EUMeTrain: Water Vapour CAL Module - Water Vapour Channels

Radiances in water vapour channels

In water vapour channels a satellite measures the intensity of infrared radiation emitted by water vapour. This radiation intensity is called radiance.

In the atmosphere the radiance measured by a satellite in water vapour bands is originally the thermal radiation coming from the Earth´s surface. When it goes upwards, it is absorbed by water vapour, and reradiated in a longer wave length that depends on

In the following picture the green boxes represent humid layers in the atmosphere. The colour shade varies according to the humidity content of the water vapour: the higher up the humid layer is, the colder it is and the less it contains water vapour.

Radiance can be converted to

The brightness temperature of humid areas is low because of the strong absorption. Therefore, in an inverted satellite image, the humid areas are light and dry areas dark.