With the term 'atmospheric windows' we refer to the regions of the electromagnetic spectrum where the interaction between the atmosphere constituents (i.e., molecules, aerosols, and cloud particles) and the electromagnetic radiation is minimized, namely the mechanisms of scattering and absorption of the radiation are less relevant than the transmission one. Therefore, the radiation collected at the sensor in these spectral regions is strictly depending on the Earth surface features, allowing to infer information about the processes/phenomena there in progress at the time of the acquisition. There are three main spectral ‘windows’ in the Earth's atmosphere. The first of these includes the visible and near-infrared (VNIR) parts of the spectrum up to the medium infrared, between wavelengths of about 0.38 μm and 3.5 μm, although it does also contain a number of opaque regions. This spectral interval includes the small portion of the electromagnetic spectrum to which human eyes are sensitive to (i.e, the visibile region between 0.4 and 0.7 μm). The second is a rather narrow region between about 8 μm and 15 μm, in which is found the bulk of the thermal infrared (TIR) radiation from objects at typical terrestrial temperatures. In this region there is only a main opaque interval, around 9.6 μm due to the presence of the ozone band. The third more or less corresponds to the microwave region, between wavelengths of a few millimeters and a few meters. Therefore, each remote sensing instrument that should be able to fully penetrate the Earth’s atmosphere has to be designed to operate in one of these three ‘window’ regions.