Santa Ana winds occur when there is a zone of high pressure in the high desert of Nevada or Utah and a zone of low pressure off of the coast of California. Air pressure always wants to equalize itself, so air from the high pressure zone moves toward the low pressure zone. The gusty wind gets funneled though the passes of the Transverse Ranges and roars to the coast. Sustained wind speeds are easily 25 to 40 miles per hour and gusts of 60 mph are common. Gusts above hurricane force have been recorded.
As the wind goes through the mountain passes, it gets compressed. This causes the temperature to rise. This is just like what happens with a tire pump. You may have noticed that the cylinder of the pump gets very hot. This not due to friction alone, but also to compression of the air. During Santa Ana conditions in the fall, when the high desert is still hot, temperatures in southern California can be the hottest in the country, even beating out old standbys like Death Valley and McAllen, Texas. The rise in temperature causes the relative humidity of the already-dry desert air to drop even lower. Humidity readings in the single digits are routine.
Santa Ana winds occur in fall and winter and often follow the passage of a low front. They last for a few days at a time and there are multiple Santa Ana events per year. As mentioned above, they are usually warm to hot, though they can be chilly in the winter.
Smoke from a Santa Ana fed fire
Though the gusts can reach gale force, the sustained winds are much less and overall, wind damage is minimal. There will be fallen trees and branches, a few shingles blown off roofs, and leaves everywhere. It is always recommended that high-profile and top-heavy vehicles stay off the roads during Santa Ana periods. They have been known to get blown over. The ocean can be very choppy during Santa Anas and small boats should stay in port. The biggest danger in Santa Ana winds is when they are accompanied by fire. The high wind, low humidity, and high temperatures all help strengthen the fire and make firefighting more difficult.
Legend has it that people become more cranky during Santa Ana periods. I have never noticed a difference, though it seems reasonable that any kind of unfavorable weather could make people more unpleasant. Staying hydrated is reported to improve one's mood. To prevent dry, itchy skin, I run a humidifier during any kind of dry weather.This tree was planted too close to the power lines and had to be severely trimmed.
Plants may need extra attention during Santa Anas. The soil dries out faster, especially with potted plants. The wind may blow over top heavy pots. I find it useful to put stones in those pots to keep them weighed down. If a pot insists on falling over, I leave it over until that particular Santa Ana period is done. A plant is more likely to be damaged in the falling than when laying down. Be sure not to over water plants in the ground. Shallow-rooted trees have more of a tendency to blow over in soft, wet soil.
Leaves of large-leaved plants like bananas and giant bird of paradise will get shredded during Santa Anas. It is recommended that plants like these be planted against the leeward side of buildings. That is probably of some help in less windy places but I doubt it is a complete solution in the more windy sites. The wind swirls around everywhere and still knocks over pots on the leeward side of my house on the leeward side of the hill.This tree blew over in a very strong Santa Ana episode.
For the safety of all - from people and pets to buildings and cars - trim dead branches from trees and have dead trees removed. Consider having removed any trees that lean dangerously. If you feel the need to trim live trees, have them pruned and thinned intelligently. Typical "topping" results in weak growth that can be broken in wind storms.
Don't forget the animals. Make sure that your pets and livestock have plenty of water. Double check automatic watering devices to make sure they are working. Make sure pets have shade during hot Santa Anas. They may appreciate a place to get out of the wind also. Keep bird baths filled.
For most people most of the time, there is nothing particularly dangerous about Santa Anas. They are at most an annoyance. They are somewhat unusual in that they are a storm accompanied by sunny weather. They are also very predictable in that they occur every year. You can't change the weather. The only thing you can change is your feelings toward it, so be prepared and you will minimize the hassle. Get your trimming done by early fall. If you have special wind chimes, windsocks, or banners, bring them inside. Close your patio umbrellas. Corral any lightweight items like bags, empty cans, and plastic flower pots. If you have allergy or asthma problems, make sure you have medicine on hand. If nothing else, go to the beach. The warmest and sunniest days of the year there are during hot Santa Anas.