Instrument Flying: Technique in Weather Page: 31
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T. O. No. 30-100D-1clouds, the pilot should seek high altitude and low
temperatures if the freezing level is close to the
ground. However, unless the altitude of the cloud top
is known, climbing through an icing condition should
not be attempted, particularly if rain or snow is fall-
ing.
CLOUDS FORMED BY CONTACT COOLING
Whenever warm moist air moves over a colder
ground or water surface, cooling of the air near the
surface will eventually bring about condensation of
the water vapor in the air, forming clouds. The type
of cloud will depend upon the wind velocity, the stabil-
ity of the air, and the presence or absence of precipi-
tation. Stratus is the most common type of cloud
formed by contact cooling. Since warm air over a
cold surface is cooled from below, the lowest air
becomes heavier, increasing the stability. When the
air is cooled below the dew point, condensation oc-
curs and clouds begin to forn. Since cooling pro-
gresses upward from the surface, stratus clouds are
ordinarily low and shallow but they may spread over
a wide area.
If the wind is strong, turbulence produced by fric-
tion will carry the cooling effect upward to a some-
what higher level. The cloud base will then be rela-
tively sharp and definite and the cloud mass as a
whole will tend to become stratocumulus.
Precipitation in the form of rain or snow seldom
falls from stratus clouds. Light rime icing may be
encountered, but it seldom reaches serious pro-
portions except in continued flight without use of de-
icers. Since the cloud is usually shallow, flight can
be made above it without difficulty. Flying in or be-
low stratus clouds is not recommended because the
cloud base is low and even small hills protrude into
it.
EXPECT LIGHT DRIZZLE
Unless the wind is strong the stability of the air
suppresses turbulence. In light winds the base of the
cloud is soft and hazy. It may be quite irregular in
height, frequently varying several hundred feet within
a short distance. -
Stratus clouds with a light wind seldom produce
icing conditions. However, drizzle may occur; and if
the temperature is below freezing, glaze will accumu-
late at a slow rate. Flight should not be prolonged
through freezing drizzle or even in the clouds above.
In the course of a prolonged flight the accumulation of
ice may become heavy enough to prevent the plane from
climbing on top. If an instrument approach is made
through a stratus layer and freezing drizzle is en-
countered, icing of the windshield will usually ob-
struct forward vision. Therefore, whenever a de-
scent is made through a stratus cloud, even though
the temperature on top is above freezing, it is wise
to have the windshield de-icer operating. If there isnone, open the forward window. Do not trust to a side
window, or "stick your neck out." The altimeter, air
speed, and other instruments cannot then be watched;
and they will be needed, particularly if scud clouds
are present, until your wheels are on the ground.
FOG AND SMOKE
Conditions that limit visibility to less than 1/2 mile
present a serious problem only when a landing must
be made or when contact flight must be maintained.
The main causes of low visibility are fog, smoke,
and dust. Several other weather elements, such as
heavy rain or snow, may also reduce the visibility,
but they do not present problems as important as the
more extensive and persistent conditions of low visi-
bility brought about by fog.
Fog is not difficult to forecast if good weather re-
ports are available. Forecasting the exact time of
formation or dissipation is tricky, however, and when
flight operations depend upon the accuracy of a fore-
cast, the advice of an experienced forecaster should
be sought.
Fog is formed by the same basic process which
forms clouds; namely, cooling of the air until its
water vapor condenses. In the case of fog, this cool-
ing must take place next to the ground. The causes
of the cooling determines the type of fog, which in
turn determines the effect upon flight operations.
THE CAUSES OF FOG
There are two principle causes of cooling which
operate near the ground, and hence two main types of
fog. Each has its local peculiarities as to terrain,
time of formation, and duration. Radiation or ground
fog is caused by radiational cooling. Advection fog is
caused by the movement of air from a warm to a cold
surface. Less common forms of fog are up-slope fog,
caused by expansional cooling of air moving upward
over sloping terrain, and frontal fog, caused by the
mingling of warm and cold air at a front and by evap-
oration of warm rain.
RADIATION FOG
Radiation fog is formed by contact cooling of the
air immediately above ground that is being cooled by
radiation. It is, therefore, strictly a night and early
morning condition, since only after sunset will the
ground radiate heat faster than it is received. Radi-
ation fog will not form when the wind exceeds 6
miles per hour. This type of fog occurs most fre-
quently on clear, relatively calm nights.
Cooling of the air by contact with the ground will
be restricted to a shallow layer. Since the cooled
air becomes heavier, it will drain into low places and
along stream beds. The fog resulting will be shallow.
In rough or mountainous terrain it will be restrictedRESTRICTED
Section 5
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31
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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/m1/37/?rotate=180: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting National WASP WWII Museum.