EM 1110-2-2907
1 October 2003
low is a summary of these three major atmospheric constituents and their significance in
remote sensing.
g. The role of atmospheric compounds in the atmosphere.
this protective layer in the atmosphere, our skin would burn when exposed to sunlight.
(2) Carbon Dioxide. Carbon dioxide (CO2) is called a greenhouse gas because it
atmosphere from radiation emitted from both the Sun and the Earth.
(3) Water vapor. Water vapor (H2O) in the atmosphere absorbs incoming long-
wave infrared and shortwave microwave radiation (22 to 1 m). Water vapor in the lower
atmosphere varies annually from location to location. For example, the air mass above a
desert would have very little water vapor to absorb energy, while the tropics would have
high concentrations of water vapor (i.e., high humidity).
(4) Summary. Because these molecules absorb radiation in very specific regions of
the spectrum, the engineering and design of spectral sensors are developed to collect
wavelength data not influenced by atmospheric absorption. The areas of the spectrum that
are not severely influenced by atmospheric absorption are the most useful regions, and
are called atmospheric windows.
h. Summary of Atmospheric Scattering and Absorption. Together atmospheric scatter
and absorption place limitations on the spectra range useful for remote sensing. Table 2-4
summarizes the causes and effects of atmospheric scattering and absorption due to at-
mospheric effects.
i. Spectrum Bands. By comparing the characteristics of the radiation in atmospheric
windows (Figure 2-15; areas where reflectance on the y-axis is high), groups or bands of
wavelengths have been shown to effectively delineate objects at or near the Earth's sur-
face. The visible portion of the spectrum coincides with an atmospheric window, and the
maximum emitted energy from the Sun. Thermal infrared energy emitted by the Earth
corresponds to an atmospheric window around 10 m, while the large window at wave-
lengths larger than 1 mm is associated with the microwave region (Figure 2-16).
Table 2-4
Properties of Radiation Scatter and Absorption in the Atmosphere
Diameter (φ) of particle
Atmospheric
relative to incoming
Result
Scattering
wavelength (λ)
φ<λ
Rayleigh scattering
Short wavelengths are scattered
φ=λ
Mie scattering
Long wavelengths are scattered
φ>>λ
Nonselective
All wavelengths are equally scattered
scattering
Absorption
No relationship
CO2, H20, and O3 remove wavelengths
2-18
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