World and Nation

Weather and Wildfire

Weather and Wildfire

The rapid growth of the wildfires in Southern California early this week was primarily facilitated by the prevailing weather conditions. In particular, there was an intense northeasterly wind and the relative humidity was extremely low, both characteristics of the Santa Ana flow regime. The Santa Ana wind is a regional example of the more general phenomenon of downslope flow. As the name implies, downslope flow occurs when wind is directed down a gradient of surface elevation. In the case of the Santa Ana, the wind blows from the plateau of the Mojave desert (elevation of roughly 1,000 meters) towards the Pacific coast. As the air travels along such a path, it descends and is consequentially compressed, as it adjusts to the higher environmental pressure at lower elevation. The compression results in heating of the air, 18 degrees (10°C) for every 1000 meters of descent. While the temperature of the descending air increases, its water vapor content remains unchanged, such that the relative (to temperature) humidity decreases. Hence, the downslope Santa Ana winds are necessarily accompanied by low relative humidity, setting the stage for explosive wildfire growth.

Extended Forecast

Tonight: Light rain likely, breezy. Low 53°F (12°C).

Saturday: Warm and windy, with showers likely. High 72°F (22°C).

Saturday night: Showers early, then clearing. Low 55°F (13°C).

Sunday: Sunny. High 63°F (17°C).

Monday: Sunny, cooler. High 49°F (9°C).