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, or at least cooler than we are in the Los Angeles (LA) Basin, which I define as spanning from Ventura to San Diego. This why they tend to occur during the cooler part of the year, from September to May. 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 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. The air flowing into Southern California, forming the Santa Anas winds, is subsiding. 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 COOL or 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 many places, the Santa Ana winds tend to behave like a mountain gap wind, increasing speed as the flow is channeled through passes and canyons. If you take a garden hose through which water is flowing, and restrict the opening, you increase the water velocity, and observe a similar effect. In other places, notably in San Diego county, the winds accelerate as they flow over the ridge, down into Southern California. These are called "downslope windstorms". Both the gap and downslope winds are capable of producing very fast speeds.
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, although very prolonged events occur occasionally. "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 May 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. Can Santa Ana winds be hot in spring, too?
A. Yes, especially in late April and into May. At this time of year, the air that builds the Great Basin high is not frigid, but is still cooler and dense than the air mass on the ocean side of the mountains. As in October, the temperatures can reach very high values in LA because the subsiding air did not start off very cold.
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". (I previously described Santa Anas specifically as katabatic winds, which is Greek for "to flow downhill", but I don't think that is a particularly good description now.)
Back to the Santa Ana Winds page
23 October 2007. Amended 22 September 2009 and May 2014.
Robert Fovell