Weather Blog

What causes the "Santa Ana" winds?

What causes the "Santa Ana" winds?
Traffic snakes up a road as residents flee their hillside homes during a fast moving, wind driven brush fire in the Sylmar area of the San Fernando Valley in Los Angeles, Monday, Oct. 13, 2008. (AP Photo/Dan Steinberg)
With the raging wildfires near Los Angeles, you've probably frequently heard that they are caused by the "Santa Ana" winds.

They are a strong, hot, dry wind that blows through the southeastern California Deserts and the eastern Los Angeles area.

Their peak season is in the fall and winter when a cold air mass slides down into the area east of the Sierra Nevada mountains from Canada or Alaska. Cold air is very dense and associated with much higher pressures. To wit: Arizona set some crazy record lows earlier this week, with Tucson dropping into the 30s for only the fourth time before Oct. 15th in their long history. Even Las Vegas was only in the low 60s for highs earlier this week.

Meanwhile, off the California coast, it's business as usual with warm air creating a lower pressure there.

Depending on how strong that high pressure is inland, this creates a tremendous difference in pressure over a short distance, and since the atmosphere likes to be equalized, you can get a very strong wind going from the inland high to the offshore low.

Here is a typical chart showing a good Santa Ana wind set-up (taken Monday).

(Note the big high in southern Idaho and those black lines of pressure tightly packed together in California. Imagine that 'H' as the top of a peak, and those lines are showing the pressure flowing down from that peak. )

But Southern California's terrain makes for a double whammy.

First of all, as that air rolls over the hills and down the western slopes, the air compresses, causing it to warm up and dry out.  (This is the same physics that brings us our summer heat waves, more on that in a moment.)  When the wind is really strong, it can really dry the air out, turning any vegetation there into a fire waiting to happen.

Second, what air tries to squeeze through the gaps in the mountains gets accelerated due to what's called the Venturi effect, also informally known by me as the "Small straw" effect. To see this in action, blow through a big straw and note how fast the air is coming out the other end. Then, using the same pressure, blow through a small straw, and note how much faster the wind speed is at the other end.

So put all that together, and you have an incredibly hot, fast, dry wind -- the hat trick of ingredients to turn the region into a tinderbox.  Have a spark or start and small fire, and it can spread, well, like wildfire.

How windy?  Ontario, Calif. reported frequent gusts between 45 and 55 mph with a peak gust of 56 mph during a 13 hour period between 2 a.m. and 3 p.m. Monday. However, in a good illustration of how gaps in mountain topography can effect the weather, just a little bit to the east in Riverside, winds were "only" gusting into the 15-25 mph range.

How dry? Dew points were in the teens across the greater Los Angeles area. Fullerton reported a relative humidity of 7 percent Monday and it was at 10 percent as of midday Tuesday. Anaheim was at 8 percent humidity  Monday and 11 percent on Tuesday. Those numbers are Phoenix-esque.

How warm? Temperatures were solidly into the upper 70s and low 80s across the Los Angeles basin, with some cities heading to the mid 80s today. You can see the latest conditions here.

And you can see that northeast wind in action by this satellite image:

Can it happen here?

Sort of. There are a few similarities that could occur here as far as the weather pattern goes, but we are lucky in that the fall and winter is our rainy season, so we aren't as susceptible to the wildfire danger.

But for those of you who live in Enumclaw, Gold Bar, North Bend and the like have experienced a somewhat similar weather set up when you get those strong east winds.

In that case, similar to the Santa Anas, we have a very strong ridge of high pressure in Eastern Washington -- typically due to a cold air mass moving south out of Canada. In the meantime, we'll have one of our regular fall or winter storms approaching from or passing by to our west off the Pacific Ocean.

This will create the huge pressure difference needed to get very strong east winds through the Cascade gaps -- sometimes as high as 70-80 mph in gusts -- that plague the Cascade foothill communities.  Just like L.A., that wind will make it very dry (humidities can drop into the teens and 20s) and typically warmer than normal as well, although not into the 70s and 80s like L.A.

Now, we do have a worst case scenario out there, and that would be for a very dry summer and early fall, and then have one of these situations develop early in the season.

In that case, we could be set up for a very dangerous wildfire situation if something were to spark over there and get fed by the east wind.  So it's not impossible to get a bad wildfire situation here, it's just takes a whole lot more to happen. Still, it's great to be prepared.