begin to build up during the late morning or early afternoon, however, you
should take your umbrella if you plan to be outside later. Gradually rising and
thickening cumulus do not usually portend severe weather, although heavy rains
sometimes occur. This kind of weather pattern is typical in the tropics and sub-
tropics during much of the year, and in the temperate latitudes in the summer.
A distant, but advancing, storm system causes increasing instability in the
upper troposphere. A clear, deep-blue sky, in which airplanes produce no con-
trails or contrails that die out quickly, indicates stability in the upper tropo-
sphere, and the continuance of fair weather for a day or two. The first noticeable
sign of an approaching low-pressure system is the tendency for aircraft to pro-
duce long contrails that span the sky from horizon to horizon.
The upper troposphere gets increasingly susceptible to cloud formation as water
vapor condenses more readily, and this tends to happen in advance of an approach-
ing storm system. As the system moves nearer, high cirrus clouds appear. They
get thicker until the sky is filled with high clouds. At temperate latitudes, the
clouds thicken toward the western horizon. The movement of the clouds is gen-
erally from west to east. In the tropics, clouds thicken toward the east; the cloud
movement is usually from some point in the eastern half of the compass toward
some point in the western half. As the atmosphere becomes more unstable, cumu-
lus congestus clouds begin to form. The wind shifts, backing several compass
points. In the temperate latitudes, the prevailing westerlies give way to breezes
that come from the direction of the equator. In the tropics, the easterly trade winds
shift, become gusty, and blow from the direction of the pole. Eventually, rain or
snow begins to fall. At temperate latitudes, the temperature can rise several
degrees Celsius over a period of a few hours. Then the sky lightens, the rain or
snow abates, and moderate weather prevails. If the storm system is intense,
however, these pleasant conditions do not last long. In the tropics, conditions are
difficult to predict without satellite information. The approaching system might
be nothing more than a weak tropical wave, but it might be a hurricane!
For those who live in the latitudes where frontal cyclones prevail, the
sequence of events just described indicates the approach and passage of a warm
front. When a warm front has moved past an area in the temperate zone, a cold
front often follows. The cold front causes large cumulonimbus clouds to form in
the summer. In the winter, the wind veers sharply and increases in speed, some-
times to gale force. If snow has fallen or already exists on the surface, it blows
in a blinding white sheet. At any particular surface location affected by the front,
the temperature falls suddenly and dramatically as the front arrives and the cold
air mass behind it takes over.
As a cyclone approaches in the tropics or subtropics, changes in temperature
are not likely to be significant. The sky gets alternately gloomy and bright. If the
CHAPTER 2 The Atmosphere
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