PCC Geography: Mr. Lee

The Esperanza Fire and Adiabatic Heating

Oct 30, 2006

The current Esperanza Fire is an object lesson demonstrating the real-life use of weather knowledge. The Santa Ana winds come from the north of the LA basin, often originating in the Mojave Desert region. Many people mistakenly think the Mojave Desert is the source of the hot winds. Well, the winds might come from that area, but more specifically, they are associated with a high pressure system that moves over the desert. The air temperature is warmed adiabatically?….by compression due to the high pressure system at a rate of about 5.5 degrees Fahrenheit per 1000 ft (or about 29 degrees per mile). Warm air has a greater capacity to hold moisture than does cool air, so the warm winds tend to remove moisture from the vegetation. The fuel become more easily combustible. As the winds flow through mountain pass and canyons, it accelerates, further increasing its ability to remove moisture. And, of course, if a fire is present, the drive the fire, increasing its intensity and the resulting heat further dries the vegetation making it easier to ignite.

The space image shown with this article was taken on 27 Oct 2006. At the time of the photo, about about 19,000 acres had burned. Notice the plume of smoke is blowing from land to sea (which is typical for Santa Ana winds).