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 c