October 2007 California wildfires
The Santa Ana winds  are warm, dry winds that characteristically appear in Southern California and Northern Baja California weather during autumn and early winter.  Santa Anas are a type of föhn 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).  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.