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Visible satellite images are very helpful for a wide variety of users. In particular, they are helpful in identifying areas of clouds and fog for general aviation pilots who must fly within sight of ground. However, visible satellite images have several major drawbacks, such as at night the visible pictures are black. Another problem is that it is sometimes difficult to distinguish between high clouds and low clouds. While the infrared channel can be used at night, frequently the low clouds and fog are near the temperature of the ground, so low clouds do not show up well on infrared images. Starting with the GOES-8 geostationary satellite series, the 3.9 micron channel has been available. At night the difference between the 3.9 and 11 micron channels detects emissivity differences rather than absolute temperature, so low clouds can be detected at night even if they are at the same temperature as the ground.


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