The Santa Ana Winds FAQ

Robert Fovell
Professor of Atmospheric and Oceanic Sciences
University of California, Los Angeles
rfovell _at_ ucla _dot_ edu

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 why they tend to occur during the cooler half of the year, from September to March. The desert is not hot at that that 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 discounts its rather more regional scope and impact. However, alternative explanations are much more 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.

In early 2008, I decided to take a look through the archives of the Los Angeles Times to see how often terms like "Santa Ana" and "santana" appeared in its pages. You can read a report on the result here.

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, ostensibly similar winds are often called "Foehn winds". In my view, the best descriptive term for the Santa Ana winds is katabatic winds, which is Greek for "to flow downhill". As mentioned above, the Santa Ana air starts out relatively colder and more dense, and thus falls downslope

Back to the Santa Ana Winds page

23 October 2007. Amended 22 September 2009.
Robert Fovell