290.

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atmospheric windows (Figure 1.3), though it should be remembered that scattering and absorption affect all wavebands in the optical spectrum to a greater or lesser degree, and these effects are variable both in space and time.

1.1.1 Atmospheric interactions

Electromagnetic radiation interacts with the Earth’s atmosphere, the degree of interaction depending on the wavelength of the radiation and the local characteristics of the atmosphere. The basic interactions are known as scattering and absorption. Scattering is more likely to occur at shorter wavelengths. The most common scattering behaviour is known as Rayleigh scattering, which is the main cause of haze in remotely sensed imagery. The atmosphere has different levels of absorption at different wavelengths. Regions of the spectrum that have a relatively high transmission are called atmospheric windows (Figure 1.3). The energy in some wavebands (e.g. from 15 to 10³ μm) is almost completely absorbed by the atmosphere. These wavelengths therefore cannot be used for remote sensing. Wavebands

Figure 1.3 Atmospheric windows (unshaded). Vertical axis is atmospheric transmission (%). Horizontal axis is the logarithm of the wavelength in micrometres.

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