Interpreting Weather-Satellite Images

I. Introduction

These exercises ask you to do the following:

The text and images below summarize some information that might help you prepare for this exercise. However, if you want to test yourself immediately without reviewing this material, go to:

Part II: Background Questions

Weather-Satellite Images: A Brief Review

Instruments carried by weather satellites can record radiation of different wavelengths, including: The background questions in Part II and the exercises in Part III will focus on images that record visible and longwave infrared (IR) radiation.

Much as your eye does, visible images record visible light from the sun that is reflected by cloud tops, land surfaces, ocean surfaces, and snow/ice surfaces. Here's an example:

Visible image example
(Click on this image
to get a bigger one.)

Cloud tops, land surfaces, ocean surfaces, and snow/ice surfaces reflect some of the visible light that strikes them, but they emit mostly IR radiation. Wavelengths of this emitted IR radiation that lie in a portion of the electromagnetic spectrum called the atmospheric window pass unaffected through the atmosphere to the satellite, which records them in ordinary infrared (IR) images. Here are two examples, one in black-and-white and the other color enhanced:

IR image example
(Click on these images
to get bigger ones.)
IR image example

Note that visible and ordinary IR images tell us little about air itself, since both kinds of image record wavelengths to which air is transparent. However, water vapor, carbon dioxide, and other gases in the atmosphere both absorb and emit IR radiation with wavelengths lying outside of the atmospheric window. Images that record IR radiation emitted by water vapor, called water vapor images, and images that record IR radiation emitted by other gases, provide information about state of the atmosphere. Here's an example:

Water vapor image example (Click on this image
to get a bigger one.)

(These exercises will not cover water vapor images.)

Weather satellites record the "brightness" or intensity of the visible and IR radiation coming from different parts of the earth or atmosphere. Black-and-white satellite images such as the examples shown above display different intensities of radiation in different shades of gray.

On visible images, brighter (that is, whiter or lighter) areas represent greater intensities of visible radiation, and darker areas represent lower intensities of visible radiation, just as your eye would see.

On IR images, since our eyes can't see IR radiation of any intensity, we have to decide arbitrarily how to translate different intensities into different shades of gray on a black and white image (or different colors on a color-enhanced IR image). By convention, we usually translate low intensities of IR emission to lighter shades of gray, and greater intensities of IR emission to darker shades of gray. Since IR emission intensity tells us about temperature (the higher the emission intensity the higher the temperature), the different shades of gray (or different colors) therefore tell us about differences in temperature.

For more information, refer to:

Proceed to Part II: Background Questions.