lower pressure. In the vacuum cleaner example, this is evidenced by the fact that air is sucked into the lower pressure inside. The magnitude of the pressure gradient force is proportional to the magnitude of the pressure gradient itself. The lower the pressure inside the vacuum cleaner, the more powerful the vacuum will be. In the atmosphere, pressure gradient is related to a quantity called geopotential height. The pressure gradient force is proportional to the geopotential height gradient.

prominence

A filamentlike protuberance from the chromosphere of the Sun. See flocculi. Compare solar flare. Prominences can be observed visually (optically) whenever the Sun's disk is masked, as during an eclipse or by using a coronagraph; and can be observed instrumentally by filtering in certain wavelengths, as with a spectroheliograph. A typical prominence is 6,000-12,000 km thick, 60,000 km high, and 200,000 km long.

PSCs

See polar stratospheric clouds.

quasi-biennial oscillation (QBO)

An oscillation of the east-west wind in the tropical stratosphere caused by the internal dynamics of tropical waves rather than the annual change of seasons cycle; the period of this wind oscillation is highly variable with periods ranging from 22 to 34 months, hence the name. The theory of why QBO exists was developed by Prof. Richard Lindzen of the Massachusetts Institute of Technology and Prof. James Holton of the University of Washington during the early 1970s. They proposed that dissipation of vertically propagating equatorial waves is the source of momentum responsible for causing the wind QBO. They used a simple model to show that the dissipation of vertically propagating Kelvin waves and Rossby-gravity waves can produce a QBO-like circulation. Although more recent 2D and 3D computer model simulations of the QBO have supported their theory, the exact sources of the momentum remains unclear.

radian

The angle subtended at the center of a circle by an arc equal in length to a radius of the circle. It is equal to 360°/2 or approximately 57° 17' 44.8".

radiance

In radiometry, a measure of the intrinsic radiant intensity emitted by a radiator in a given direction. It is the irradiance (radiant flux density) produced by radiation from the source upon a unit surface area oriented normal to the line between source and receiver, divided by the solid angle subtended by the source at the receiving surface. It is assumed that the medium between the radiator and receiver is perfectly transparent; therefore, radiance is independent of attenuation between source and receiver. If the radiant source is a perfectly diffuse radiator (that is, emits exactly according to Lambert law), then its radiance is equal to its emittance per unit solid angle. The radiance of a light source is termed luminance (formerly, brightness).

radiancy

The rate of radiant-energy emission for a unit area of a source in all the radial directions of the overspreading hemisphere. Also called radiant emittance.

radiant energy

1. The energy of any type of electromagnetic radiation. Also called radiation.

2. Infrequently, any energy that may be radiated, as, for example, acoustic energy.

radiant energy density

The instantaneous amount of radiant energy contained in a unit volume of propagating medium.

radiant flux

The rate of flow of radiant energy.

radiant flux density

The radiant flux per unit area. When applied to a source, it is called radiancy or radiant emittance (symbol W). When applied to a receiver, it is called irradiancy or irradiance (symbol H).

radiant intensity

Radiant flux per unit solid angle.

radiation

1. The process by which electromagnetic energy is propagated through free space by virtue of joint undulatory variations in the electric and magnetic fields in space. This concept is to be distinguished from conduction and convection. A group of physical principles known as the radiation laws comprise, to a large extent, the current state of practical knowledge of the complex radiative processes.

2. The process by which energy is propagated through any medium by virtue of the wave motion of that medium, as in the propagation of sound wave through the atmosphere, or ocean waves along the water surface.

3. = radiant energy.

4. = electromagnetic radiation, specifically, high-energy radiation such as gamma rays and X-rays.

radiative cooling

The cooling process of Earth's surface and adjacent air; occurs when infrared (heat) energy radiates from the surface upward through the atmosphere into space. Air near the surface transfers its thermal energy to the nearby ground through conduction, so that radiative cooling lowers the temperature of both the surface and the lowest part of the atmosphere. See also infrared cooling.

radiative heating

The warming process of Earth's surface and adjacent air due to the absorption of radiative energy.

radiative transfer

Theory dealing with the propagation of electromagnetic radiation through a medium.

radical

1. A highly reactive molecule or atom with an unpaired electron. The species is often represented by a formula with a single dot as the unpaired electron

2. An atom or group of atoms that contains one or more unpaired electrons (usually very reactive species).

radiosonde

An instrument, usually balloon borne, for the simultaneous measurement and transmission of meteorological data as it moves vertically through the atmosphere. The instrument consists of transducers for the measurement of pressure, temperature, and humidity; a modulator for the conversion of the output of the transducers to a quantity that controls a property of the radiofrequency signal; a selector switch that determines the sequence in which the parameters are to be transmitted; and a transmitter which generates the radiofrequency carrier. See also rawinsonde.

Rankine temperature scale

A scale of absolute temperature; the temperature in degrees Rankine is equal to 9/5 of the temperature in Kelvins and to the temperature in degrees Fahrenheit plus 459.67.

rawinsonde

Amethod of observation of temperature, pressure, relative humidity, and winds aloft by means of a balloon-borne radiosonde tracked by a radar or radio direction finding equipment.

Rayleigh scattering

1. Any scattering process produced by spherical particles whose radii are smaller than about one-tenth the wavelength of the scattered radiation. Compare Mie scattering. See also size parameter. In Rayleigh scattering, the scattering coefficient varies inversely with the fourth power of the wavelength, a relation known as the Rayleigh law. The angular intensity polarization relationships for Rayleigh scattering are conveniently simple. For particles not larger than the Rayleigh limit, there is complete symmetry of scattering about a plane normal to the direction of the incident radiation, so that the forward scatter equals the backward scatter.

