The Santa Ana Winds FAQ Robert Fovell
Professor of Atmospheric and Oceanic Sciences
University of California, Los Angeles
Q. What are Santa Ana winds?
A. Santa Ana winds are dry and warm (often hot) winds in the Southern
California area that blow in from the desert -- which includes the
Great Basin of the western United States, incorporating Nevada and
part of Utah.
Q. Are Santa Ana winds hot owing to their desert origin?
A. NO. These winds blow when the desert is COOL. This is why they tend
to occur during the cooler half of the year, from September to March.
The desert is not hot at that time of year.
Q. Why do the Santa Anas start blowing?
A. During the winter half-year, the Great Basin tends to be cooler
than the Los Angeles (LA) Basin. Periodically, high pressure builds
over the Great Basin. In the Northern Hemisphere, air flows clockwise
around high pressure systems. For highs located in the Great Basin,
that clockwise flow pushes air into the LA Basin from the northeast
and the east. Thus, the Santa Ana winds start in the Great Basin as a
cool or even cold wind.
Q. So why are Santa Anas warm or even hot?
A. The Great Basin resides at a higher elevation than the LA Basin,
which is near sea level. Thus, in flowing towards Southern
California, the Santa Anas flow DOWNSLOPE. When air descends, it is
compressed, and its temperature rises. Dry (unsaturated) air warms on
descent at a rate of 10C/km or almost 30F per mile -- an incredible
rate. That means if you take a piece of air located a only mile above
your head, and brought it down to your feet, it would wind up 30
degrees Fahrenheit warmer than when it started. You don't need to
change the altitude of air very much to alter its temperature
significantly.
Q. So dryness is a critical characteristic of Santa Anas. Why are
they so dry?
A. The Santa Ana wind tends to have very low relative humidity (RH),
often registering below 10%. RH depends on two factors: how much
vapor is in the air (vapor supply) and how much the air can hold
(vapor capacity). Vapor capacity depends mainly on temperature; warm
air can "hold" more vapor than cold air. One way to decrease RH of
air is to raise its temperature. Thus, as the Santa Ana winds blow
downslope, they're not just getting hotter, their relative humidity
is also decreasing. Relatively dry air is thirsty air, and takes
moisture from wherever it can, including your skin and plant life.
Increase its speed, and very dry air can desiccate vegetation very
quickly.
Q. Can Santa Ana winds exist without the topography of the Western
United States?
A. If you flattened the mountains of the Western U.S., the Santa Anas
would still blow, but they'd be COLD winds, and not especially dry
either... and you would not be reading this FAQ.
Q. Why do Santa Ana winds also tend to be fast winds?
A. In addition to flowing downslope, Santa Ana winds tend to be
channeled through passes and canyons, locally increasing their speed.
If you take a garden hose through which water is flowing, and
restrict the opening, you increase the water velocity. In the same
way, winds pick up speed when channeled through topographic features.
Q. When Santa Anas start blowing, how long do they last?
A. After formation, the high pressure systems associated with Santa
Ana wind conditions tend to move eastward with time. As the highs
move away from Southern California, their influence on the area
diminishes. Thus, Santa Ana episodes tend to last only a few
days. "This, too, shall pass."
Q. Santa Ana winds only occur in October, right?
A. No. Santa Ana conditions can exist at any time in which the Great
Basin tends to be cooler than Southern California -- typically the
September to March period. However, the winds garner the most
attention around October because of unique aspects of Southern
California climate which enhances fire danger in the autumn season.
Q. So why is the fire danger larger in the fall?
A. Southern California has a Mediterranean climate, which means
precipitation falls during the winter and the hot summer season is
also dry. By the time the Great Basin starts cooling in September and
October, Southern California has already experienced an extended
period of hot, dry weather. In October, the Great Basin is cooler
than the LA area but not yet very cold -- so the Santa Ana winds can
start at a higher temperature and thus reach sea level very hot and
extremely dry after compression warming. That heat, in combination
with prolonged summer drought, produces an especially high fire
hazard. As the winter takes hold in Nevada, however, decreasing
temperatures there mean the winds won't be as hot and dry once they
descend to sea level. In most years, the rains have already started
in Los Angeles by that time, replenishing plant moisture. The fire
threat never completely vanishes - especially during dry winters --
but it usually decreases as the winter wears on.
Q. Where did the name for the Santa Ana winds come from?
A. Sources vary. The most common explanation has the wind being named
for the Santa Ana Canyon in Southern California's Orange County,
which would seem to belittle its rather more regional scope and
impact. However, alternative explanations also seem dubious. It is
often claimed that "Santa Ana" is a corrupted version of "Santana"
which is purported to mean "devil" in Spanish or an Indian language.
Yet, the Spanish word for devil is "diablo" and its Satan
is "Satana", conspicuously missing an extra "n". The Indian language
in which "Santana" means devil has not yet been identified. I am not
a linguist, but I suggest it is more likely for a term like "Santana"
to have evolved from "Santa Ana" than vice-versa (think "San
Francisco" becoming "Frisco" or "New Orleans" devolving
into "N'orleans"). References to Mexican General Santa Anna have also
been made. All I'm fairly sure of it's not likely named after the
feast day of Santa Ana in the Catholic calendar as that falls in
July, outside of Santa Ana season.
Q. What other nicknames has the Santa Ana earned?
A. Santa Ana winds occupy an important place in Southern California
literature. In Raymond Chandler's stories, which were set in Los
Angeles, Santa Anas appear by name and also by nicknames such
as "devil wind" and "red wind". The winds also figure in Joan
Didion's and Michael Connelly's works.
Q. Are Santa Ana-like winds unique to Southern California?
A. No. Warm, dry downslope winds can occur anywhere where weather
patterns force originally cold air to flow downslope. In the Front
Range of the Rockies, downslope winds during winter can cause snow to
disappear owing to their heat and dryness, and are variously
dubbed "Chinooks" or "snow-eaters". In Europe, these winds are often
called "Foehn winds".
