Earth's itself represents the second (after Sun) most powerfull natural source of e.m. radiation for EO. Even if very less powerfull than Sun such a source is available for EO day and nigth. Its average emittance can be approximated by that of a blackbody at about 290 K. The maximum of its emission, following the Wien's Law, falls then around 10 micron (in the Thermal InfraRed - TIR spectral range) being Earth's emission trascurable in the VIS-SWIR range.
Most of Earth's thermally emitted radiation falls in the spectral range 8-14 microns where it benefits of a quite high atmospheric transmittance (TIR atmospheric spectral window) in standard atmospheric conditions. However thick clouds prevent TIR radiation to reach satellite sensors (adsorbing and/or reflecting backward the radiation leaving Earth's surface) so that ground resolution cells affected by clouds are usually identified (cloud-mask) in the image pre-processing phase and not considered for further elaboration devoted to investigate surface properties. Even if very low in intensity, Earth's emitted radiation in the Far InfraRed (FIR) and in the MicroWaves (MW) spectral ranges are also used for quite important investigation related to the Earth's Energy balance (FIR) and for meteo-climatological applications. The complete transparence of Earth's atmosphere to the MWs, even in presence of meteorological (not precipitating) clouds make this Earth's emitted signal particularly important for application (e.g. climatological) requiring temporal continuity (all weather) of observations of Earth's surface properties like Temperature, Soil wetness, etc.. However, due to the weakness of the Earth's emitted signal in the MW ranges, such products can be achievable just at quite low spatial resolution (e.g. > 10km) by passive EO MW sensors