2. The scattering of light by a body with a particle diameter (Dp) less than 0.03 micrometers is termed Rayleigh Scattering. The wavelength of light scattered is dependent on the Dp, and the amount of light scattered is dependent on the number of particles present per unit volume. Shorter visible wavelengths, such as blue, are scattered by smaller particles than are the longer wavelengths like red.

red noise

Refers to low frequency noise. See noise.

regression analysis

A statistical technique that uses the relation between a dependent variable and one or more independent variables such that the dependent variable can be predicted by the independent variable. When a regression model includes only one independent variable, it is known as a simple regression model; when a regression model includes more than one independent variable, it is known as a multiple regression model. See also regression coefficients. As an example of a regression analysis, consider Company X, which sells teddy bears. The number of teddy bears sold per day may depend on the price of each bear and the amount of money company X spends on advertising. In this example, the number of bears sold is the dependent variable, and the cost of each bear and amount of advertising money are the independent variables. When doing their bookkeeping, Company X will construct a regression model to determine the best mathematical relationship between the cost of each bear, the amount of advertising money spent, and the number of bears sold per day. They will base this model on trial cases where the company varied the cost of the bear and amount of advertising money spent, and monitored the change in the number of bears sold. Using these results, they can predict the number of bears that will sell at a particular price and level of advertising, and they can adjust these factors such that they meet their daily bear selling quota. If we are studying the variability in total ozone amount at some location, we will want to express this variability in terms of the individual processes that cause it. We will need to construct a model where the dependent variable is the ozone time series and the independent variables are time series that represent the individual components of ozone variability. The change of the seasons and the shifting of tropical stratospheric winds associated with the QBO can affect ozone amounts. These would be among the components included in our ozone time series. Regarding the components, although we don't know the magnitude of each component, we do know what each component looks like from other sets of data. These data sets are termed "proxies" for the model components, because each is used in the regression model as a proxy (substitute) for a particular component of variability. The one assumption that we make is that of a linear relationship between the proxy and the variation in our dependent variable believed to be caused by the proxy. In our ozone example, we assume that the ozone response to each proxy has the same shape (time dependence) as the proxy itself. For example, the QBO in the Singapore wind time series has a period of about 29 months. Therefore, the model ozone QBO component will also have a period of about 29 months. For the seasonal cycle, we employ a combination of sine and cosine waves that repeat every year as the proxy. For a long-term trend, we assume a linear function of time as the proxy.

regression coefficients

In regression analysis, it is the scaling factor between the dependent and independent variable. In a simple regression model, the model takes the form of the equation for a line,

      y = mx + b

Here y is the dependent variable, x is the independent variable, m is the regression coefficient, and b is the y intercept. The term "simple linear regression model" is sometimes used synonymously. In a multiple regression model, the equation has the more general form

      y = m1x1 + m2x2 + m3x3 +...+ mnxn + b

where n is the total number of proxy (see regression analysis) terms in our model and b is the value of y when [x1, x2, x3, ..., xn] = 0. The term "multiple linear regression model" is sometimes used synonymously. There are several statistical techniques to determine the regression coefficients, including the least squares technique. In this technique, the regression coefficients are determined such that the variance of the residual is minimized.

regression equation

A linear equation that is the “best” linear equation representing the points of a scatter diagram, using the least-squares criterion to find the best fitting line, meaning that the sum of the squares of the vertical distances from the points to the line be made as small as possible.

regression model

Same as statistical regression model.

relative vorticity

The spin of the air associated with weather systems as air spirals away from high pressure and into low pressure. It is related to the horizontal (north-south and east-west) components of velocity of the air. When the rotation is counterclockwise or cyclonic, as it is around a low pressure system, the relative vorticity is referred to as positive vorticity. Such positive spin is associated with rising motions, as occurs in the center of a low pressure system. When the rotation is clockwise or anticyclonic, as it is around a high pressure system, the relative vorticity is referred to as negative vorticity. Such negative spin is associated with sinking motions, as occurs in a region of high pressure (hence the suppression of clouds and precipitation). The positive component of relative vorticity is a central concept in meteorology. Often it is simply referred to as "vorticity." The two quantities of planetary vorticity and positive (relative) vorticity added together define the quantity of absolute vorticity. For fuller, mathematical explanation of how planetary, relative, and absolute vorticity relate to each other, see vorticity.

reservoir species

Chemical compounds that store a particular species in a nonreactive form (like a reservoir).

residuals

The remaining unexplained variability of the ozone time series. See statistical uncertainty.

Rossby wave

Waves whose restoring mechanism is the latitudinal (north-south) gradient of potential vorticity; named for the Swedish-American scientist Carl Gustav Rossby. Such waves exist because of potential vorticity and the conservation of angular momentum. Consider an air parcel moving south. The planetary component of potential vorticity (represented by the Coriolis parameter) will decrease. Since the total potential vorticity remains nearly constant, the local (positive) vorticity of the air must increase. This results in a counterclockwise local rotation of the air mass. But as the air mass begins to turn north again, the planetary component of the potential vorticity increases, resulting in a decrease in the local vorticity. The air mass turns south yet again. The result is a wave pattern in the motion of the air parcel as viewed from above Earth. Large scale topographical features create undulating Rossby wave patterns around the globe in the northern hemisphere in the 30-60°N band. The lack of significant topography in the southern hemisphere leads to a more zonal flow with fewer Rossby waves.

Rossby-gravity waves

Waves that have both Rossby and inertiogravity characteristics; their restoring force is the gradient of potential vorticity, static stability, and the Coriolis parameter.

rotational energy level

Energy associated with rotational motion of a molecule or atom.

rotational motion

1. Motion of a body about an axis within the body.

2. Refers to the motion of a molecule or atom about an internal axis; spin.

SAGE I, II, and III

Acronym for Stratospheric Aerosol and Gas Experiment; satellite instrument employing a solar occultation technique to measure trace gases such as ozone, nitrogen dioxide, and water vapor, as well as stratospheric aerosols (like sulfuric acid droplets) in the UV and visible regions. The SAGE II instrument aboard the Earth Radiation Budget Satellite (ERBS) has been measuring ozone since October 1984. The next generation SAGE III, scheduled for initial launch in mid-1999 aboard the Russian Meteor-3M spacecraft will have expanded capabilities to measure in the near infrared region. It will also be able to use the Moon as a light source. This lunar occultation mode will increase the spatial coverage of the occultation technique. Measurements in the infrared will allow for characterization of more atmospheric constituents.

