Published: 2007-08-26 10:48:33
Updated: 2007-08-26 10:48:33
Posted August 26, 2007
MIKE MOSS SAYS: James, There are a couple of primary satellite image types, and the appearance of clouds depends on the technology involved. Satellite images that use a sensor in the visible wavelength range ( the same part of the electromagnetic spectrum that our eyes process) tend to produce images that look similar to a photograph taken from space, so clouds that are thick and well-lit by sunlight from above will appear bright white and those that are thin/translucent or are located in the shadow of higher clouds or are located near the "terminator" that separates the sunlit portion of earth from the dark side, appear some shade of gray.
The other primary satellite sensor is one that uses infrared wavelengths in order to provide imagery that is available 24-hours a day and that also includes information about temperature and, by inference, cloud height. Infrared images use a system in which the colors represent temperatures. There are many different color schemes, sometimes called enhancement curves, that can be used to highlight certain meteorological phenomena (to make fog and low clouds more noticeable, make overshooting tops on thunderstorms stand out, etc) but a common one is to simply make higher temperatures darker and lower temperatures brighter shades of gray. This usually involves making temperatures that are typical of the earth's surface almost black, and temperatures that are typical of the tops of thick clouds almost white. In this case, the medium or dark gray clouds are usually low to mid level clouds, while the brightest whites are usually associated with either thick high clouds or with the upper portions of convective cells such as heavy showers or thunderstorms. Of course, this scheme can become a little tricky to interpret at times in the winter when the earth's surface in some locations may become notably colder than some of the air at certain altitudes above it.