AOPA Pilot Magazine - February 2014 - 85

P&E

WX WATCH

Wind wise
Get ready to batten down the hatches!
BY THOMAS A. HORNE

WELL, WE'RE COMING OUT of the
depths of winter, and staring March
in the face. For pilots-and, well, just
about everybody else-thoughts now
turn to the strong winds so characteristic of the colder months, and
especially the transition from winter
to spring. What do we need to know to
be wind wise about this time of year?
UP HIGH

It all starts with the band of strong
high-altitude winds (typically above
30,000 feet, although low-level conditions can occur as low as 5,000 feet!)
that encircle the globe. This is the
jet stream, and its influence on the
weather below cannot be overemphasized. The jet stream is more than just
a river of fast-moving air. It's the delineation between the colder, Arctic air
masses to its north and the comparatively warmer air masses to its south.
Don't believe it? Look at a 240-millibar constant pressure chart for the
winds at about FL340. Then compare
it with a chart showing surface high
temperatures. The jet stream boundary will be quite apparent.
Jet streams-along with their
cores of strongest winds, often called
jet streaks-bulge southwards, creating
low pressure troughs. These troughs
bring huge pockets of colder air southwards. This means a strong potential
for icing conditions, to be sure. But
the farther south the troughs travel,
the greater the temperature contrasts
between the air on either side of the
jet stream boundary. Because temperature differences create pressure
differences, jet streams create strong
winds-often all the way to the surface during the colder months.

Jet streaks also create lifting
forces that can penetrate to the surface. Lifting means lower barometric
pressures and cooling, cooling means
condensation, and that means precipitation and instrument weather-as
well as the surface lows and fronts that
create them. That's true any time of
year. In the late weeks of winter, as the
sun's rays begin to strike the northern
hemisphere more directly, the areas
to the south of the jet stream and its
streaks increase in temperature. Now
the contrasts across the jet stream
boundary increase as well. Winds pick
up at altitude (150 knots, anyone?) and
at the surface as well (check the area
forecasts for the "WIND" note that
indicates sustained surface winds or
gusts of 20 knots or more).
IN THE TEENS

Strong winds in the middle altitudes (say, 12,000 to 24,000 feet)
have an equally big say in adverse flying weather. As with winds at the jet
stream level, any horizontal change in
wind speed above 20 knots across a
distance of 100 nm can be taken as a
warning sign of turbulence.
Changes in wind direction with
altitude are big factors in the formation of supercell thunderstorms,
which also begin to rear their heads in
March and April. If winds are backing
(moving in a counterclockwise direction) with altitude, this can set up the
rotation necessary for creating mesocyclones-mini-lows that can create
tornadic conditions.

THE CONVECTIVE SIGMETS (top) are the direct result

of the low pressure system moving into the midwest
(center), which is in turn a result of the jet stream's
trough above the same area (above).

DOWN LOW

Propelled by the dynamics above
them, surface winds associated with
www.aopa.org/pilot AOPA PILOT | 85
http://www.aopa.org/pilot
AOPA Pilot Magazine - February 2014

Table of Contents for the Digital Edition of AOPA Pilot Magazine - February 2014