SBUV

Acronym for Solar Backscatter Ultraviolet instrument; satellite instruments flown aboard the Nimbus-7 and NOAA polar orbiting series that employ the backscatter ultraviolet (BUV) technique for measuring total ozone. The Nimbus-7 SBUV instrument was operational from November 1978 to July 1990.

scattering

The process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. In scattering, no energy transformation results, only a change in the spatial distribution of the radiation. Also called scatter. Along with absorption, scattering is a major cause of the attenuation of radiation by the atmosphere. Scattering varies as a function of the ratio of the particle diameter to the wavelength of the radiation. When this ratio is less than about 1/10, Rayleigh scattering occurs in which the scattering coefficient varies inversely as the fourth power of the wavelength. At larger values of the ratio of particle diameter to wavelength, the scattering varies in a complex fashion described by the Mie theory; at a ratio of the order of 10, the laws of geometric optics begin to apply.

scattering coefficient

A measure of the attenuation caused by scattering of radiation as it traverses a medium containing scattering particles. Also called total scattering coefficient.

scattering cross-section

The hypothetical area normal to the incident radiation that would geometrically intercept the total amount of radiation actually scattered by a scattering particle. It is also defined, equivalently, as the cross-section area of an isotropic scatterer (a sphere) that would scatter the same amount of radiation as the actual amount. Also called extinction cross-section and effective area.

seasonal cycle

The annual cyclical pattern in any atmospheric variable, whether temperature or trace gas concentration, caused by the seasons. Also called an annual cycle. The seasonal cycle or seasonal variability is one type of variability. Other types of variability include short-term (day-to-day or week-to-week), interannual (year-to-year), or long-term (decade-to-decade or longer). Thus, the amount of ozone in a particular location has a short-term variability, a seasonal variability, an interannual variability, and a long-term variability. The point of a multiple regression model is to determine how much variability in some quantity (like ozone amount) is the result of these different types of variability.

Second Law of Thermodynamics

An inequality asserting that it is impossible to transfer heat from a colder to a warmer system without the occurrence of other simultaneous changes in the two systems or in the environment. It follows from this law that during an adiabatic process, entropy cannot decrease. For reversible adiabatic processes entropy remains constant, and for irreversible adiabatic processes it increases. Another equivalent formulation of the law is that it is impossible to convert the heat of a system into work without the occurrence of other simultaneous changes in the system or its environment. This version, which requires an engine to have a cold source as well as a heat source, is particularly useful in engineering applications. See First Law of Thermodynamics.

semiannual oscillation (SAO)

The oscillation in upper level stratospheric winds over the tropics between easterly and westerly directions roughly every 6 months. It is due to the passage of the Sun over the equator twice a year.

sensible heat

Heat energy that can be felt or measured directly with a thermometer.

sensible heat exchange

See sensible heat flux.

sensible heat flux

The rate of transfer of sensible heat across a unit area. That is, the rate of energy transfer resulting from sensible heating. The temperature of an air mass moving over a relatively warmer surface increases because of sensible heat flux from the surface to the air.

SH

Abbreviation for southern hemisphere.

shortwave heating

Heating caused by shortwave radiation; see also radiative heating.

shortwave radiation

In meteorology, a term used loosely to distinguish radiation in the visible and near-visible portions of the electromagnetic spectrum (roughly 0.4 to 1.0 micron in wavelength) from long-wave (infrared) radiation.

sigma uncertainty level

See statistical uncertainty.

simple regression model

A statistical regression model whose mathematical form is given by the equation for a line

      y = mx + b

Here y is the dependent variable, x is the independent variable, m is the regression coefficient, and b is the y intercept. Also called a simple linear regression model. Compare to multiple regression model. See also regression analysis.

sine wave

1. A wave that can be expressed as the sine of a linear function of time or space or both.

2. A smoothly varying wave that repeats itself; its frequency is the rate at which the fundamental shape repeats itself. Any waveform can be distilled into a combination of pure sine waves of varying frequencies and amplitudes.

single scattering albedo

The fraction of incident radiation that is not absorbed.

size parameter

The ratio of circumference of the sphere (of an aerosol particle) to the wavelength of incident radiation. For Rayleigh scattering, the particle radii are smaller than about one-tenth the wavelength of the scattered radiation. For Mie scattering the particle radii are on the order of the size of the wavelength of the incident radiation. See both Rayleigh scattering and Mie scattering.

SOLSTICE

Acronym for the Solar Stellar Irradiance Comparison Experiment that was launched aboard the Upper Atmospheric Research (UARS) satellite in 1991. SOLSTICE measures the magnitude of solar spectrum irradiance of the total solar disk in the ultraviolet (UV) wavelength region from 115 to 430 nm. Its purpose is to determine solar UV variability on three time scales. These include short-term variations spanning time periods of minutes to hours (exemplified by solar flares), intermediate-term variations lasting days to weeks (characterized by the solar rotation and the development of active regions), and long-term variations (associated with the 11-year sunspot cycle or the 22-year magnetic field cycle).

