very large lake such as Superior, or in areas with vast, dense forests. Severe heat
is worsened by open plains, prairies, or steppes, especially if there is not much
vegetation. Large cities are the worst of all.
Most of the time, the interior of a city is somewhat warmer than the sur-
rounding countryside. This happens for two reasons. First, concrete, asphalt, and
other human-made materials do not cool off by evaporation as effectively as
vegetation or soil, because they do not absorb much water to begin with. Second,
there are sources of heat within a city, such as automobiles, electric lights, and
power plants, that are much less concentrated in rural places. In the downtown
area of a large city, the temperature can be several degrees Celsius higher than
in the countryside a few miles away. The highest temperatures are observed near
the center of a city. The heat zone in a city is known as a heat island.
Heat islands are of obvious benefit in extremely cold weather. The expense of
heating during the winter is generally lower in a metropolitan area than in rural
areas, but in the summer during a heat wave, the heat island makes the situation
worse. Because of this, urban people flock to rural areas during heat waves.
EL NIÑO
Global weather patterns are influenced by factors we do not completely under-
stand. Hot and cold spells, fluctuations in precipitation, and frequency of storms
are affected by ocean temperatures. One of the most interesting, noticed for hun-
dreds of years by natives of the eastern equatorial Pacific, is a strange rise in the
water temperature. It occurs at unpredictable intervals, lasting from 1 to 3 years.
This oceanic heat wave, given the Spanish name El Niño (The Child), is associ-
ated with climatic variability around the world.
An El Niño event is actually a combination of atmospheric and oceanic phe-
nomena. Semipermanent areas of atmospheric high and low pressure over the
Pacific reverse their positions, and the ocean temperature in some locations,
especially off the coast of Peru, rises by several degrees Celsius. Atmospheric
winds and ocean currents change course, bringing wet weather in some areas
and drought to others. Floods, triggered by tropical storms and excessive rain-
fall, batter Peru and Ecuador in South America, parts of the west coast of North
America, and the southeastern United States. Dry weather causes heat waves,
dust storms, and crop losses in Mexico, southern Africa, Australia, and Indonesia.
In southern Asia, the seasonal monsoon rains may fail.
Normally, high atmospheric pressure prevails in the eastern Pacific, and low
pressure dominates the western Pacific. This is why the western mountain slopes
of South America are chronically dry, while Indonesia is covered by rain forests.
CHAPTER 8 Abnormal Weather
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