Flight Training - April 2015 - 45

Stable or Unstable?" April 2014
Flight Training.)

Average Number of Thunderstorm Days Per Year

POP-UP. You might hear a television meteorologist refer to the
probability of "pop-up" thunderstorms. Obviously thunderstorms
don't just pop up out of nowhere.
The air needs to be relatively
humid and conditions have to
be right to make the atmosphere
unstable. Meteorologists can predict a day or two in advance when
the atmosphere is likely to be
humid and unstable enough to
KEY
create thunderstorms. They are
Average number of days
with thunderstorms per year
also becoming better at forecastper 10,000 square miles
ing two or even three days in
30-50
Fewer
70-90
advance when conditions will
© 1999 Oklahoma Climatological Survey
than 10
More
be ripe for widespread or fierce
All rights reserved.
50-70
10-30
than 90
thunderstorms. In other words,
most of the time you should have
FLORIDA HAS the highest number of days with thunderstorms in a given year per 10,000 miles.
a good idea the evening before you go
flying whether thunderstorms could be
a concern.
Pop-up thunderstorms refer to those
When the updrafts fade away, leaving
MHz, for en route weather advisories or
that don't form in clusters or lines,
with onboard weather on the instrument only downdrafts, a thunderstorm has
such as along an advancing cold front.
reached its dissipating stage. Even so, a
panel or a tablet.
While forecasters can say a day or two in
fading thunderstorm is still dangerous,
THREE STAGES. All thunderstorms go
advance which parts of the country are
since potentially dangerous downdrafts
through three stages, and all the stages
likely to have thunderstorms, they can't
are possible anywhere around a thunsay exactly where individual storms will should be avoided. A thunderstorm
derstorm-even when it's dying.
begins as a growing, puffy cumulus
form until weather radar detects thunWATCH FOR GROWING CUMULUS
cloud, which forms as humid air rises
derstorms actually forming.
CLOUDS. Turbulence is likely to be fierce
and cools.
If a cold front is heading toward a
before a cumulus cloud produces its first
Pilots should be weather watchers;
region where the atmosphere is primed
lighting stroke, which officially makes
watching from the ground as clouds
for thunderstorms, forecasters can do a
pretty good job of saying when the thun- grow can be instructive, especially when the cloud a thunderstorm.
By the way, lightning by itself poses
the cloud is growing rapidly. Think
derstorms are likely to begin. It's easier
little danger for anyone in an airplane.
about what it would be like inside a
to predict the movement and timing of
relatively large weather feathers such as fast-growing cloud. You can see from far Pilots are more likely to be struck
by lightning while standing outside
away that it is turbulent there.
fronts than it is to predict smaller-scale
performing a preflight inspection. (See
When rain or hail begins falling from
events such as individual thunderstorms.
a growing cumulus cloud, it has reached "Weather: Too Hot to Handle," June
While a forecast made a few hours or
2013 Flight Training.)
its mature phase, which is the most
even a day before a planned flight can
A thunderstorm's strength depends
dangerous of the three phases. The fallgive you some idea of what to expect,
on the temperature difference between
ing precipitation drags air down, which
you need to obtain a preflight briefair rising into the thunderstorm and the
creates downdrafts. Extremely violent
ing shortly before takeoff to ensure a
temperature of the surrounding air at
thunderstorm isn't likely to catch you in turbulence results from currents of air
all altitudes. Patterns of winds aloft also
that are racing up-sometimes faster
the air.
help to determine where thunderstorms
If you're flying away from your airport than 100 mph-adjacent to air that's
will form and how strong they are likely
coming down at roughly half the speed
you need to keep up with the weather
to be.
by checking with Flight Watch, on 122.0 of the updrafts.

APRIL 2015 FLIGHT TRAINING

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Flight Training - April 2015

Table of Contents for the Digital Edition of Flight Training - April 2015