Lesson 4: Background

Interpreting Satellite Infrared Images

The simplest cloud measurement technique is the threshold method, in which an equivalent blackbody temperature or a spectral reflectance threshold is selected which distinguishes between cloud and non-cloud in infrared or visible satellite images. Information on cloud top temperature is obtained by comparing the observed brightness temperature with an atmospheric temperature profile - this approach usually underestimates the cloud height. Using a visible or near-infrared reflectance threshold works well for determining clear-sky ocean scenes that are free of sun glint. For example, you might classify a pixel as cloudy if the reflectance at a visible wavelength is greater than 8%.

Another straightforward approach employs two channels in combination. For example, the split window technique makes use of observations near 11 and 12 um to detect clouds over oceans. Cloud classification is accomplished by considering the 11 um blackbody temperature and the difference between the 11 and 12 um. Clear scenes have warm temperatures and brightness temperature differences that are negative, usually less than about -1°. Another simple two channel technique uses visible and infrared observations. In this approach observed visible reflectance and equivalent blackbody temperature are compiled in a 2-D array, and observations are then classified based on their relative brightness and temperature. For example, clear sky oceans would be warm and dark while convective clouds would be cold and bright. Automated classification of clouds is accomplished by either assigning thresholds or by employing maximum likelihood statistical techniques.

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