Specific molecules absorb radiation at specific wavelengths. Figure 2.03 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’s only toward the ultraviolet -- wavelengths greater than 0.3 µm -- that the atmosphere begins to transmit the radiation to a degree useful for remote sensing. The amount of black or gray at a given wavelength in Figure 2.03 represents the degree to which the atmosphere transmits electromagnetic radiation at that wavelength. The further the area of gray extends away from the x-axis, the greater the tendency for the atmosphere to transmit those wavelengths. The closer the area of gray to the x-axis, the greater the tendency for the atmosphere to absorb, and not transmit, the radiation at those wavelengths.

Before discussing how the atmosphere absorbs and transmits infrared electromagnetic energy, there are three other characteristics of the atmosphere that bear mentioning here.

Note that the atmosphere generally does not transmit infrared electromagnetic radiation (~3-20 µm) as well as visible or microwave wavelengths. However, there are four “windows” in the electromagnetic spectrum in which the atmosphere absorbs or emits little thermal infrared radiation based on the chemical composition of the atmosphere (the nitrogen, oxygen, and water vapor molecules). These window are

  1. 2.5-2.8 µm
  2. 3.5-4.0 µm
  3. 8.0-9.0 µm
  4. 10-13 µm

Satellite sensors designed to measure infrared radiation from the sea surface, like the AVHRR, operate in one of the above “windows,” where there is minimal absorption or emission by the gas molecules in the atmosphere (hence, maximum transmission). Therefore, surface thermal infrared emissions would be clearly visible from the satellite.