What is Atmospheric Windows?

The earth sunlight energy come from solar radiation. And the sun energy shines on the atmosphere, and most sunlight will be absorbed by the atmosphere and only small portion of sunlight shine on the earth.

This is a simple sketch describes the atmosphere can absorb the sunlight.

 The higher atmospheric opacity, the less the sunlight can penetrate through the atmosphere to fall down to the earth.

 The lower atmospheric opacity, the more sunlight can penetrate through the atmosphere to fall down to the earth.

From 300nm to 1um, or even till 2.5um, this range has a most sunlight shone on the earth.

 Our eyes can see only this range of sunlight remarked in colors, and they are where photosynthesis orginates from, and earth plants and animals live by energy. If the atmosphere Absorbs more sunlight, the earth living can not live by it without energy.

 And in the 10um range also a good range for analysis, and 10cm to 10m the transmittance is also very high, They are for electricmagnetic wave and mobile communication. So the the lower absorbance can penetrate the atmosphere, the more a sun lights can shine on the earth. 

 Here define atmospheric windows, those light with a low absobance can penetrate the atmosphere to shine on the earth. Including our fieldspec spectroradiometer and remote sensing can be working in the atmospheric windows.