Solar Backscatter Ultraviolet

See SBUV.

solar constant

The solar flux that reaches the top of Earth's atmosphere on a unit area perpendicular to the incident radiation at the mean Earth-Sun distance. It has a value of about 1370 watts per meter² or 1.934 calories per cm² per minute.

solar cycle

See sunspot cycle.

solar flare

A bright eruption from the Sun's chromosphere. Compare prominence. Also called a flare. Solar flares may appear within minutes and fade within an hour. They cover a wide range of intensity and size, and they tend to occur between sunspots or over their penumbrae. Solar flares are related to radio fadeouts and terrestrial magnetic disturbances. Solar flares eject high energy protons that present a serious hazard to astronauts in unshielded spacecraft.

solar flux

The flow of shortwave electromagnetic energy from the Sun per unit of area per unit time, typically measured in watts per meter squared. See solar constant above.

solar insolation

1. In general, solar radiation received at Earth's surface.

2. The rate at which direct solar radiation is incident upon a unit horizontal surface at any point on or above the surface of Earth. Compare solar constant.

solar maximum

The period when sunspot activity is at its greatest during the 11-year sunspot cycle. Sunspots tend to cluster on one side of the Sun during such periods.

solar minimum

The period when sunspot activity is at its least during the 11-year sunspot cycle.

solar occultation technique

A particular type of occultation technique in which only solar radiation is used for vertical profile measurements of atmospheric trace gases, including ozone. Such instruments measure solar radiation directly through the limb of the atmosphere when the Sun appears to rise above or set behind the limb of the Earth from the perspective of the orbiting satellite. That is, the instrument measures solar radiation during sunrise and sunset events directly through the limb of the atmosphere. The latest generation of occultation instruments are also able to use lunar and stellar radiation directly through the limb of the atmosphere during Moon and star rise and set events (depending on which celestial radiator is being used by the satellite instrument). By measuring the amount of absorption of radiation through the atmosphere at different wavelengths (e.g., UV, visible, infrared), occultation instruments can infer the vertical profiles of a number of trace constituents, including ozone.

solar protons

Protons emitted by the Sun, especially during a solar flare.

solar wind

1. The outflow of charged particles from the solar corona into space. Because of the high temperature of the particles of the corona (mostly protons and electrons), they are moving at speeds higher than the solar escape velocity. At the orbit of Earth, these particles are moving at about 500 km per second. Some of these particles are captured by the magnetic fields of the planets, forming their magnetospheres.

2. Streams of plasma (ionized gas) flowing approximately radially outward from the Sun.

solar zenith angle

The highest angle that the Sun achieves in the sky at some Earth-bound location. It is a function of terrestrial latitude and season.

sonde

1. Abbreviation of ozonesonde.

2. Abbreviation of radiosonde and rawinsonde.

Southern Oscillation

A large-scale atmospheric and hydrospheric fluctuation centered in the equatorial Pacific Ocean. It exhibits a nearly annual pressure anomaly, alternatively high over the Indian Ocean and high over the South Pacific. Its period is slightly variable, averaging 2.33 years. The variation in pressure is accompanied by variations in wind strengths, ocean currents, sea-surface temperatures, and precipitation in the surrounding areas. The Southern Oscillation is coupled directly with the El Niño phenomenon, giving rise to the El Niño-Southern Oscillation (ENSO) phenomenon. See El Niño events and ENSO.

Southern Oscillation Index (SOI)

A measurement of difference in sea level pressure (averaged over some period of time) between the island of Tahiti in the central tropical Pacific and Darwin, Australia. It is the Tahiti minus Darwin normalized sea level pressure. In the normal or non-ENSO situation, sea level pressure at Tahiti is higher than that at Darwin (just as it is higher at the Galapagos Islands than it is at Tahiti). In a warm ENSO event, the situation reverses, with sea level pressure at Tahiti lower than that at Darwin, so that the SOI has a negative reading. See also ENSO and Southern Oscillation.

spectrophotometer

A photometer that measures the intensity of radiation as a function of the frequency (or wavelength) of the radiation. Also called spectroradiometer. See Dobson spectrophotometer. In one design, radiation enters the spectrophotometer through a slit and is dispersed by means of a prism. A bolometer having a fixed aperture scans the dispersed radiation so that the intensity over a narrow wave band is obtained as a function of frequency.

spectrum

1. In physics, any series of energies arranged according to wavelength (or frequency).

2. The series of images produced when a beam of radiant energy is subject to dispersion.

3. Short for electromagnetic spectrum or for any part of it used for a specific purpose as the radio spectrum (10 kilocycles to 300,000 megacycles).

speed of light

The speed of propagation of electromagnetic radiation through a perfect vacuum; a universal dimensional constant equal to 2.9979 x 10⁸ meters per second. Also called velocity of light.

stability parameter

A measure of the static stability of the atmosphere, that is, a measure of how stable or unstable the air is. The stability of the air is determined by its lapse rate. When the lapse rate is less than the adiabatic lapse rate, the atmosphere is said to be stably stratified or statically stable.

standard error

A measure of the dispersion (scatter) of data points with respect to a regression curve. It is a measure of the variation to be expected in making predictions from the regression equation.

standard temperature and pressure

A temperature of 0°C and a pressure of 1 atmosphere (760 torrs).

statistical regression model

A model that employs regression analysis to study long-term trends and other, shorter term sources of variability in a time series of some dependent variable.

statistical uncertainty

The remaining variability in a statistical regression model that is not explained by any of the known sources of variability; that is, the amount of unexplained variability or residual in the model. In this sense, statistical uncertainty is a measure of the goodness of our model fit to the actual data. It is measured in terms of the mathematical quantity called standard deviation and is denoted by s. Another type of uncertainty arises from the precision of the instruments used. This is denoted i. The overall uncertainty is given as the square root of the sum of the squares of the both types of uncertainty measurements:

       = ( s² + i²)^1/2

See standard error.

steady state

The condition of a substance or system whose local physical and chemical properties do not vary with time.

Stefan-Boltzmann constant

A universal constant of proportionality between the radiant emittance of a blackbody and the fourth power of the body's absolute temperature; 5.6697 X 10^-5 erg centimeter squared second degrees K⁴.

