AOPA Pilot Magazine - August 2019 - 87

P&E WX WATCH

Satellite
scanning
The view from orbit
BY THOMAS A. HORNE

IF YOU'RE LIKE ME, the first step in your
preflight briefing is to check the satellite
imagery. It's often the most intuitive snapshot of recent weather, it updates every 20
minutes or so, and it makes seeing trends
in cloud cover easy-even at night. It can
even be looped if you visit the Satellite page
on the Observations drop-down menu of
the Aviation Weather Center's website
(aviationweather.gov). The imagery comes
from the two Geostationary Operational
Environment Satellites (GOES): GOESEast, parked at 75 degrees west longitude,
and GOES-West, at 137 degrees west longitude. From their perches some 22,300 miles
above the Earth's surface in geostationary
orbits, these satellites use radiometers to
measure the radiation emanating from the
clouds and Earth below. The wavelengths
they sense show the visible, infrared, and
water vapor spectrums. Each spectrum has
its strengths and weaknesses, which is why
it's useful to compare imagery for all three
when scouting out the weather.
Visible. The visible imagery channel
measures the reflected light, or albedo, in
the same wavelengths as those used by the
human eye-about 0.5 microns. The higher
the reflective power of clouds, the brighter
they will appear. This makes it easy to spot
the highly reflective tops of a thunderstorm's
cumulonimbus tops. Less dense clouds
don't reflective as much light, so they're less
bright. The Earth's surface doesn't reflect
much light at all, unless it's covered in snow.
Imagery in the visible channel has great resolution, so with a little experience it's fairly
easy to learn the signatures of different cloud
formations-the thin, parallel rows of wave
clouds downwind from a mountain range,
the widespread small puffy clouds of fairweather cumulus, the dark "dry slot" that
marks relatively cloudless post-cold frontal weather, or the distinctly wispy look of a
shield of cirrus clouds ahead of a warm front.
The contours of a supercell thunderstorm's
IMAGES FROM MAY 29, 2019, a day that

spawned tornados in the mid-Atlantic.
Water vapor imagery (top) shows high
moisture content in cloud tops. Visible
imagery (center) gives more detail of
the weather system's cloud composition,
extent, and structure. Black-and-white
infrared imagery (bottom) renders the
highest cloud tops in the brightest shades.

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AOPA Pilot Magazine - August 2019

Table of Contents for the Digital Edition of AOPA Pilot Magazine - August 2019