Nearly all energy on Earth is supplied by the sun through radiation.

This radiation occurs at wavelengths ranging from many meters (radio waves) to nanometers (X-ray).

Molecules and particles in the atmosphere and on the surface interact in different ways with radiation of different wavelengths. Measurements at different wavelengths can therefore be used to derive information on different aspects of the atmosphere and surface.

Microwave wavelengths, 10 cm..1 mm

At microwave wavelengths, scattering by air molecules and cloud droplets is negligible, and therefore active satellite measurements in this spectral region can provide information — even under cloudy conditions — about surface properties such as ice and snow coverage, surface roughness, ocean wave height and thus wind speed.

Many molecules also have emission lines at these wavelengths. This fact can be used for passive remote sensing of trace gases such as O₃ or ClO in the atmosphere. Because the width of the emission line depends on pressure, the shape of the lines provides information on the vertical profile of the species in the atmosphere. However, such measurements are limited to the upper atmosphere.

Infrared wavelengths, 1 mm..740 nm

The infrared part of the spectrum can also be used for passive remote sensing of many atmospheric species, in particular those which are known as greenhouse gases. As in the case of microwave spectroscopy, the shape of the emission line contains profile information.

Thermal emission of the surface and atmosphere is observed in the thermal infrared. This emission can be used for the remote sensing of temperature and altitude, e.g. when determining cloud-top height. This, however, requires knowledge of the vertical temperature profile in the atmosphere.

Because ice and water have quite different absorption characteristics in the infrared, the phase of cloud particles can also be determined.

Remote sensing of trace species in the thermal infrared cannot reach below the middle troposphere because of the lack of thermal contrast between the lower atmosphere and the surface.

Visible wavelengths, 740 nm..380 nm

In the visible part of the spectrum, radiation can penetrate to the surface and is reflected back to a satellite, at least in the absence of clouds. This data can therefore be used to study the Earth's surface and determine, for example, cloud cover, vegetation type or land use.

At visible wavelengths both Mie and Rayleigh scattering are important, as well as absorption by a number of relevant molecules such as O₃, NO₂ or water vapour. While measurements in the visible spectral range are sensitive down to the surface, the absorption does not provide direct information on the vertical distribution of the trace species.

Ultraviolet wavelengths, 380 nm..10 nm

At UV wavelengths Rayleigh scattering becomes very strong and with decreasing wavelength photons penetrate less and less deeply into the atmosphere. This fact can be used to derive vertical profiles of ozone, which absorbs strongly in the UV.

There are a number of other, weaker absorbers in this spectral range which can also be retrieved, albeit without vertical information.

Aerosols

Some aerosols have strong absorptions at UV wavelengths and can be measured in the same way as trace gases, but usually aerosols are retrieved by analysing the extinction as a function of wavelength at dedicated spectral points from the UV to the IR. In order to extract the weak aerosol signal, one has to stay away from absorption features and cloud contaminated measurements. In addition, the spectral reflectance of the surface must be well known.