Stefan-Boltzmann Law

One of the radiation laws that states that the amount of energy radiated per unit time from a unit surface area of an ideal blackbody is proportional to the fourth power of the absolute temperature of the blackbody. This law was established experimentally by Stefan and was given theoretical support by thermodynamic reasoning of Boltzmann. This law may be deduced by integrating Planck's Law over the entire frequency spectrum.

steradians

The unit of measurement for solid angles; it is equal to the solid angle subtended at the center of a sphere by a portion of the surface of the sphere whose area equals the square of the sphere's radius.

sticking coefficient

Also called "accommodation coefficient," it is one of several parameters that occur in heterogeneous chemical reactions. It represents the probability that a molecule in gas phase will stick to a particle, such as an aerosol or polar stratospheric cloud, after a collision. This probability ranges from 0 to unity (100% chance). It is a function of both particle type and temperature. In general, the colder the temperature, the greater the likelihood that sticking will occur.

stratopause

The boundary between the stratosphere and the mesosphere. It occurs at an atmosphere height of approximately 50 km; however this depends on latitude. The atmosphere is characterized by a decrease in pressure with respect to increased altitude. More important, regions within the atmosphere, like the troposphere, stratosphere, and mesosphere, are distinguishable because of distinct temperature gradients with relatively well-defined starting and ending points. The stratopause is the highest portion of the stratosphere, with a temperature of approximately 0°C; the stratopause can also be described as the warmest region between the mesosphere and the stratosphere.

stratosphere

The thermal atmospheric region of the atmosphere between the troposphere and the mesosphere. The lower boundary of the stratospheric region is marked by the tropopause and begins at approximately 13 km; however, this altitude of the troposphere depends on latitude. The upper limit of the stratosphere is marked by the stratopause at approximately 50 km. The stratosphere is characterized by relatively stable temperatures (between -80°C and -50°C) in the lower regions, and begins warming near 20 km, reaching its maximum temperature of approximately 0°C at the stratopause. Stratospheric chemistry is of particular interest to scientists because ozone, the principal substance that shields Earth from incoming solar ultraviolet radiation, is found in the stratosphere. It should also be noted that wind currents in the stratosphere are primarily horizontal in nature.

stratospheric clouds

Clouds found in the cold, dry stratospheric region of the atmosphere, including two chief types, nacreous clouds and polar stratospheric clouds.

stratospheric sudden warming

An event in which the zonal-mean wind and temperature fields of the polar stratosphere undergo a significant disruption as the zonal (westerly) wind field, which increases with height and peaks in the polar night jet decelerates or even reverses direction and temperatures increase rapidly inside the polar vortex region. It is defined as a sudden major warming if at a 10 mb or below the zonal-mean temperature increases poleward from 60 latitude (i.e. the temperature gradient reverses) and the zonal-mean zonal wind reverses (becomes easterly). It is defined as a minor warming if the temperature gradient reverses but the circulation doesn't. The event is believed to be initiated by an anomalous growth of a planetary wave disturbance that propagates vertically from the troposphere into the stratosphere and interacts with the preexisting circulation there. This is especially true of sudden major warmings. This wave growth may be due to an independently generated forcing mechanism in the troposphere or some process that disturbs both troposphere and stratosphere together. The major warming can be diagnosed in terms of wave mean-flow interaction or in terms of synoptic maps. The former suggest and quantify a dynamical scenario in which the planetary waves are diverted from their climatological equatorward propagation by the anomalous refractive properties of the preconditioned flow. Such a preconditioned flow refers to a polar vortex that is tigher than its usual form and perhaps displaced off the pole itself. The planetary waves are focused into the polar region where they lead to a mean-flow deceleration of the polar night jet and temperature rises in the stratosphere, followed by the associated mean meridional circulations (including the Brewer-Dobson circulation). The synoptic map approach, on the other hand, describes the major warming events in terms of distortion and displacement off the pole of the polar vortex, replaced by a high pressure ridge. Maps of potential vorticity and chemical tracers (such as ozone) give particularly vivid descriptions of the development of observed and modeled warmings.

stratospheric-tropospheric exchange (STE)

Refers to the transport of material, such as trace gases and water vapor, between the troposphere and stratosphere through the tropopause. Because of the huge difference in potential vorticity between the stratosphere where it is very high and the troposphere where it is much lower, "exchange" requires that air parcels of tropospheric origin gain potential vorticity through mixing or diabatic processes when entering the stratosphere and that air parcels of stratospheric origin likewise lose potential vorticity when entering the troposphere. Transport across the tropopause may be caused by a number of processes. These include the large-scale mean diabatic circulation (i.e., Brewer-Dobson circulation), transverse secondary circulations associated with subtropical and polar jet streams, cumulonimbus clouds penetrating into the stratosphere, tropopause folds and subsequent mixing caused by upper-level cyclogenesis, turbulent mixing associated with gravity wave breaking or with shear instabilities at the tropopause, and local radiative cooling in the vicinity of high-level cirrus anvil (thunderstorm top blow-off) clouds.

subtropical jet

One of two tropospheric wind speed maxima; it is confined to latitudes equatorward of about 30°. Like its polar jet counterpart, the region of greatest wind speed or jet core tends to migrate eastward and become involved in middle latitude weather disturbances. Unlike the polar jet, it is completely confined to the high troposphere and its associated frontal zone joins with the stratosphere at very high levels. In this way, it is one of the mechanisms for stratospheric-tropospheric exchange.

sunspot

A relatively dark area on the surface of the Sun consisting of a dark central umbra surrounded by a penumbra which is intermediate in brightness between the umbra and the surrounding photosphere. Sunspots usually occur in pairs with opposite magnetic polarities. They have a lifetime ranging from a few days to several months. Their occurrence exhibits approximately an 11-year period; see sunspot cycle.

