Observations from instruments flown on satellites are used extensively in weather analysis. Flown onboard satellites are instruments that measure electromagnetic energy that is either reflected or emitted by our planet. These instruments are called radiometers. Two common types of radiometers are used in satellite meteorology. One type measures the amount of visible light from the sun reflected back to space by the Earth's surface or by clouds. The second measures the amount of radiation emitted by these entities.

The radiometers flown on satellites are not cameras. Moving mirrors make the radiometer view different regions of the earth. The instrument begins by looking off in one direction and then scans across a scene making observations as it proceeds. In this manner the radiometer scans a scene line-by-line. Data is recorded in a two-dimensional format which, when displayed on a computer monitor or television screen, makes up an image. The smallest part of this image is the pixel -- which stands for "picture element."  The radiation coming from each pixel is presented in an image format in terms of a grayscale. An image consists of 256 gray shades, ranging from 0 for pure black to 255 for pure white.

All objects emit electromagnetic radiation. The infrared radiometers on satellites measure radiation with wavelengths of 10 to 12 mm. For easy interpretation, the radiant energy measured by infrared radiometers is converted to a temperature. Cold objects are white and hot surfaces appear black. Time sequences of infrared images are animated and shown on television news programs.

The following interactive exercise is an introduction to interpreting satellite weather images typically seen on evening news.  To view this exercise you will need to have Java® properly installed on your web browser.
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This page, and the accompanying exercise, is an example of an interactive web exercise that can enhance teaching and learning.  Please send us comments, suggestions, or queries on how to use this in your class.