Santa Anas are a type of
f?hn wind, the result of
air pressure buildup in the
high-altitude Great Basin between the
Sierra Nevada and the
Rocky Mountains. This high energy wind spills out of the Great Basin and is pulled by gravity into the surrounding lowlands. The air circulates clockwise around the high pressure area bringing winds from the east and northeast to Southern California (the reverse of the westerly winds characteristic of the latitude).
It is often said that the air is heated and dried as it passes through the
Mojave and
Sonoran Deserts, but according to meteorologists this is a popular misconception. The Santa Ana winds usually form during autumn and early spring when the desert is relatively cold, although they may form at virtually any time of year. The air heats up due to
adiabatic heating while being compressed during its descent. While the air has already been dried by
orographic lift prior to reaching the Great Basin, the relative humidity of the air declines rapidly as it descends and warms in its final stages as it passes over the
Transverse and
Peninsular Ranges.
The air is then
forced down the mountain slopes out towards the
Pacific coast; the air mass is further heated by
compression as it drops in altitude before reaching the
Los Angeles Basin western
San Diego County and
Tijuana (
Baja California) at typical speeds of 35 knots. The southern California coastal region gets some of its hottest weather of the year during autumn while Santa Ana winds are blowing. During Santa Ana conditions it is typically hotter along the coast than in the deserts and the
humidity plummets to less than 15%.
QuikSCAT image showing the speed of the Santa Ana winds (m/s).
As the Santa Ana winds are channeled through the mountain passes they can approach hurricane force. The combination of wind, heat, and dryness turns the
chaparral into explosive fuel for the infamous
wildfires for which the region is known. Wildfires fanned by Santa Ana winds burned 721,791 acres (2,921 km?) in two weeks during October 2003.
