Interpretation of IR Imagery

IR imagery indicates the temperature of the radiating surfaces. In black and white image warm areas are shown in dark tones and cold areas in light tones. Clouds will generally appear whiter than the earth surface because of their lower temperature. In this respect IR and VIS images have some similarity.

Because cloud top temperature decreases with height IR images show good contrast between clouds at different levels. This is not possible in VIS imagery.

Coast lines show up clearly in IR images whenever there is strong contrast between land and sea surface temperatures. During the day the land may appear darker (warmer) than the sea but at night may appear lighter (cooler). At times when land and sea temperatures are almost same, it becomes impossible to detect coastlines in IR imagery. The most marked contrast between land and sea is normally found in Summer and Winter and is least in Spring and Autumn. Thin Ci which is often transparent in the VIS can show up clearly in IR especially when it lies over much warmer surface.

IR imagery is inferior to VIS in providing information about cloud texture as it is based upon emitted and not scattered radiation. Low clouds and fog can rarely be observed in IR at night because they have almost the same temperature as the underlying surface. But during day, such clouds are easily detected in.

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