Flight Training - November 2013 - 43

or a mixture of rain, snow, and ice. The
darker green area shows where heavier
precipitation is falling.
A STORM IS BORN. Extratropical storms
often form along stationary fronts—where
a kink or wave forms along the front, often
with the help of an upper-air disturbance
that encourages air to rise from the surface. As air rises, some of the surrounding
air flows in to replace it. Earth’s rotation
causes this moving air to begin turning
in a counter-clockwise direction in the
Northern Hemisphere; the opposite way
south of the equator. As air flows into the
low-pressure center, it rises, and more air
flows in to replace it.
Figure 2 (right) is a three-dimensional
view showing how an extratropical
cyclone begins forming as the warm air on
the right flows over the heavier cold air. At
the same time the cold air begins to wedge
under the warm air, lifting it.
As all of this is going on, the entire
system is moving across the countryside
with the atmosphere’s general west-toeast flow.
At its height an extratropical cyclone
has distinct cold and warm fronts, which
can make the storm quite large. If the
storm’s center is over the Midwest, the
cold front can stretch south across Kentucky, western Tennessee, and Mississippi
to the Gulf of Mexico. Strong thunderstorms can occur in the South while snow
showers, followed by bitter cold air, are
falling on the Midwest. Figure 3 represents a storm at this stage.
The warm front can stretch eastward
to the Appalachians with widespread
low clouds, rain, freezing rain, sleet, and
snow stretching to the north across the
Great Lakes.
Most of the time an extratropical
cyclone’s most violent surface weather
occurs along the cold front, but the warm
front often causes a widespread area of
weather with solid clouds with a widespread area of minimum visibilities.
An extratropical cyclone reaches its
final phase and begins to fade away when
an occluded front forms. Figure 4 shows
an occluded front with the violet line with

AT ITS HEIGHT, AN
EXTRATROPICAL CYCLONE
HAS DISTINCT COLD AND
WARM FRONTS, WHICH CAN
MAKE THE STORM QUITE
LARGE.

FIGURE 2

triangles and half circles between the cold
front and the storm’s center.
MORE TO STORMS THAN YOU MIGHT
THINK. Unless you take college-level
courses in meteorology, you could end up
thinking that a storm’s fronts present the
greatest concern for pilots.
This is because the FAA’s sources of
training materials, such as the “Weather
Theory” chapter in The Pilot’s Handbook
of Aviation Knowledge, focus on the fronts
with little or no mention of other parts of
extratropical cyclones.
An ordinary weather briefing would
tell you that widespread clouds are
occurring and expected to continue over
the large area north of a storm’s center,
and you might think it should be easy to
handle, especially if you have an instrument rating.
Don’t count on it. The generally
counterclockwise winds around the low
pressure center bring warm, humid air
over the warm front and wrap it around to
the north side of the storm’s center. Atmospheric sciences refer to this air motion
as a “warm conveyor belt.” Like a warm
front, the air here is warm only in relation
to its surroundings, and the air could be
below freezing with enough humidity to
deposit thick ice on any aircraft that flies
into this part of the system.
When an extratropical cyclone
approaches, meteorologists might be
looking forward to a couple of days of
“interesting” weather. On the other hand,
the last thing a pilot wants to hear from a
weather briefer is, “Looks like you’ll have
some interesting weather.”

WARM AIR on the right flows over heavier cold air as
the cyclone begins to form.

FIGURE 3

DISTINCT cold and warm fronts present a variety of
weather over a large area.

FIGURE 4

THE FINAL PHASE of an extratropical cyclone commences when an occluded front forms between the
cold front and the storm's center.

Jack Williams is an instrument-rated private pilot.
His latest book is The AMS Weather Book: The Ultimate
Guide to America’s Weather.

NOVEMBER 2013 FLIGHT TRAINING

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Flight Training - November 2013

Table of Contents for the Digital Edition of Flight Training - November 2013