Most people in the US, especially California, have heard of the Santa Ana winds. These are the baneful, powerful west-blowing winds that spread devastating forest fires throughout Southern California throughout the summer. But very few people know what they are, or what causes them.
The Santa Ana winds are the most famous and powerful versions of a type of wind known meteorologically as katabatic winds. They are more commonly called drainage winds.
The winds occur predominantly in spring and autumn, due to the temperature issues involved. During these times of year, air in the Great Basin and the Mojave Desert begins moving due to changing seasonal temperatures and air moving north from Mexico and south from Canada. These are the same atmospheric movements that clash and create tornadoes.
As more air funnels into the lower-elevation areas, it pushes the rest of the air up towards the higher elevations. As it moves upwards, the air compresses and cools, becoming more and more humid and eventually dropping most or all of it’s moisture as rain. It then becomes trapped in these higher elevation plateaus, held in by the wall of the Sierra Nevada mountain range.
The presence of air in these areas does not preclude continued movement in lower areas. Air continues to funnel in, forcing more and more air into higher elevations, drying as it goes. This is mostly held there by the mountains, too.
However, there are a few escapes for this compressed air. That is the few small gaps in the dense Sierra Nevada and San Gabriel mountain ranges, and in between the two. As the pressure in the plateaus rises, it pushes the air sufficiently to overcome the more minor elevation differences between the plateaus and these gaps.
These gaps are comparatively quite narrow, further compressing the dry air coming through them. The air then bursts out and drops into the lower Central Valley. However, the Central Valley is also a much lower elevation than the high Midwestern plateaus. Air being the way it is, the compressed air immediately expands and decompresses as it is pushed further westward by more escaping air.
Gravity affecting air as it does everything else, it immediately yanks the already rapidly moving air downwards. Air pulled downwards in turn re-compresses, becoming very dense. When it re-compresses, it also heats up, getting warmer the farther into the Valley it gets.
Remember, if you will, that this air has now been compressed three times in this process: once when forced upwards to the plateau, once when it was further forced through the mountains, and again when it finished passing over the mountains. This means that by the time of the third compression, coming down the Valley, virtually all of the moisture has essentially be squeezed out of the air like wringing a sponge.
The totality of these effects is that the resultant winds are very fast (due to the rapid change in elevation), very hot (because of the rapid re-compression), and extremely dry (because of the prior compression). This makes the winds the perfect vector for carrying forest fires.
So, the next time you hear about the Santa Ana winds inflaming wildfires in Southern California, you’ll know exactly what they’re talking about.