sunspot cycle

A cycle with an average length of 11.1 years but varying between about 7 and 17 years in the number and area of sunspots, as given by the relative sunspot number. This number rises from a minimum of 0 to 10 to a maximum of 50 to 140 about 4 years later and then declines more slowly. Also called solar cycle or 11-year solar cycle.

surface albedo

The albedo of the surface of a body. In connection to Earth, it refers to the amount of incoming solar shortwave radiation reflected by the surface. It is approximately 34%, meaning that the Earth has an albedo of 0.34 as a global average. Different types of surfaces (ice cap, forest cover, open grassland, ocean, farmland, city, and clouds) have different albedos. The 34% is a global average.

surface boundary layer

The thin layer of air adjacent to Earth's surface, extending to the Ekman layer. Thus, it is the lowest layer of the troposphere. Within this layer the wind distribution is determined largely by the vertical temperature gradient and the nature and contours of the underlying surface; shearing stresses are approximate constant. Also called surface layer, friction layer, atmospheric boundary layer, ground layer. Together with the Ekman layer it forms the planetary boundary layer.

synoptic scale wave

Atmospheric waves hat have horizontal dimensions (wavelengths) of 1,000 to 4,000 km. Subplanetary scale Rossby waves make up such synoptic scale waves. See Rossby waves.

termolecular reaction

A reaction containing three particles. In the atmosphere an example of a termolecular reaction is one in which a molecule acquires the excess energy released by a reaction so that the products created do not convert back to the original state of the reactants. The reaction of molecule oxygen with an oxygen atom (radical) to produce ozone is a good example of an important atmospheric termolecular reaction. In the atmosphere this third body is most often molecular nitrogen because statistically this is the most likely next collision since atmospheric N2 = 78% by volume.

ternary solutions

A solution composed of nitric acid (HNO3), water vapor (H2O), and sulfuric acid (H2SO4); such solutions are believed to make up some Type I polar stratospheric clouds. See polar stratospheric clouds. Certain heterogeneous reactions involved in stratospheric ozone loss occur on such solutions.

terrestrial radiation

The total infrared radiation emitted by Earth and its atmosphere in the temperature range of approximately 200-300K. Because Earth is nearly a perfect radiator, the radiation from its surface varies as the fourth power of the surface's absolute temperature. Terrestrial radiation provides a major part of the potential energy changes necessary to drive the atmospheric wind system and is responsible for maintaining the surface air temperature within limits for livability.

thermal damping

A radiative process by which warmer regions of the atmosphere will radiatively cool to space at greater rates than the colder regions and restore the atmosphere to a more uniform temperature field. In general, this process becomes more significant with increasing altitude in the stratosphere. Thermal damping is associated with thermal dissipation.

thermal dissipation

Refers to the process of wave dissipation in which radiative heating and cooling lessen the temperature differences that are associated with Rossby wave formation. These Rossby waves have associated large scale areas of warm and cold temperature perturbations. The warm regions will radiatively cool to space at greater rates than the colder regions and restore the atmosphere to a more uniform temperature field in a process of thermal damping. See also Rossby wave.

thermal equilibrium

In the absence of an external heat source, the radiative condition in which objects in a confined space eventually reach the same temperature. Although electromagnetic energy is still being exchanged between objects that are in a state of thermal equilibrium, the objects are absorbing and emitting this energy at exactly the same rate so no net heat exchange takes place. An example of this is a hot stone left in a closed room. The heat emitted by the stone will warm the room and in the process the stone will cool. Because the room is so much larger, the overall air temperature change will be very small. The stone will cool down considerably though. Eventually, the stone will reach the temperature of the room. The stone is emitting blackbody radiation that is causing it to cool. See blackbody radiation.

thermal radiation

The electromagnetic radiation emitted by any substance as the result of the thermal excitation of its molecules. Thermal radiation ranges in wavelength from the longest infrared radiation to the shortest ultraviolet radiation.

thermal wind

A relationship that derives from the balance between horizontal temperature gradients and vertical gradients of the zonal wind. It is a relationship that shows how the wind speed and direction change with altitude. In this sense, the thermal wind is not an actual wind as it does not refer to air parcels in motion. The zonal (east-west) component of the thermal wind depends on the latitudinal (equator-pole) gradient of the average temperature between the two levels in the atmosphere. Likewise, the meridional (north-south) component of the thermal wind depends on the longitudinal (i.e. along a circle of latitude through different longitudes) gradient of the average temperature between two levels in the atmosphere. To better understand the concept of the thermal wind, begin by considering that above a certain altitude the atmosphere is in a state of geostrophic balance. This means that the winds that arise are geostrophic in nature. The existence of horizontal temperature gradients result in the existence of a wind field. More specifically, the existence of an equator-pole temperature gradient results in the existence of a zonal wind field with the region of fastest winds referred to as a zonal jet stream. See zonal wind. The nature of the jet stream as a high-altitude, fast moving ribbon of air means that there must exist a vertical gradient in the geostrophic wind from relatively slow winds at the surface to fast winds inside the jet stream at the top of the troposphere and in the stratosphere. The thermal wind relationship says that the magnitude of the vertical gradient of the geostrophic wind is larger AND positive where the magnitude of the equator-pole temperature gradient is larger (either positive OR negative). In the northern hemisphere, the temperature gradient is defined as negative; i.e., decreases from equator to pole. Where the magnitude of the gradient increases, that is, the equator-pole temperature changes faster, the geostrophic wind increases with height. In the southern hemisphere, the temperature gradient is defined as positive (i.e., again decreases from equator to pole.) Again, where the magnitude of the gradient increases, the geostrophic wind increases with height. In both cases, the geostrophic (zonal) wind increases with height. The vector difference between the geostrophic wind at two levels is called the thermal wind.

thermocline

A transition layer of water in the ocean, with a steeper vertical temperature gradient than that found in the layers of ocean above and below. The permanent thermocline separates the warm mixed surface layer of the ocean from the cold deep ocean water and is found between 100-1000 m depths. The thermocline first appears at the 55-60° N and S latitudes where it forms a horizontal separation between temperate and polar waters. The thermocline reaches its maximum depth at midlatitudes and is shallowest at the equator and at its northern and southern limits. The thermocline is stably stratified, and transfer of water and carbon dioxide across this zone occurs very slowly. Thus, the thermocline acts as a barrier to the downward mixing of carbon dioxide.

thermonuclear

Pertaining to a nuclear reaction that is triggered by particles of high thermal energy.

thermosphere

The outermost shell of the atmosphere, between the mesosphere and outer space; where temperatures increase steadily with altitude.

