Written by: Scott Hetsko

Wildfires have been ravaging Southern California the past few days. Unusually strong Santa Ana winds are to blame. The Santa Ana's develop when the desert is cold, and are thus most common during the cool season stretching from October through March. High pressure builds over the Great Basin (e.g., in Nevada) and the cold air there begins to sink. However, this air is forced downhill which compresses and warms it at a rate of about 10C per kilometer (29F per mile) of descent. As its temperature rises, the relative humidity drops; the air starts out dry and winds up at sea level much drier still. The air picks up speed as it is channeled through passes and canyons. Santa Ana's can cause a great deal of damage. The fast, hot winds cause vegetation to dry out, increasing the danger of wildfire. Once the fires start, the winds fan the flames and hasten their spread.