In autumn, we often hear about Santa Ana winds exacerbating wild fires in the western states. The fires and their path of destruction are appropriately the focus of the news stories, leaving many to ask, "What are Santa Ana winds?"
A meteorological and geographic phenomenon, the Santa Ana winds blow warm air from east to west across southern California each autumn. They are named after the Santa Ana Canyon in southern California, but are often referred to as "red winds" or "devil winds."
The Santa Ana Winds form in Utah, Nevada, and Arizona between the Sierra Nevada and Rocky Mountains. During the fall, air pressure builds in cold temperatures at high altitudes in this region and forms a drainage wind, which eventually blows down the mountains and out the Great Basin toward the southern California coastline. The winds tend to blow at 35 to 45 mph, but can gust at hurricane speeds.
Although the winds form in cold weather at high altitudes in the mountains, they blow warm, dry air across southern California. As the winds drain out of the mountains, they descend and heat up as the air is compressed in a process called adiabatic heating. The wind also dries on its descent, often leaving its humidity below 10% by the time it reaches the coastline. During the autumn when the Santa Ana winds blow, southern California is at its warmest, often hotter than the deserts.
When wildfires burn in southern California during the Santa Ana winds, the fires are fueled by the hot, dry winds, causing even greater destruction. In October 2003, 721,791 acres burned in two weeks of wildfires in southern California, fanned by the Santa Ana winds. During wildfires in October 2007, the Santa Ana winds also contributed to the burning of 426,000 acres. Some areas saw wind gusts of 40 to 60 mph, which is equivalent to tropical storm speeds. Other areas had winds traveling at category two hurricane speeds of over 100 mph. In 2011, Santa Ana wind events in southern California recorded record gusts of 140 mph, producing "the strongest easterly wind event in the past several years," according to the national weather agency.
Although the Santa Ana winds can cause damage, they benefit the environment in other ways. The winds cause cold water to rise to the surface of the ocean, which brings nutrients to the surface. The Santa Anas also make the air in southern California more pleasant by blowing air pollution out to sea.
When the Santa Ana winds begin to recede, a fog settles over southern California in its wake. Where dry air prevailed in the lower atmosphere during the Santa Ana winds, a cool moist layer forms quickly after the winds stop, creating a dense fog.