AOPA Pilot Magazine - December 2013 - 80

P&E

WX WATCH

completely out of luck. The picture will be
totally dark.
INFRARED

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The infrared channel is keyed to temperature. The colder the clouds, the brighter
the returns, which are shown with color
contours to help identify the coldest cloud
tops. Colder surfaces are given the brightest colors, and warmer surfaces are given
darker colors.
The good: Because temperatures
decrease with altitude, we can infer that
the highest clouds and the most severe
thunderstorms will have the coldest tops.
These are easily identified on an infrared
image. In the absence of visible imagery at
night, infrared is a fairly good stand-in.
The not-so-good: If cold clouds happen
to be in thin layers, and Earth's surface is
cold, then some of that terrestrial radiation
can pass through those thin cloud layers.
The satellite averages out the radiation
from the cloud tops and the ground, and
the result is a picture that can show cloud
tops warmer and lower than they really are.
And while infrared is nice to have at night,
it does a lousy job of detecting low clouds
and fog because the radiating temperatures
of any low clouds can be the same as those
of the nearby ground.
WATER VAPOR

The water vapor channel detects the emission and absorption of Earth's emitted
radiation by water vapor-which, to get
technical about it, happens right around
the 6.7-micron wavelength. The channel
shows water vapor in the atmosphere with
shades of white or gray; drier air appears
as black areas.
The good: Day or night, the water
vapor channel shows moisture distribution patterns associated with fronts and
low pressure. In animated views, it's easy
to see the progress of large-scale weather
events, so water vapor imagery is helpful in
observing and forecasting them.
The not-so-good: Because the satellite
looks for water vapor's quite narrow wavelength, any high-altitude concentrations
will block out the signatures of any lowerlevel concentrations-or clouds, for that
80  |  AOPA PILOT  December 2013

SAME DAY (October 15, 2013), different

analyses. Infrared (top) view shows high
cloud tops over the Great Lakes. Visible
shot (middle) shows the extent of lower,
thinner cloud layers. Surface analysis chart
(above) shows how fronts correlate with
satellite imagery.

matter. You could have a situation where
a black zone of drier air appears on water
vapor imagery, but an infrared or visible
view shows clouds in the same area. Yes,
thanks to gravity, water vapor concentrations drop with altitude, but it's still there
nonetheless-just in lower concentrations.
For this reason, water vapor imagery is best
at depicting water vapor in the middle altitudes, say from 10,000 to 25,000 feet.
Playing on each image's strengths, it's
best to use satellite views in combination.
Use visible imagery to determine if clouds
are thick or thin. Use infrared to tell how
high they are. Use water vapor to watch for
system movements and the rise and fall of
moisture inflows and outflows.
AOPA
EMAIL tom.horne@aopa.org
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AOPA Pilot Magazine - December 2013

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