Third Law of Thermodynamics

The statement that every substance has a finite positive entropy, and that the entropy of a crystalline substance is 0 at the temperature of absolute zero. Compare First Law of Thermodynamics and Second Law of Thermodynamics. See also entropy and absolute zero.

TOMS

Abbreviation for Total Ozone Mapping Spectrometer, an instrument that employs a backscatter ultraviolet (BUV) technique to make ozone measurments. The TOMS instrument flown aboard the Nimbus-7 satellite measured backscattered UV at six wavelengths and provided a contiguous map of total column ozone. It provided almost complete global coverage of ozone outside the polar night region from November 1978 to May 1993. The Nimbus-7 TOMS scanned back and forth through a set of angles about the nadir angle. Its nadir angle field of view was a square 50 km on a side. The METEOR-3 TOMS instrument measured ozone from August 1991 to December 1994. It had a nadir angle field of view of about 60 km on a side. Other TOMS instruments are continuing to provide global coverage. One such is the Earth Probe (EP) TOMS instrument, launched in July 1996. Its nadir angle field of view is about 39 km on a side. Total ozone maps are created once a day.

torr

Provisional international standard term to replace the English term millimeter of mercury and its abbreviation mm of Hg (or the French mm de Hg). The torr is defined as 1/760 of a standard atmosphere or 1,013,250/760 dynes per square centimeter. This is equivalent to defining the torr as 1333.22 microbars and differs by only one part in 7 million form the International Standard millimeter of mercury. The prefixes milli and micro are attached without hyphenation.

total column ozone

The total amount of ozone that is found in a column of air above Earth from the surface to the top of the atmosphere. The majority of this amount is typically found in the stratosphere.

trace gas

A minor constituent of the atmosphere. The most important trace gases contributing to the greenhouse effect are water vapor, carbon dioxide, ozone, methane, ammonia, nitric acid, nitrous oxide, ethylene, sulfur dioxide, nitric oxide, dichlorofluoromethane or Freon 12, trichlorofluoromethane or Freon 11, methyl chloride, carbon monoxide, and carbon tetrachloride. Such trace gases are sometimes referred to as trace species.

trace species

See trace gas above.

trade winds

The wind system occupying most of the tropics that blows from the subtropical highs toward the equatorial trough; northeasterly in the northern hemisphere and southeasterly in the southern hemisphere.

trajectory

In general, the path traced by any body moving as a result of an externally applied force, considered in three dimensions. Trajectory is sometimes used to mean flight path or orbit, but orbit usually means a closed path and trajectory, a path that is not closed.

translation energy

In a gas, the energy associated with random straight line motion of the molecules. Such motion is referred to as translational motion. Compare to vibrational motion.

translational motion

Refers to translation or movement in a straight line without rotation. At the molecular level, it refers to the random straight line motion of molecules. The energy associated with such motion is referred to as translation energy.

tropopause

The boundary between the troposphere and stratosphere, usually characterized by an abrupt change of lapse rate. The change is in the direction of increased atmospheric stability from regions below to regions above the tropopause. Its height varies from 15 to 20 km in the tropics to about 10 km in polar regions. In polar regions in winter it is often difficult or impossible to determine just where the tropopause lies, since under some conditions there is no abrupt change in lapse rate at any height.

tropopause folding

Refers to a stratospheric intrusion of air that sinks into the baroclinic zone beneath an upper tropospheric jet stream. Such folds are formed by a steepening of the tropopause and the associated isentropes (surfaces of constant potential vorticity) at a jet core. Tropopause folds are the dominant and most efficient form of stratospheric-tropospheric exchange in the middle latitudes. Folds usually occur of the western flank of cutoff low systems. Clean, dry stratospheric air, rich in ozone and potential vorticity, is transported downward to tropospheric levels.

troposphere

That portion of the atmosphere from Earth's surface to the stratosphere; that is, the lowest 10 to 20 km of the atmosphere. The troposphere is characterized by decreasing temperature with height, appreciable vertical wind motion, appreciable water vapor content, and weather. Compare to stratosphere. Dynamically, the troposphere can be divided into the following layers: planetary boundary layer, which includes both the surface boundary layer and the Ekman layer, and the free atmosphere. See free atmosphere, Ekman layer, planetary boundary layer, and surface boundary layer.

UARS

Abbreviation for the Upper Atmosphere Research Satellite, deployed by NASA to study the chemistry in the upper atmosphere of Earth.

ultraviolet radiation

Often referred to as UV radiation. Electromagnetic radiation of shorter wavelength than visible radiation; roughly, radiation in the wavelength interval from1 nanometer to 400 nanometers. Also called just ultraviolet. Ultraviolet radiation from the Sun is responsible for many complex photochemical reactions characteristic of the upper atmosphere; e.g., the formation of the ozone layer through ultraviolet dissociation of oxygen molecules followed by recombination to form ozone.

