Q&A: Santa Ana Winds

Question: I’ve lived in California all my life, and every year we hear about the Santa Ana Winds. They often accelerate the spread of wildfires. Could you explain what causes that wind? Thanks! — JR, San Diego, CA

Answer: Sure thing JR! The Santa Anas are a specific instance of what are called catabatic winds (sometimes “katabatic”). They happen all over the world and have different names in different regions. What qualifies these winds as catabatic is that they result from cool dense air falling down a topographic slope. As the air descends it’s heated by adiabatic compression and arrives as a hot blast of wind.

The Santa Anas can also be a Föhn wind if there’s precipitation on the windward side of the mountain. That releases even more heat into the air. The graphic shows that scenario. Either way, when air falls to a lower elevation it increases in temperature. And like a ball rolling down a hill, it increases the speed of the wind. Note: ΔH in the diagram is not necessarily to scale. The difference in elevations can be thousands of feet, but I needed the mountain larger to show other details.

With a Föhn wind, the incoming air is humid and releases rain as it rises and cools. When water vapor turns back into a liquid, it releases its latent heat of condensation. That warms the air. And because the sky is usually clear on the leeward side of mountains, the Sun can cause additional heating.

The end result is a blast of hot winds that can dry out vegetation, setting the stage for wildfires. With the Santa Anas, the air temperature can rise by 10°C (18°F) or more. That’s enough to turn dew-covered grass into dry tinder in a matter of hours. In one case, catabatic winds caused the air temperature in Loma, Montana to rise from -48°C to 9°C (-54°F to 49°F) in just 24 hours — a swing of 57°C (103°F)! 

And when fires do happen, catabatic winds will accelerate their spread. We can’t do much about the Santa Anas, save for predicting them. But we can increase wildfire mitigation protocols, like clearing dead fuel on the ground and increasing preparedness. Still, if you live near a mountain range, you’re going to occasionally get catabatic winds.

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