Flight Training - July 2021 - 26

PREFLIGHT WEATHER
BY THOMAS A. HORNE
ON THE LOOKOUT: COLD FRONTS
SCANNING THE SKY FOR SIGNS OF CHANGE
PILOTS RELY ON TEXT AND GRAPHICAL
forecasts for weather information, but it
helps to cultivate your senses for signs
of change. Like sailors, we need to stay
aware of our surroundings to become
weather savvy. Of all the weather events,
fronts-boundaries between two different
air masses-probably attract the
most attention. Warm fronts occur
when warmer air replaces colder air.
Stationary fronts are, well, stationary,
and don't move much until a new
system comes along.
Cold fronts get a lot of attention
because they can generate plenty of
lifting forces and cause a lot of adverse
weather in a short time. How can you,
standing on a ramp at your airport, sense
that a cold front is approaching?
The first clue will be the wind
direction. It will be out of the southerly
points of the compass, and increasing in
strength. Air is being drawn toward the
cold front's parent low pressure center
somewhere to the north, and now its
direction parallels the approaching cold
front's northeast-southwest orientation.
As the front gets closer, surface winds
will pick up, and reach 20 knots-maybe
more, if the pressure gradient is strong
JARGON
METAR
26
FLIGHT TRAINING JULY 2021
enough. Is the altimeter setting high at
your airport? Say, 30.20 inches of mercury,
and falling rapidly? That's when
you'll see the " PRESFR " (pressure falling
rapidly) note in any nearby METARs,
that's a sign of a big pressure difference,
so the gradient's steep, making strong
surface winds a pretty sure thing. If
there's a barometer in your flight school
office, you'll see that its needle confirms a
drop in pressure.
You'll notice that high temperatures
accompany those prefrontal winds, which
is what you might expect from winds out
of the south. It will be humid, too. You're
in the warm sector of the frontal complex,
and it can get muggier as the cold
front makes its entrance.
A shift in wind direction is a first sign
that the front has arrived. Where once
winds blew out of the south, a check
of the windsock will show it swinging
around to the west or northwest. Now
the front is barging ahead, pushing all
that warm, muggy air ahead of it. The
arrival of cooler temperatures is another
sign that the cold front has arrived. The
colder air wedges beneath the warm air,
creates strong lifting motions. The result:
The rising air cools as it rises, causing
condensation aloft-and plenty of clouds.
If there's enough moisture, count on
rain-and perhaps thunderstorms.
After an hour or two the cold front
will have passed to the east. Surface
temperatures are noticeably cooler, and
as atmospheric pressure rises, gusty westerly
winds could easily reach 20 knots,
with gusts to 30 or more knots. Now's not
the time to practice takeoffs and landings,
unless you have an instructor aboard.
And that barometer in the office? It's
moving to the right with determination-
confirming the front's passage. Want to
see it in writing? You might just see a new
note in the METARs: FROPA, short for
frontal passage. Soon, skies will clear, and
the stage will be set for a few days worth
of good VFR weather. Until the next front
comes along, that is.
No two fronts are alike, but the scene
we just visited is fairly typical of summertime
cold fronts. It's good to know that
the faster a cold front moves, the bigger
the temperature and pressure differences
across the front, and the higher the
prefrontal moisture ahead of it, the more
violent the weather.
tom.horne@aopa.org
A METAR is an aviation routine weather report. The observation
of current surface weather is reported in a standard international
format, regulated by the International Civil Aviation Organization
(ICAO). Weather stations around the world report weather
conditions every hour.
Flight Training - July 2021

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