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TYPES OF IMAGERY

Today’s weather satellites are equipped with sensors capable of providing us with visual and infrared pictures. This dual sensing capability permits picture-taking both day and night. It also permits imagery switching from one picture to the next in order to compare the IR and visual pictures of the same region. You will note as you look at compared pictures that there can be quite a bit of contrast between visual and IR pictures of the same features. Therefore, to cor-rectly analyze satellite pictures, you must have an understanding of the various types of imagery.

Visual Imagery

Visual satellite photos look very much like standard black-and-white photographs. Actually, they are constructed line by line from digi-tal information gathered by the satellite sensor. The pictures are produced in black and white and various shades of gray. They are a measure of the Sun’s reflection off clouds, land, and sea surfaces. The amount of re-flectivity is dependent, on (1) the position of the satellite in relation to the Sun, (2) the reflective power of the surfaces being scanned by the camera, and (3) to some extent, moisture. With regard to a satellite’s relative position to the Sun, early morning and late evening visual pictures sometimes show up partially or totally black because the Sun is not high enough above the horizon in relation to the area of Earth being scanned by the camera.

Figure 10-1-1.—Partial picture owing to darkness (sunset).

Figure 10-1-1 is an example of such an occur-rence. There is simply not enough sunlight over the eastern portion of North America to permit a picture of this area. The reflective power of a surface or body (albedo) is a factor that makes features highly visible or practically invisible. Surfaces that are highly reflective (high albedos) show up bright white in pictures, while surfaces with low albedos appear dark. Table 10-1-1 lists the reflective power of some features. The last factor affecting reflectivity is moisture. Drier areas of Earth, such as deserts, reflect more sunlight than moist areas and in comparison appear brighter.

Table 10-1-1.—Reflectivity of Various Surfaces

Figure 10-1-2.—Unenhanced IR picture.

In addition to the required sunlight, there are other factors that control the appearance of features in visual imagery. The height of the satellite above Earth controls the area of coverage and the size of the features seen in imagery; the higher the satellite, the greater the coverage and the smaller the features. Camera resolution controls imagery detail; the smaller the resolution, the greater (finer) the detail. The focal point of the camera system defines the most useable area of the picture; features at the edges of pictures are not as easily interpreted as those near the picture’s center (the focal point).

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