Flight Training - June 2023 - 28

PREFLIGHT WEATHER
BY THOMAS A. HORNE
HIGH PRESSURE'S TRICKS
THAT BIG 'H' ISN'T ALWAYS A GOOD THING
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DURING YOUR PREFLIGHT WEATHER
briefing a briefer might say that high
pressure is in control of the weather
along your route of flight. You look at the
surface analysis chart and see that indeed,
there's a big " H " parked over a huge
section of the country. There's no mention
of airmets or convective sigmets, so you
make the " go " decision to fly.
Yes, big highs usually mean fair
weather, but there are some qualifiers.
They mostly have to do with the nature
and direction of the wind circulation
around areas dominated by high pressure.
We all learned that wind blows in a
clockwise direction around highs (in the
northern hemisphere, that is), and this is
the key to being savvy about the weather
they can make. The location and movement
of a high-pressure system is also
important. Sometimes you hear people
say that there's " no weather " when high
pressure's around. Not so!
Weather systems mainly move from
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west to east, so think about a big high
moving east. At its leading edge-or " front
side " -winds blow from the north or
northwest. Wind and weather are reflections
of their origins, so this means they are
affected by conditions to the north. In this
case, colder temperatures, lower dew points
and humidity, and strong winds are the first
signs that strong high pressure is moving
in. Expect strong winds out of the northerly
points of the compass, and a fair amount
of turbulence. This is especially true if the
high is following a cold front ahead of it.
There have been warmer temperatures
ahead of cold front, and the warmed ground
it created will generate rising air with the
arrival of a high's cold winds. That, and
the shear from a change in wind direction
after frontal passage often means a bumpy
flight-and perhaps a workout during
takeoffs and landings as you correct for
crosswinds and airspeed fluctuations.
When the center of high pressure
moves overhead, expect little or no
wind at the surface. Remember, high
pressure consists of sinking air, so there
is usually less chance of turbulence or
28
FLIGHT TRAINING JUNE 2023
cloud buildups. Clear skies are often the
rule-especially at night when surface
heating drops. Temperatures may drop
low enough to match the dew point and
create visibility-restricting fog in the early
morning hours. During the day, the high's
descending air can create a temperature
inversion that can trap air near the surface,
causing it to stagnate, and trap haze,
smog, and smoke.
After two days or so, the high will
have moved east. Now we're affected
by the " back side " of the high, and the
clockwise flow brings in warm, moist air
out of southerly directions. Many times it
comes in the form of a warm front, as the
warmer air rides up and over the colder,
higher-pressure air it replaces.
The back side of the high can set
the stage for the entry of low-pressure
systems from the west. Their attendant
warm fronts bring widespread cloud
cover as soggy air from the Gulf of Mexico
is drawn northward. As the ground
warms under this influence, cloud
buildups form to create thunderstorms.
Some systems can cover territory across
several states, making diversions difficult
or impossible for VFR operations. The
north-south alignment of most of these
cloud and storm systems means that
flights east or west can be impacted.
Better to make a 180-degree turn before
going there.
Once the warm front and its clouds
move east, colder air follows as the typically
inevitable cold front passes, winds
switch around from the north, and cold
air arrives. The next high-pressure system
makes its entry, and the cycle starts all
over again.
The exception is the Bermuda high,
which is a quasi-stationary high-pressure
system that parks itself over the Atlantic,
off the United States' east coast. Get ready
to see its back side clouds and storminess
regularly affect the weather east of the
Mississippi for most of the summer. Still
think high pressure is no big deal?
tom.horne@aopa.org
http://www.aopa.org/financemytraining
Flight Training - June 2023

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