Flight Training - January 2014 - 43

is more common than arctic air masses.
Unlike arctic air masses, these form during
summer.
Maritime polar: Cool and humid; forms
over the northern Atlantic and Pacific
oceans.
Maritime tropical: Warm and very
humid; forms over the Gulf of Mexico and
the Atlantic Ocean south and east of the
Gulf Stream.
Continental tropical: Hot and very dry;
forms over the U.S. Southwest and northern Mexico deserts during summer.
MOVING AIR MASSES CREATE FRONTS.

When an air mass-for example, cold, dry
air from northern Canada-moves south, it
eventually runs into warmer, often morehumid air that's less dense. Instead of
merging with the warm air, the dense, cold
air mass shoves under the lighter warm
air mass, pushing up the lighter air. When
warm air is on the move, it rides over
heavier cold air.
In both cases, as warm, humid air rises,
it cools and its moisture begins condensing
to form clouds and precipitation. The kind
of front determines the kinds of clouds
that form and precipitation that falls.
The fronts we're most interested in
and the ones shown on weather maps
are where air masses meet at the Earth's
surface. Air masses are domes of highpressure air with the highest pressures
in the centers of the air masses. Fronts-
where air masses meet-are elongated
areas of low pressure known as troughs.
(On weather maps you usually see
"trough" spelled "trof.")
Let's look at the different kinds of fronts.
Each of the images at right shows a crosssection of a kind of front with locations
on the ground that give you an idea of
the extent of potential weather generally
associated with particular fronts.

On weather maps, cold fronts are
shown as a blue line with triangles on
one side, pointing in the direction of the
front's movement. Weather map lines
for all types of fronts show the front's
surface location, not what could be
happening aloft.
Often the clouds and precipitation with
a cold front are confined to a few miles
ahead of and behind the location of a
front on a map, but sometimes you'll find
clouds and rain behind a cold front.
The violence a cold front produces
depends in part on the temperature
contrast between the air masses ahead
of and behind the front. The greater
the differences, the higher the odds of

violent weather. This is one reason why
cold-front thunderstorms often are most
violent in the spring when the contrasts
are the greatest.
WARM FRONTS. Warm fronts, especially
in the winter, can bring some of the year's
worst weather for those on the ground
and in the air. As warm air advances, it
rides over the cold air since it's less dense.
Warm air is replacing the cold air at the
ground where you see a warm-front
symbol on a weather map-a red line with
red half-circles indicating the direction
of movement. The half-circles are on the
side of the front where it's moving into
the cold air.

OCCLUDED FRONT

COLD FRONT

WARM FRONT

COLD FRONTS. Cold fronts are usually the
most dangerous to pilots because they normally cause thunderstorms. Cold air that's
charging across the region can give warm,
humid air a vigorous shove upward. If
conditions are right, this can lead to strong
thunderstorms along the front.

JANUARY 2014 FLIGHT TRAINING

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Flight Training - January 2014

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