Flight Training - July 2015 - 45

MESOSCALE WEATHER VARIES. While
many pilots haven't heard the term
"mesoscale weather," knowing at least
a little about it could help them avoid
trouble. Middle-sized weather systems
can make life difficult for pilots, especially
from May into August from the Rockies
to the Appalachians-and in some parts of
the world outside the United States.
The American Meteorological Society's
Glossary of Meteorology defines mesoscale weather as "A cloud system that
occurs in connection with an ensemble of
thunderstorms and produces a contiguous precipitation area on the order of 100
km or more in horizontal scale in at least
one direction." The word "convection"
tells you that thunderstorms are involved,
and "system" indicates that the thunderstorms are organized, not just a random
collection.
Squall lines are only one form of mesoscale weather. They have worried pilots
since the early 1920s, when airplanes
began regularly flying over the horizon
and meteorologists started taking their
first, stumbling steps toward aviation
forecasts. Today, most pilot weather education includes squall lines (see "Weather:
A Line of Trouble," May 2015 Flight
Training).
Other kinds of mesoscale convective systems include polar lows, small
but intense storms over polar oceans;
lake-effect snow; tropical cyclones, such
as hurricanes; and mesoscale convective
complexes (MCCs), which are common
during warm weather on North America's
Great Plains-but sometimes move east
from there.
MCC HISTORY. By the 1970s, atmospheric
scientists using improved weather radars
and satellites were learning in more detail
about weather features that had always
been around, but weren't well understood. The discovery of MCCs is a perfect
example.
In the 1960s and early 1970s, Robert Maddox was an Air Force weather
officer who, along with other forecasters,
struggled to predict the large areas of
overnight thunderstorms that often form

after sunset over the central and northern
ARE YOU WEATHER WISE?
The AOPA Air Safety Institute offers five
different online safety courses to help you
become "Weather Wise":
* Weather Wise: Air Masses and Fronts
* Weather Wise: Ceilings and Visibility
* Weather Wise: Precipitation and Icing
* Weather Wise: VFR into IMC
* Weather Wise: Thunderstorms and ATC
www.aopa.org/Education

Plains during warm weather. After he
left the Air Force and was working at the
NOAA Laboratory in Boulder, Colorado-
and studying for a doctorate at Colorado
State University in Fort Collins-weather
satellite improvements were showing the
temperatures of the cloud tops, which
are a good measure of cloud heights. The
colder the cloud tops, the higher they are.
Until Maddox began intensely studying
satellite images from over the Plains on
rainy nights, most forecasters had thought
the storms were random.
"The thing that struck me was the
coherent structure of the [cloud top]
temperatures," Maddox says. "It wasn't

a cold blob here, a cold blob there. What
really astounded me is how frequently
[organized systems of thunderstorms]
occurred."
In November 1980 he published a study
in the Bulletin of the American Meteorological Society that described how he
had "identified, defined, and contrasted
with other types of convective weather
systems" a system he called mesoscale
convective complexes. Maddox and other
meteorologists also used the then-new
Doppler weather radars and other data
to work out the picture of what's going on
in a MCC.
By definition, a MCC is a nearly circular cluster of thunderstorms with an area
of cloud top temperatures colder than
minus 26 degrees Fahrenheit that covers
an area of about 39,000 square miles-
roughly the size of Ohio. They usually
begin forming from disorganized clusters
of thunderstorms in the late afternoon,
and reach their peak in the middle of the
night. Low-level jet streams from the
south feed warm, very humid air into the
complex, supplying the energy that keeps
the thunderstorms going.
MCCs normally die down sometime
around dawn, when the low-level jet

WHEN LOW-LEVEL JETS die in the early morning, the rain ends and clouds begin to evaporate. The
mesoscale vorticity center continues to rotate as it moves east, where it can help a new MCC form the next
evening. (Illustration from The AMS Weather Book by Jack Williams. Used with permission of the American
Meteorological Society.)

JULY 2015 FLIGHT TRAINING

/ 45

http://www.aopa.org/Education

Flight Training - July 2015

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