Umkehr effect

Due to the presence of the ozone layer, an anomaly of the relative zenith intensities of scattered sunlight at certain wavelengths in the ultraviolet as the Sun approaches the horizon.

uncorrelated noise

Two random -- that is, "noisy" -- time series that are completely unrelated (i.e. uncorrelated) to each other. See noise. See also red noise and white noise.

variability

The state or condition of having no fixed quantitative value.

velocity

1. Speed.

2. A vector quantity equal to speed in a given direction.

In sense 1, velocity is often used synonymously with speed, as in the velocity of the airplane, but in such contexts speed is properly the preferred term; except in the compound airspeed, velocity is preferred to speed in reference to motion of air or other fluid.

vibrational energy level

The energy associated with the vibrational motion of an atom.

vibrational motion

Refers to the back and forth motion of the atoms in a molecule. In general, the greater the thermal infrared energy absorbed by a molecule, the greater is this back and forth motion of the constituent atoms.

vorticity

A vector measure of local rotation in a fluid flow, defined mathematically as the curl of the velocity vector V as the velocity field shrinks in size to zero. That is, the vorticity component normal to a small plane element is the limit of the circulation per unit area as the area of the element approaches zero.

Vorticity is mathematically defined as:

       = operator x V

There are two types of vorticity, relative vorticity and absolute vorticity. We begin by considering the wind relative to Earth. We define it as a velocity vector V. It consists of the change with respect to time t of three components, the x (east-west) component, y (north-south) component, and z (vertical) component. Thus,

      V=(dx/dt, dy/dt, dz/dy)

The dx/dt component defines the zonal wind, u=dx/dt

The dy/dt component defines the meridional wind, v=dy/dt

The dz/dt component defines the vertical wind, w=dz/dt (typically, it is very small). Thus, we may rewrite V as

      V= (u,v,w)

Returning to our vorticity formula,

      (relative) vorticity= {[partial w/partial y]-[partial v/partial z], [partial u/partial z] - [partial       w/partial x], [partial v/partial x]- [partial u/partial y]}

The first term represents the zonal component of relative vorticity, the second term represents the meridional component of relative vorticity, and the third term represents the vertical component of relative vorticity. It is the third term, the vertical component of relative vorticity that is of primary concern in large-scale dynamic meteorology. This term is often denoted by .

       = [partial v/partial x]- [partial u/partial y]

Absolute vorticity is defined the same way, except we account for the rotation of Earth via the Coriolis parameter, f. Absolute vorticity is denoted by .

       = + f

where f= 2 x x sine , as defined in Coriolis parameter. That is, the vorticity of a solid rotation is twice the angular velocity vector. In meteorology, the vorticity usually refers to the vertical component of the vorticity as defined above.

Walker Cell

A zonal circulation of the atmosphere confined to equatorial regions and driven principally by the oceanic temperature gradient. In the Pacific, air flows westward from the colder, eastern area to the warm, western ocean, where it acquires warmth and moisture and subsequently rises. A return flow aloft and subsidence over the eastern ocean complete the cell.

watt

The unit of power in the MKSA system; that power which produces energy at the rate of 1 joule per second.

wave

A disturbance that is propagated in a medium in such a manner that at any point in the medium the quantity serving as measure of disturbance is a function of the time, while at any instant the displacement at a point is a function of the position of the point. Any physical quantity that has the same relationship to some independent variable (usually time) that a propagated disturbance has, at a particular instant, with respect to space, may be called a wave. In meteorology, a wave is also defined as a deviation in the zonal mean flow of the atmosphere. (See wave-mean flow interaction for further details.) Waves include gravity, (mixed) Rossby-gravity, inertio-gravity, Rossby, planetary, and equatorial (Kelvin), all of which have been defined in this glossary.

wave crest

Peak of a wave, opposite of the wave trough.

wavelength

In general, the mean distance between maximums (or minimums) of a roughly periodic pattern. Specifically, the laest distance between particles moving in the same phase of oscillation in a wave disturbance. The wavelength is measured along the direction of propagation of the wave, usually from the midpoint of a crest (or trough) to the midpoint of the next adjoining crest (or trough). The reciprocal of wavelength is the wave number.

wave-mean flow interaction

Refers to the way in which a wave alters the zonal mean flow. Waves existing in the zonal mean flow represent departures from the basic state flow. These waves, referred to as eddies, play a big role in transporting energy and momentum across latitudes. Transient eddies are departures from the time mean state. Standing eddies, on the other hand, arise from the mean field itself. These standing eddies are found by subtracting out the zonal mean. Wave-mean flow interaction drives many stratospheric motions. Dissipation of the equatorial (Kelvin) and Rossby-gravity waves force the westerly and easterly phases, respectively, of the QBO. If wave amplitudes grow, the waves become nonlinear and may "break" in a process quite analogous to ocean waves at the seashore. Above the stratosphere, breaking gravity waves force the semiannual oscillation (SAO). Nonlinear breaking gravity waves also cause a strong forcing of the zonal mean circulation in the upper mesosphere. Finally, the mean meridional circulation is driven by breaking planetary waves. Wave-mean flow concept may be applied to understanding the causes of the stratospheric sudden warming event.

wavenumber

The reciprocal of wavelength; the number of waves per unit distance in the direction of propagation; or, sometimes 2 times this quantity. In spectroscopy, wave number is usually expressed in reciprocal centimeters, as 100,000 cm (100,000 per centimeter).

westerly

Any winds with components from the west; usually applied to broad currents or patterns of persistent westerly winds.

white noise

A sound or electromagnetic wave whose spectrum is continuous and uniform as a function of frequency. Compare to red noise.

Wien displacement law

The wavelength at which a blackbody radiation curve reaches a maximum is related to its temperature inversely as shown by

      (max) =A/T

where

(max) = wavelength of maximum spectral radiant exitance in micrometers, A = 2898 micrometers K, T = temperature K.

zonal

In the east-west direction, along latitude zones.

zonal average

A average of some atmospheric variable, such as wind or temperature, averaged about a circle of constant latitude.

zonal mean

A zonal average of some atmospheric variable.

zonal mean flow

The time-averaged zonal average of some atmospheric variable, such as wind or temperature.

zonal wind

The component of the wind field blowing parallel to lines of latitude; i.e., from west to east or vice versa.