Flight Training - January 2014 - 44

WEATHER»

As the warm, humid air rises over the
cold air, its humidity condenses into
widespread areas of clouds and precipitation. If it's a winter warm front,
the precipitation can include snow, sleet,
freezing rain, and ordinary rain. The
clouds and precipitation can stretch 300
or more miles from the surface front. The
bad weather can last for a couple of days,
since warm fronts move slower than cold
fronts. Hazards include widespread areas
of poor visibility and low ceilings, and
supercooled water drops in clouds that
can coat aircraft with ice.
STATIONARY FRONTS. As their name
indicates, stationary fronts aren't moving.
Often a stationary front forms when an
advancing cold air stalls and the differences between the air masses are fading
away. Sometimes the front disappears
over a few hours. New storms often form
on stationary fronts.

A stationary front can remain mostly
in place for a few days, maybe briefly
moving toward the north as a warm
front and then back to the south as a
cold front. Surface weather maps depict
stationary fronts as alternating red and
blue lines with red semicircles on the
cold-air side and blue triangles on the
warm-air side.
Upper-level disturbances frequently
travel along stationary fronts, creating
low clouds and precipitation. Stationary fronts can create a few days of poor
flying conditions over wide areas (see
"Weather: Don't Go With the Flow,"
November 2013 Flight Training).
OCCLUDED FRONTS. An occluded front
is an area where warm, cold, and cool
air masses are in conflict and can bring
a combination of the kinds of weather
found in both warm and cold frontal
zones. The image on the previous page

shows a mass of cold air moving into
another mass of cold air, which has
pushed the warm air up off the ground.
The cold air to the left should be called
something like "frigid" air to indicate
that it's colder and thus denser than the
cold air it's replacing at the surface.
The main thing pilots should know
is that when they see a purple line with
alternating purple triangles and half-circles on the same side of the map there's
an occluded front in the area with the
possibility of the most dangerous features of both warm and cold fronts.
Dangerous weather can be found far
away from fronts. But, a front across
your planned route of flight is always a
sign to be alert for potentially dangerous
weather.
Jack Williams is an instrument-rated private pilot.
His latest book is The AMS Weather Book: The Ultimate
Guide to America's Weather.

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Flight Training - January 2014

Table of Contents for the Digital Edition of Flight Training - January 2014
