Santa Ana Weather
Santa Ana days typically occur from September through February. Literally and figuratively, Santa Ana days can be headache days in San Diego. They dramatically increase the risk for serious wild fires, for the following reasons:
- low humidity (below 15% inlands and not much above this near the coast); sometimes relative humidity can drop into the single digits inland
- strong winds (20-30mph, with gusts up to 50mph), especially in inland valleys and the mountains
- unusually high temperatures during the day
Santa Anas increase the wildfire risk particularly in October and November before the first significant winter storms arrive. If winter storms arrive late, the fuel becomes drier with every Santa Ana, so the fire risk increases.
Additional Factors
While the average daily temperature may be elevated only slight during a Santa Ana, day-night temperature differences are much more extreme than on normal days. During the day, temperatures are much higher, sometimes breaking temperature records for that day. During the night, temperatures can be much lower than normal.
The reason for this is the low humidity level in the air. Because of the high heat capacity of H2O, water vapor serves as temperature moderator: day-time temperatures in moist air rise more slowly than in dry air; night-time temperatures in moist air sink more slowly than in dry air. In fact, a Santa Ana late in the year dramatically increases the risk of reaching freezing temperatures later in the night, and farmers of sensitive crop have to take precautionary measures.
El Niños
The fire risk is increased after periods of long dryness, especially when they were preceded by an unusually wet year that promoted extensive growth of vegetation. For example, strong El Niño winters typically bring increased precipitation to southern California and after a few seasons, the vegetation that grew extensively during the El Niño winter starts to die off. For example, the strong 1997/98 El Niño promoted growth of vegetation in San Diego county that then burned in the 2003 wildfires. The 2005/06 winter was one of the wettest on record, nurturing the vegetation before it burned in the 2007 wildfires.
Low Air Quality For San Diego's coastal areas, Santa Anas also cause relatively low air quality because the smoggy air from inland gets blown westward by the easterly winds. Particularly bad are Santa Anas with northeasterly winds because then the smog from the L.A. basin gets blown toward San Diego. On Santa Ana days, a thick brown smog layer can be see on the horizon above the ocean. Some of this air extends on land, providing hazardous conditions for people with asthma and other respiratory ailments.
The October 2003 Southern California Fire Disaster
What Makes the Santa Ana condition so Unique?
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The Right-Hand Rule
The right-hand rule gives the air flow on the Earth's surface in cyclonic (low pressure) and anti-cyclonic (high pressure) systems in the Northern Hemisphere. Point your thumb in the direction of vertical flow in a system (down for high pressure, up for low pressure). The fingers show the direction of winds along the Earth's surface.