Introduction: Interpreting Weather-Satellite Images

Interpreting Weather-Satellite Images

I. Introduction

These exercises ask you to do the following:

Review a series of questions about visible and infrared (IR) weather-satellite images and about the features of the earth typically distinguishable on them;

Apply your knowledge of basic physical principles, and of properties of clouds and the earth's surface, to account for differences in the brightness of features recorded on a set of visible and IR satellite images; and

Try to formulate guidelines to help you distinguish a visible image from an IR image.

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:

ultraviolet radiation;
visible light;
near-infrared radiation;
longwave infrared (IR) radiation; and
microwave radiation.

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

(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:

Infrared
Image

(Click on these images
to get bigger ones.)

Color
Enhanced
Infrared
Image

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

(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:

Some Mostly Common-Sense Properties of Features Distinguishable on Weather-Satellite Images; and

Weather Satellite Images

Proceed to Part II: Background Questions.