There are three ways that the atmosphere can influence the amount of radiation arriving at the satellite.

  1. Attenuation by atmospheric absorption.
  2. Augmentation by atmospheric emission.
  3. Attenuation or augmentation by atmospheric scattering.

Specific molecules absorb radiation at specific wavelengths Figure 2.04 shows the degree to which molecules in the atmosphere transmit electromagnetic radiation ranging from ultraviolet to microwave wavelengths, with wavelength on the x axis and percent transmission through the atmosphere on the y axis. Very short wavelength radiation, such as x-rays, is not shown on this figure. These wavelengths are not useful for measurements within Earth's atmosphere because most x-rays are absorbed by molecules in the atmosphere, its clouds, and other constituents. It is only toward the ultraviolet -- wavelengths greater than 0.3 mm -- that the atmosphere begins to transmit electromagnetic radiation at a specified wavelength. The further the area of gray extends away from the x axis, the greater the tendency for the atmosphere to transmit at those wavelengths. The closer the area of gray to the x axis, the greater the tendency for the atmosphere to absorb, not transmit, the radiation at those wavelengths.

Before discussing how the atmosphere absorbs and transmits microwave energy, there are three other characteristics of the atmosphere that bear mentioning here. First, the atmosphere protects us from harmful ultraviolet radiation by strongly absorbing ultraviolet radiation (wavelengths < 0.3 mm), allowing very little ultraviolet radiation to reach Earth's surface. Second, the atmosphere transmits (i.e., is "transparent") electromagnetic radiation over most of the visible portion of the electromagnetic spectrum (wavelengths 0.4-0.7 mm). This is why we can see the Sun on a cloudless day. Third, with the exception of four atmospheric "windows" (2.0-2.5, 3.5-4.0, 8.0-9.0, and 10-13 mm), the atmosphere generally does not transmit infrared electromagnetic radiation (~3-20 mm) as well as the visible wavelengths.

Figure 2.04 shows that the atmosphere readily transmits most wavelengths of microwave electromagnetic radiation (wavelengths > 1.0 cm). At microwave wavelengths, significant atmospheric absorption is limited to

For typical atmospheric conditions, scattering is minimal at microwave wavelengths. The only exception is scattering by liquid water drops varying in size from small drops in clouds and fog (1-10 mm in diameter) to large rain drops (102-104 mm).

There are minimal atmospheric influences (absorption, scattering, atmospheric emissions) on microwave radiation leaving Earth's surface, and one of the big advantages of using sensors operating at microwave wavelengths is that the measurements can be obtained in day, night, and nearly all weather conditions. Regions of thick clouds and heavy rain are the only regions where the surface microwave emissions would not be capable of reaching the satellite. Fortunately, the atmosphere over the North and South Pole regions is very dry and virtually cloud free, particular in winter.

Therefore, given the typical atmospheric conditions in the polar regions, the microwave radiation arriving at the satellite is now due to just two sources: direct surface emissions and extraterrestrial sources. If we further assume that the extraterrestrial sources are much smaller than the direct surface emissions and are therefore negligible, the microwave radiation arriving at a sensor monitoring the polar regions is solely from direct surface emissions.