SIGNS OF A STORM
Even without a barometer or a hygrometer (a device for measuring relative humid-
ity), you can often tell when a storm system is approaching. The wind changes direc-
tion. In the tropics of the northern hemisphere, the prevailing east wind turns and
blows from a northerly quarter. At northern temperate latitudes, the prevailing west
wind shifts counterclockwise to southwest, then south or southeast; this is called a
backing wind. In the southern hemisphere, the wind also shifts, but in the opposite
direction; an east wind becomes southerly, and a west wind becomes northerly.
An approaching storm is commonly preceded by rising temperature and humid-
ity, but not always. A storm coming up the Atlantic coast towards New England will
produce an easterly wind off the sea, raising the dewpoint temperature while the
barometric pressure falls. The same happens in the Midwest when winds ahead
of a storm system blow from the south, bringing moisture from the Gulf of Mexico
into the continental interior. However, a hurricane approaching the east coast of
Florida from the east may bring dry air in ahead of itself, as backing winds (from
easterly to northerly) “pull air down” from the cool North Atlantic, where the air,
because of its lower temperature, is capable of holding less moisture. This drives
the relative humidity down. It can also produce an unusually clear sky and balmy,
pleasant weather, which lulls inexperienced people into complacency. This phe-
nomenon has been called “the calm before the storm.” Anyone who has spent a
number of years in hurricane-prone regions knows better than to be deceived by this!
OTHER INSTRUMENTS
Meteorologists have sophisticated apparatus for forecasting the weather. Satellites
orbit the earth, their cameras scanning the surface for weather patterns. Large sys-
tems, which can harbor thundershowers, tornadoes, high winds, heavy snows, or
other adverse conditions, have a characteristic signature that the meteorologist
recognizes immediately.
As a storm system approaches a weather station, the meteorologist can look
at it with a radar set. Large balloons, equipped with instruments to measure tem-
perature, humidity, pressure, wind direction, and wind speed, can be sent aloft to
detect atmospheric changes that indicate approaching or developing weather
systems. Aircraft pilots can fly their aircraft near (or even into) a storm and see
for themselves how bad it is. Meteorologists all over the region, the country, and
the world share information, and from this combined effort intricate diagrams
are made, showing the locations of air currents, temperature regions, pressure
zones, and other parameters.
CHAPTER 2 The Atmosphere
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