Flight Training - April 2013 - 43

computer models indicated that a derecho
was brewing. By 2:30 p.m. the derecho was
going strong in the Midwest, with 91-mph
gusts in Indiana. The SPC increased the
risk of high winds in the Washington, D.C.,
area; local NWS offices and news media
forecasters began issuing alerts.
The lesson for pilots is this: Earlymorning forecasts aren’t the last word for
the day—especially when thunderstorms
are involved.
SQUALL LINES AND DERECHOS. Any squall
line moves across the countryside with
damaging winds, rain, hail, and sometimes tornados. As the line moves, usually
toward the southeast or east, some storms
die out as new ones sprout and grow.
Most squall lines come and go in a
day, usually starting in the late morning
or afternoon and fizzling out sometime
after dark. Any squall line is dangerous,
with the usual thunderstorm hazards
of extreme turbulence and icing in the
clouds, heavy rain, hail, strong winds
that blast down from the clouds to hit the

ground and spread out for miles, and the
threat of tornados.
At times, however, squall lines such
as the one in June 2012 begin producing
a series of downbursts with 57 mph or
stronger winds hitting the ground at several places along a path that was at least
240 miles long over a period of hours.
This makes it a derecho—pronounced
deh-RAY-cho.
While many weather-wise pilots have
never heard the term, it’s far from new.
Gustavus Hinrichs, a physics professor
at the University of Iowa, introduced the
term in an 1888 scientific journal article.
He decided to use the term derecho to
connote strong, straight-line thunderstorm winds to differentiate such storms
from tornados with their rotating winds.
Derecho is a Spanish word that can mean
“straight ahead.” Hinrichs believed, as
some still do, that the word “tornado”
comes from the Spanish word “tornar,”
which means to turn.
The word derecho fell into disuse and
forecasters ignored it until 1987, when

Robert Johns and William Hirt—meteorologists with the predecessor of today’s
Storm Prediction Center—published
an article in a meteorological journal
describing the term’s history and how it
was used. The article inspired meteorologists to begin using the term again, and
they developed the criteria to separate a
derecho from an ordinary squall line: 57
mph or stronger winds along a path at
least 240 miles long.
KNOW THE THREAT’S NAME. In a way,
for a pilot it doesn’t matter whether he
or she runs into a derecho or the only
severe thunderstorm over Kansas on a
particular day. It’s important to stay well
away from any type of convective weather.
But, knowing that a day’s threat is from
a squall line instead of isolated thunderstorms is important.
If you’re planning a flight across the
Midwest and the forecast is for isolated
thunderstorms, you could be safe taking
off as long as you can be sure you’ll be
able to see any thunderstorm in time

AP IMAGES

KNOWING THAT A DAY'S THREAT IS FROM A SQUALL LINE INSTEAD
OF AN ISOLATED THUNDERSTORM IS IMPORTANT.

A SQUALL LINE
moves in (left) and
brings damaging
winds, rain, hail,
and sometimes
tornados, such as
the case of the
derecho, that
struck Washington,
D.C., in June 2012.

APRIL 2013 FLIGHT TRAINING

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