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The graph on the right shows a weighting function for the carbon dioxide channel in a standard atmosphere. The brightness temperature that is sensed can be thought of as a weighted mean value over this pressure range.
Most of the energy that the satellite detects in this channel originates from middle and lower portions of the troposphere. Since the weighting function peaks near the earth's surface, clouds and surface features are still visible. However, as the contribution to observed brightness temperature due to carbon dioxide becomes more dominant lower in the atmosphere, lower clouds and surface features appear less distinct.
Comparing the 13.3 and 10.7 micron imagery on the left shows how infrared emission from carbon dioxide partially masks particularly the lower cloud features and surface.