Atmospheric Pressure of the Santa Ana Winds

Santa Ana Winds

• Santa Ana winds are strong dry winds that flow to the southwest from the Mojave Desert into the Los Angeles basin. These winds are also warm or, often, hot. They have a very low relative humidity, typically below 25 percent but often below 10 percent. They commonly flow at speeds of 28 miles per hour, with gusts to over 56 miles per hour. Their heat is not due to desert heat. In fact, they develop when the desert is cool, from October through March.

Atmospheric High Pressure

• The primary trigger for Santa Ana winds is atmospheric high pressure over the Great Basin, consisting of Nevada and parts of Utah. High-pressure systems are characterized by sinking air that produces an outflow of surface winds in a clockwise direction. On the southern side of the high, this circulation pushes winds through the San Gabriel and San Bernardino Mountains and into the Los Angeles Basin.

Compression Effect

• The Great Basin has a higher elevation than the Los Angeles basin. This means that as the air sinks beneath the high and flows to the southwest, it is forced downhill. The descending air is compressed by the atmosphere, causing it to warm and dry out -- a process known as adiabatic heating. Air temperature is greatly affected by altitude. If you take a parcel of air only one mile above your head and lower it to your level, it will warm nearly 30 degrees Fahrenheit. This means a 60 degree desert wind will become a 90 degree wind as it flows into the Los Angeles basin.

Channeling Effect

• The speed of these winds is greatly increased when they are channeled through passes and canyons in the San Gabriel and San Bernardino Mountains. This increase is explained by the Bernoulli principle, which describes the conservation of energy for fluids, such as air. The effect is similar to placing your finger over the end of a garden hose.

Atmospheric Low Pressure

• The Santa Ana winds can also be further amplified by an area of low pressure off the coast of California. Air always moves from high pressure to low pressure, seeking a natural balance. This movement is known as wind. An area of low pressure will literally suck winds through the mountain passes and canyons, from the Great Basin high and into the Los Angeles basin.

Santa Ana Effects

• Santa Ana winds pose a great threat to the areas they flow through. The hot, dry winds dry out vegetation, raising the risk of wildfires. Once a fire starts, the winds intensify and spread the flames. Furthermore, the fire heats the air, causing it to rise. This sucks in more air, thereby increasing the winds even further.