1959 US Army Vietnam War Terrain Intelligence ... - Survival Books

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53. Fog
a. Fog is defined as a mass of minute water droplets suspended
in the atmosphere at the surface of the earth. It is formed by the
condensation of water vapor in the air.
b. The most favorable conditions for the formation of fog are an
abundance of water vapor, high relative humidity, and a light surface
wind. A light wind tends to thicken fog. Increasing wind
speeds will cause fog to lift or to dissipate.
c. Fog usually is more prevalent in coastal areas than inland because
there is more water vapor in the atmosphere. Inland fogs may
be very persistent in industrial regions. In most areas of the world,
fog occurs more frequently during the colder seasons of the year than
it does in the warmer seasons.
54. Storms
a. Thunderstormts.
(1) A thunderstorm is a local storm accompanied by lightning,
thunder, strong gusts of wind, heavy rain, and sometimes
hail, usually lasting for no more than an hour or two. In
most cases, a thunderstorm is characterized by turbulence,
sustained updrafts and downdrafts, precipitation, and lightning.
(2) When a thunderstorm reaches its mature stage and the rain
begins, a downdraft starts in the lower and middle levels of
the storm. This large body of descending air causes strong,
gusty surface winds that move out ahead of the main storm
area, often resulting in a radical, abrupt change in wind speed
and direction termed the first gust. In general, the strongest
thunderstorm winds occur on the forward side of the storm
where the downdraft first reaches the surface. The speed of
a thunderstorm wind may reach 50 to 75 miles per hour for a
short time.
b. Tornadoes. Tornadoes are circular whirlpools of air which
range in size from about 100 feet to one-half mile in diameter. A tornado
appears as a rotating funnel-shaped cloud extending toward the
ground from the base of a thundercloud. The low pressure and the
high wind speeds encountered in the center of the tornado are very
destructive. The paths of tornadoes over the ground usually are only
a few miles long and the tornadoes move at speeds of 25 to 55 miles per
hour. Although the maximum wind speeds associated with tornadoes
never have been measured directly, property damage and other effects
indicate thal; they may exceed 500 miles per hour. When they occur
over water, tornadoes are termed waterspouts.
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