| What Is a Desert? |
| Some sources define a desert as an area receiving no more than ten inches
of precipitation annually. However, many areas receiving this amount of
precipitation are not deserts. This simple definition is not complete. Both
the timing and type of precipitation determine the environment established.
In a desert, rain isn’t evenly distributed throughout the year. |
|
| Weather patterns often create short, violent downpours. Flash floods,
characteristic of some desert areas, are produced. Much of the water runs
off before it can soak into the soil. A lot of moisture is also lost to
evaporation. Many deserts lie in areas of high pressure systems where there
is little cloud cover. At least 90 percent of the sun’s rays reaches
Earth’s surface, producing seasonal hot temperatures. (For comparison,
the surface of more humid lands, covered with more vegetation, receives
only 40 percent of possible solar radiation.) The hot, dry air causes any
available water to evaporate quickly. |
| When temperatures are extremely hot, rain can evaporate before it reaches
Earth. The conditions producing high daytime temperatures reverse the process
after sundown. Approximately 90 percent of the day’s accumulated heat
radiates back toward the sky. In moister climates only about 50 percent
of this heat is lost. These conditions produce the wide range of daily temperatures
characteristic of deserts. This range is often 50 degrees or more. The rapid
heating and cooling of air create another characteristic of most deserts:
strong winds. These winds, circulating air that is often hot and dry, increase
the already high rate of evaporation. Evaporation in American deserts ranges
from 70 to 160 inches per year. |
| A desert then is not so easily defined. All these characteristics: seasonal,
high temperatures; low, sporadic rainfall; a high rate of evaporation; wide
temperature ranges; and strong winds are part of the definition. |
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http://www.nps.gov/jotr/nature/features/desert/defined.html
last modified: 03/26/02
web editor: Sandra kaye |
