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ADDS - Satellite Help Page (3 of 3)
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More details in plain English, please:
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The visible images display the earth very similar to how humans
see it with their eyes or how typical cameras view it. Clouds
and snow appear bright white (high albedo/reflectance) but oceans
and trees are much dimmer.
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The infrared images display the earth in a manner that correlates
with temperature. Generally speaking, the warmer an object, the
more infrared energy it emits (gives off). The satellite sensor
measures this energy and calibrates it to temperature using a
very simple physical relationship (Planck's Law). In the real
world, clouds that are very high in the atmosphere are generally
quite cold (perhaps -50°C) whereas clouds that are very near the
earth's surface can be quite warm (perhaps +5°C). Likewise,
the land may be even warmer than the lower clouds (perhaps +20°C).
Those colder clouds emit much less infrared energy than the
warmer clouds and the land emits more than those warm clouds.
The data measured by the satellite are calibrated and colorized
according to the temperature with red shades representing higher
(warmer) temperatures and blue shades representing lower
(cooler) temperatures. If the temperature of the atmosphere
decreases with height (which is typical), a user can get an idea
of which clouds are high-level and which are low-level based
on the cloud top temperature.
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The water vapor images display the earth in a manner that
correlates to quantity of water vapor in the upper portions of
the atmosphere (25,000 feet and higher in general). The actual
numbers displayed on the water vapor images correspond to
temperature but there is no direct relationship between these
values and the temperatures of clouds (as is the case for infrared
images) since this channel doesn't really "see" clouds but "sees"
high-level water vapor instead. The most useful tidbit to be
gained from the water vapor images is the locations of storm
systems and the jet stream. Another useful tidbit is aided by
the color scale used on the images. In general, regions displayed
in shades of red are VERY dry in the upper atmosphere and MAY
correlate to crisp blue skies from a ground perspective. On the
contrary, regions displayed in shades of blue or green are
indicative of lots of high-level moisture and may also indicate
cloudiness. This cloudiness could simply be high-level cirrus
types or serious storms. That determination cannot be gained
from this image by itself but could easily be determined when
used in conjunction with the other channels.
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