effective in deserts because of the lack of vegetation to hold soils and small pieces
of weathered rock in place. Although sand blown by the wind polishes some desert
surfaces, water is a far greater shaper of desert landscapes. In the driest deserts the
plentitude of dry stream channels is ample evidence that water erodes and deposits
much more total material than wind.
Desert precipitation is noticeably scarce. How dry is a desert? Quantitative
definitions such as that of Ko ̈ppen are based on precipitation amounts that change
with temperature. For instance, Albuquerque, New Mexico garners a meager
precipitation of about 300 mm per year. This renders Albuquerque quite dry with
a natural cover that was scattered bushes and grasses in response to the large
evapotranspiration of the summer. The northeastern portion of the province of
Alberta, Canada has similar yearly precipitation, but it is a land of forest and
swamp because of considerably lower evapotranspiration. A hallmark of deserts
is the unreliability of precipitation. There is huge year-to-year variability in the
amount and timing of precipitation. In both the subtropical and middle latitude
deserts precipitation is intense when it does come because a great percentage of
it is from convectional storms.
Desert 97Meager life is supported in Death Valley, California, which is the hottest place in the Western
Hemisphere. (Photo courtesy of Steve Stadler)