Geoscience Reference
In-Depth Information
smaller ranges, the Cascades and the Sierra Nevada, run roughly parallel to the main
Rockies.
The two largest desert areas are the Chihuahuan Desert (450,000 km
2
), which
extends from north-central Mexico into southern NewMexico, and the Sonoran Desert
(300,000 km
2
), which extends from north-west Mexico into southern Arizona. The
Sonoran Desert is bounded to the east by the Sierra Madre Occidental, and receives
most of its rain from the Pacific during the summer. The Chihuahuan Desert lies
between the Sierra Madre Occidental and the Sierra Madre Oriental, with most of its
rainfall in summer, supplemented by sporadic tropical cyclones from the Caribbean.
The smaller Mojave Desert (140,000 km
2
) lies north-east of Los Angeles and south-
west of Las Vegas, within the rain shadow of the Transverse Ranges. Precipitation
occurs mainly in winter, with snow on the higher ranges. The Colorado Plateau
(375,000 km
2
) and the Great Basin (410,000 km
2
) are also arid, although their higher
elevation means that they are slightly cooler in summer than the less-elevated southern
deserts. The Baja California peninsula is also very dry and extends slightly south of
the Tropic of Cancer. We now consider the causes of aridity in North America and
northern Mexico.
20.3 Present-day climate and causes of aridity
Much of North America lies within the zone of the westerlies, but the Rockies are
aligned perpendicular to these moist air masses and provide a formidable barrier to
their eastward passage. As a consequence, the regions east of the Rockies are in a
vast rain shadow, extending in the north from North Dakota through South Dakota,
Nebraska, Kansas and Oklahoma to Texas in the south. The Great Basin of Nevada
and much of Utah, the Colorado Plateau and Arizona also lie in the rain shadow
of the Rockies. The Great Salt Lake Desert of Utah is an eloquent witness to this
aridity. The climate of much of inland North America is best described as continental,
with hot, dry summers and very cold winters. The absence of any east-west aligned
mountain barriers also means that outbreaks of polar air can penetrate far to the south
in winter, bringing snow and extreme cold as far south as Florida, Louisiana and Texas
(Kendrew,
1961
, p. 412, fig. 127).
The west coast lies in the direct path of the westerlies, which bring rain throughout
the year to Oregon and Washington and winter rain to California (
Figure 20.5
). An
additional factor accentuating the summer aridity of California and Baja California is
the presence offshore of the cold California Current, which brings coastal fog to Los
Angeles and San Francisco. The Sonoran and Chihuahuan Deserts and the southern
tip of Baja California lie beneath the descending arm of the tropical Hadley Cell,
which serves to intensify their aridity (see
Chapter 1
). Because the prevailing winds
are blowing offshore along the east coast, the zone of high coastal rain is relatively
narrow. An exception is the well-watered region around the Gulf of Mexico, which
