A body, in area temperature, emits thermic energy in form of different wave-length radiations. The 40% of spectrum of such emissions is concentrated between 8 and 13 μ and it is just in these wave-length limits that atmosphere presents, in particular conditions, a good transparence. More precisely the atmosphere transparence depends from the water vapor and pollution contents as well as from the zenithal angle. When the sky is cloudy or when the emitting surface is orientated towards the horizon, the atmosphere results practically opaque to the infrared. When the sky is clear and the surface is orientated towards the zenith, the atmosphere transparence can reach 80% and for wave-length between 8 and 13 μ the so named "transparence window” open ...