Visible/Fog
The vis/fog satellite images are generated from visible
geostationary satellite images during local daylight hours, and from a
derived "fog" image at night that emphasizes the low clouds
(Note the saw-tooth boundary between the different images).
During the day the visible image is brightness normalized by
dividing by the cosine of the solar zenith angle. This removes most of
the image dependence on sun angle, such as brightness changes at sun
rise and sun set and differences between winter and summer image
brightness. The visible brightness depends primarily on the thickness
of the cloud. The visible image is used when the sun is higher than 3
degrees above the horizon.
At night there are no visible images, so a derived "fog"
image is substituted for the visible. The "fog" image is
generated from the temperature difference between the 3.7 micron
images and the 11 micron infrared images. The temperature difference
depends primarily on emissivity differences caused by different
physical characteristics of the radiating surfaces. The brightness of
the image is set up so that low clouds are white, ground is gray, high
clouds are black, and very high cold thunderstorm tops are a salt and
pepper black and white. The "fog" image is not very
sensitive to the temperature of the low clouds except for extremely
cold surface temperatures below -40 degrees, when it starts to show
some of the salt and pepper appearance. The ground generally shows up
as a gray color, except for a few areas in the west, such as the delta
of the Colorado River which shows up white because of the soil
emissivity there. The white "fog" low clouds start to show
up on the images when the low clouds are wider than 2 miles, and are
thicker than about a hundred feet. Hence it will generally pick up
widespread IFR condition, but may not show small, thin local fog or
haze conditions.
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