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Adapted from An Introduction to Satellite Image Interpretation, Eric D. Conway and the Maryland Space Grant Consortium, ©1997, Johns Hopkins University Press, Baltimore, 255 pp with Interactive CD-ROM.
For more information about this book and how to order copies go to the JHU Press On-line Catalog
Environmental satellites provide data in several different formats. The most commonly used channels on weather satellites used are the visible, infrared, and water vapor satellites. Each of these channels on the satellite sensors is sensitive to energy (electromagnetic energy) at a particular range of frequencies, therefore each type provides a different view of the Earth, its atmosphere, and its oceans. Researchers rely on all three types of data, and often use them together to better understand the interactions between the atmosphere, oceans, and the Earth's surface.
In this activity, you will read about each type of image and compare visible, infrared, and water vapor imagery to observe the differences. You will be able to describe the use of each type of satellite imagery and use all three together to describe the atmospheric conditions over a portion of the Earth. There are several images to compare that consist of different views of the same scene.
Comparison of Infrared, Water vapor, and Visible satellite imagery:
*Infrared image of North America [G8IR.GIF]
*Visible image of North America [G8VIS.GIF]
*Water vapor image of North America [G8WV.GIF]
Comparison of visible and water vapor imagery [mar20_vi.gif] [mar20_wv.gif]
Comparison of visible and water vapor imagery [nov18_vi.gif]
[nov18_wv.gif]
VIS imagery indicates the amount of solar radiation reflected
from the Earth. A VIS image is an approximation of the Earth's albedo, that
is, the percentage of 
incoming sunlight reflected
by a surface. In satellite VIS imagery, light tones represent areas of high
reflectivity and darker tones represent areas of low reflectivity. Features
on the surface of the Earth or in the atmosphere vary in their reflectivity
and can therefore be discerned on a VIS image. In a VIS image, the large,
thick clouds appear white since they have a high albedo. Thinner clouds
appear light to medium gray. The ocean, with a very low albedo, appears
nearly black. The land, characterized by albedos that depend on the nature
of the surface, appears as various shades of gray.
The IR sensors on board the polar orbiting and geostationary
satellites measure the amount of infrared energy emitted by the Earth and
the atmosphere. Because the amount of energy emitted depends on the 
temperature of the surface, IR imagery is essentially a picture
of the surface and cloud top temperatures portrayed in black, white, or
gray shades. This information can be used to observe thermal properties
of the Earth and the atmosphere. In conventional IR imagery, colder areas
appear as white or light gray tones and warm areas appear black or dark
gray.
On most display systems, the gray scale of an IR image is composed of 256 gray shades ranging from white (coolest temperatures) to black (warmest temperatures). The data correlates temperature with gray shade in a simple linear relationship. In an infrared image, the highest (and therefore coldest) cloud tops appear white. Lower clouds appear as lighter shades of gray, and warmer land and water surfaces appear as darker shades of gray.
As the Earth and the atmosphere emit energy, specific wavelengths
are absorbed by the atmosphere, especially by clouds and suspended water
vapor. At other wavelengths, the energy is not absorbed and is 
transmitted through the atmosphere. Most IR sensors on meteorological
satellites take advantage of the infrared bands that are transmitted through
the atmosphere. This allows accurate measurements of the temperatures of
the Earth and cloud tops to be made. Some satellite sensors, however, study
radiation at wavelengths that are readily absorbed by the atmosphere. Studying
the IR energy at these wavelengths allows atmospheric gas concentrations
to be studied without interference from surface features.
Two widely used applications of this concept are channel 9 (7.3 microns) and channel 10 (6.7 microns) on the GOES VISSR sensors. Energy emitted at these particular wavelengths is readily absorbed by water vapor in the atmosphere. Images that are taken in these channels are used to locate large concentrations of water vapor and water vapor gradients in the middle and upper troposphere (this is the lowest layer of the atmosphere and the location of the most significant weather). The darker regions in water vapor imagery are areas where very little water vapor exists in the middle and upper troposphere, and the lighter regions are very moist. Water vapor imagery has become a very valuable tool for weather analysis and prediction in the last ten years because water vapor imagery shows moisture in the atmosphere, not just cloud patterns. This allows meteorologists to observe large-scale circulation patterns even when clouds are not present.
