Annual autumn winds dehydrate plants, increase fire danger
"Here come the Santa Ana winds."
We hear this term from TV weathermen all the time, but how many of us really know what it means? While they haven't yet occurred this year, we've all felt the effects of these hot, dry, fast, fire-fanning winds in autumns past. But how do they get like that? Where do they come from? And who is Santa Ana, anyway?
It turns out the Santa Ana winds originate when high pressure zones build up in the Great Basin area, which is generally cooler than the Los Angeles Basin during the winter half of the year, from September to March. Because air flows clockwise around high pressure in the Northern Hemisphere, winds spinning off the southern edge of the Great Basin are sent flying westward across southern California, tipping over your lawn chairs and stealing your homework on their way to the Pacific.
As these cold winds traverse the desert, they also lose elevation, which is what really gives the Santa Anas their dangerous reputation. John Makevich, Meteorology Instructor at COC, says that when a gas is put under pressure its temperature increases through what's called adiabatic heating.
"The adiabatic process is what happens when the air is sinking and it compresses, and as it compresses it heats up," says Makevich.
Molecules are closer together in compressed air, causing more friction and heat. Have you ever felt an aerosol can get cold as you spray? That's the reverse effect of the same principle. The gas expands to fill the can with fewer molecules; less pressure, less heat.
Robert Fovell, Professor of Atmospheric and Oceanic Sciences at UCLA, says that as the Santa Anas fall, they heat up at an incredible rate. A hot wind sounds like enough bad news when fighting a wildfire, but you know the old adage - 'It's not the heat, it's the humidity!'
"As the air comes down slope it's compressed and it will actually warm up to 30 degrees Fahrenheit per mile of descent," Fovell explains. "So you don't have to move the air down very far before it becomes much hotter. And then as the temperature goes up, the relative humidity goes down very quickly."
We think of cold air as being less humid than warm air, because vapor condenses to water at cold temperatures. That's true in a sense, but Professor Fovell says that what is really important as far as the weather is concerned is not the vapor supply (the amount of water in the air), but the relative humidity (the ratio of how much water is in the air compared to how much could be).
"As the air comes down slope from Nevada, there's not necessarily more or less water vapor in that air. But because the air's temperature is increasing because you're squeezing it ... the vapor capacity goes way up so the relative humidity goes way down," says Fovell. "It's basically the same air and it may even have the same moisture content. But the relative humidity determines what the moisture stress is on plants. If you blow very dry air past a plant it's just going to draw all the moisture out of the plant. We saw this in the Station Fire. On days when the relative humidity goes up just a little bit, the firefighters have a much easier job."
Warm air has higher vapor capacity (it can 'hold' more water) than cold. So as the cold Santa Anas heat up, they become 'thirstier' and begin sucking moisture from anywhere they can get it. That could be your lawn, your chapped lips, or the brush behind your house. Now we start to see why these winds have such dire consequences for the already fire-prone chaparral climate of southern California (but it's a dry heat!).
Finally, the Santa Ana winds are strong and fast-moving. This is due in part to the rugged terrain they cross over as they enter the Los Angeles area. As the falling winds are forced through narrow canyons and around mountains, they speed up, like a river flowing through narrow rapids.
So high pressure in the Great Basin creates cold westerly winds which warm as they fall, speed up through canyons, and desiccate local vegetation, creating fuel for wildfires and then spreading them. This is especially dangerous in October because the wind's starting temperature is higher. At that time, the Great Basin may still be only slightly cool or even warm, but Los Angeles is even warmer, so the winds still blow, but instead of going from cold to hot, they start hot and end even hotter. This also coincides with the end of another 'perfect' rainless SoCal summer, meaning the Santa Anas make an already dry landscape even drier.
Southern California isn't the only place where this weather phenomenon occurs. In Europe, they are known as foehn winds, a term originating in the Alps which occurs when cold winds run up over mountains and are warmed by rapid acceleration down the leeward, or downwind, slope. Similar phenomena are called 'Chinooks' when they occur in the Rocky Mountain region.
Theories abound as to the origin of the name Santa Ana for the SoCal variety of such winds, but there is no clear consensus. Everything from the Mexican General and President to ancient Native American words has been proposed. It is also suggested that the term derives from a corrupted version of the Spanish for 'devil winds' or 'Satan's winds,' as they are also sometimes known. The term could simply refer to the region of Orange County bearing the same name, though the winds affect a far greater geographic area.