Instrument Flying: Technique in Weather Page: 33
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T. O. No. 30-100D-1
SHOWERS AND SQUALLSWhere rain or snow is falling, ice accretion will
almost always be found at some level. Where heavy
rain is occurring, and particularly in showers, turbu-
lence will also be found at the level at which the pre-
cipitation is originating. This is because the physical
process which forms rain requires the presence of
ice crystals and water droplets at subfreezing tem-
peratures in the same region of a cloud. These con-
ditions also favor ice accretion. To form heavy rain,
extensive lifting of the air is necessary, implying the
presence of strong vertical currents and turbulence.
However, it would be a mistake to assume that icing
and turbulence are not present simply because pre-
cipitation is not occurring. Both icing and turbulence
frequently occur without rain or snow or any apparent
threat of them.
Showers and squalls, because they are relatively
small and usually surrounded by clear air, are too
often regarded lightly by inexperienced pilots. This
is a dangerous attitude, and the history of aviation
records its disastrous results. Some of the most
severe turbulence and worst icing conditions are en-
countered in small, localized squalls.
WHYS OF SHOWERS AND SQUALLS
The distinction usually made between a shower and
a squall is afine one,based upon the absence or pres-
ence of strong, gusty surface winds and the intensity
of precipitation. These are characteristics which can
best be observed from the ground, but they are likely
to be meaningless for the pilot. Showers and squalls
will, therefore, be discussed together in this section.
Both showers and squalls occur in conditionally
unstable air; that is, in air which .is stable when un-
saturated but becomes unstable when saturation oc-
curs. Either type of storm may be produced by sur-
face heating or released by such localized upward
displacement of air as occurs over a mountain ridge.
Except for a difference in intensity, these two typesof storm present similar flight conditions and require
the same flying technique.
The intensity of a local storm will be governed by
the amount of moisture available and by the amount of
energy released when this moisture condenses. The
amount of energy released will be large when a steep
lapse rate, through a comparatively deep layer of air,
indicates that air can rise unstably for a considerable
distance. The production of cloud and precipitation
will proceed rapidly under such conditions, and the
resulting weather is described as a squall. A layer
of unstable air at least 10,000 feet thick is usually
necessary before a heavy squall can be produced.
If upper air observations of temperature and hu-
midity are available, the forecaster can usually pre-
dict the occurrence and intensity of squalls or show-
ers by noting the steepness of the lapse rate and the
amount of water vapor available to provide energy
for the storm.
IT'S A QUESTION OF TEMPERATURE
Unstable rising air currents may be started through
heating of the surface air by contact with sun-warmed
ground, or by upward displacement of a portion of the
air. When both factors are active, as on a hot day
over the barren mountainous terrain from central
Texas westward, violent squalls may be produced.
Such storms often remain stationary over a given
hill or peak throughout the period of their activity.
The slope of most frontal surfaces is too slight
to provide the upward thrust necessary to initiate the
formation of squalls; but cold fronts, having relatively
steep slope and high speed, form squalls more readily
then warm fronts. Showers are more common along
warm fronts.
One physical difference between a squall and a
shower is in the vertical distance between the baseRESTRICTED
Section 6
33
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Army Air Forces. Instrument Flying: Technique in Weather, book, January 1, 1944; Ashland, Ohio. (https://texashistory.unt.edu/ark:/67531/metapth873973/m1/39/?q=+date%3A1941-1945: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting National WASP WWII Museum.