|Santa Ana wind damage / Flickr|
Around the world there are what are called “down slope winds”, and they have unique names in their region. Here in the North America there are two general areas of these down slope winds. On the eastern slope of the Rocky Mountains in Alberta, Montana, Wyoming and Colorado, they are called Chinook winds. And in Southern California they are called the Santa Ana winds.
Santa Ana Winds
To the east of Los Angeles are the Santa Ana Mountains and the Santa Ana canyon, which the Santa Ana winds are named for. When a wind blows from the east towards the ocean, they are down slope winds coming off of the Santa Ana Mountains. These winds are hot and dry as they move downward towards the ocean.
You might be thinking these are just winds like any other wind. Not true, something different happens with a Santa Ana wind. Something happens to the air and atmosphere in a down sloping wind.
The area around Los Angeles is not far above sea level, when just to the east in the Angeles National Forest the elevations are above 6,000 feet (1,830 meters) and even higher to the north. As the wind blows towards the ocean out of the east, they move downhill. The wind blowing down the slopes of the Santa Ana Mountains heats up by a process known as adiabatic heating or actually the dry adiabatic lapse rate.
The heating of the air occurs at a rate of 5.5 degrees F per 1,000 feet. Or for every 180 feet downhill the temperature can increase by 1 degree F. This means that the temperature at the base of the Santa Ana Mountains can be as much as 33 degrees F warmer. With the winds starting at 6,000 feet or higher in elevation to almost sea level, the air compresses and heats up rapidly.
As the Santa Ana winds blow downhill and the temperature is increasing, the humidity is dropping. The humidity can drop to as low as 10% and less. This can dry out the vegetation in a small amount of time, literally within hours. And it’s not only the down slope winds that increase the temperature; they also increase in speed as they blow down hill.
According to the Los Angeles National Weather Service, the wind has to blow at least 29 mph to qualify as a Santa Ana wind. The wind is usually much stronger than that because of what is called the Venturi effect. The Venturi effect is when the wind blows through the canyons, which can increase the wind speeds greatly.
So, what actually causes these Santa Ana winds to blow? Pressure, high and low pressure is what causes wind. A high-pressure area will always try and fill a lower pressure area and this causes the wind. A Santa Ana wind can start to blow if a high-pressure system were to be east or northeast of the Los Angeles area.
In the northern hemisphere the wind circulates around a high-pressure system in a clockwise motion. If the high pressure area is to the east of Los Angeles the wind will blow out of the east up the Santa Ana Mountains and then race down the Los Angeles side of the Santa Ana Mountains causing the Santa Ana winds to blow.
|Satellite photo of dust blowing towards the Pacific / NASA|
The Santa Ana wind blowing, drying and warming the air can easily cause any small fire to blow out of control very quickly. This is why you almost always hear the words Santa Ana winds in conjunction with southern California wildfires.
The Santa Ana winds blowing are already drying out the air and the vegetation and can also cause power lines to spark and any spark in these conditions can start a wildfire. Once the fire starts in these hot, dry and windy conditions they spread rapidly. A wildfire can also create its own weather, increasing the wind speed with rapidly changing wind directions.
These down slope Santa Ana winds also produce positive ions which can affect your mood and how your feel. That’s why when these winds blow your nerves feel on end as well. Yes the wind can also affect your health and your moods.