Interpreting IR Satellite Images
The infrared (IR) sensor on the satellite detects electromagnetic radiation in the "thermal IR", at wavelengths between 10 and 12 micrometers (about 20 times longer than the wavelength of visible light). The intensity of the IR radiation reaching the satellite depends on the warmth of the object emitting that radiation.
The most intense radiation comes from regions where the ground or the ocean is warm. Such regions of intense emission are shown as dark shades of gray. The IR radiation emitted by clouds in the the upper atmosphere, where it is very cold, is much less intense. These regions of low intensity IR emission are shown as white and light gray in the IR photo.
Thus, high clouds are white. Low clouds, with temperatures near that of the surface, are often medium gray. The surface and the oceans in the lower latitudes appear dark gray. On a sunny afternoon, the land heats up sufficiently to appear almost black.
Because IR is constantly emitted by the earth and by clouds, it is possible to obtain IR satellite imagery even when the scene is not illuminated by the sun (and thereby construct loops that extend for a full 24 hours). In contrast, visible satellite imagery, which relies on sunlight reflected up to the satellite, can only be obtained during the daylight hours.