Figure 15.22: Air cools and loses moisture as it rises over a mountain. It descends on the
leeward side and warms by compression. The resulting warm and dry winds are Foehn winds
or Chinook winds. If the air loses precipitation over the mountain, the leeward side of the
mountain will be dry, experiencing rainshadow effect. ( 13 )
Santa Ana Winds
“Deadly” is a term often used to describe theSanta Ana windsin Southern California
(Figure15.23). These winds are created in the late fall and winter when the Great Basin
east of the Sierra Nevada cools. The high pressure is created when the Great Basin cools
forces winds downhill and in a clockwise direction. The air sinks rapidly, so that its pressure
rises. At the same time, the air’s temperature rises and its humidity falls. The winds blow
across the Southwestern deserts and then race downhill and westward toward the ocean. Air
is forced through canyons cutting the San Gabriel and San Bernadino mountains. The winds
are especially fast through Santa Ana Canyon, which gives them their name.
The Santa Ana winds often arrive at the end of California’s long summer drought season.
The hot dry winds dry out the landscape even more. If a fire starts, it can spread quickly,
causing large-scale devastation. In late October 2007, Santa Ana winds fueled many fires
that together burned 426,000 acres of wild land and more than 1500 homes (Figure15.24).
The 2003 Santa Ana winds contributed to the loss of 721,791 acres to wild fires.
Desert Winds
The image of a lonely traveler battling a dust storm in the desert is one most people have
seen, at least in cartoon form. Desert winds pick up dust because there is not as much
vegetation to hold down the dirt and sand. Hot winds blow across many deserts and most
are given local names. Across the Sahara there are winds known as leveche, sirocco and
sharav.