(Source: www.forbes.com)

The numbers are staggering from the California wine country fires (as of Wednesday morning): 17 dead, 180+ missing, 2000 buildings destroyed, 25000 fleeing their homes, and 120,000 acres burned from these fires. Even as I write this, those numbers are certainly going to go up as many fires in the region are only minimally contained. Meteorology and physics are helping to fan possibly some of the worse fires in California history. What are Diablo Winds?

The answer lies within a complex mix of meteorology, physics, geography, and topography. Dry, nearly hurricane-force winds are fanning the flames associated with the California fires in this region. Early Wednesday morning, the National Weather Service issued the following,

236 am PDT Wednesday October 11, 2017…RED FLAG WARNING for Wednesday afternoon through Thursday…Increasing northerly winds and drying conditions are forecast to return to the region later today and persist through Thursday.

Jitze Couperus/State of California Coastal Conservancy

This is related to the passage of a dry front and is likely to amplify fire conditions. However, the Diablo Winds have been a driving force with these fires. While not the “cause” of the fires, these winds situated in the peak of California fire season do not help.  In the map below, you can see the region of the western United States called the Great Basin (the brownish-yellowish region). If an area of high pressure is situated over that region, winds blow from the central Great Basin toward the Pacific coast. In the northern hemisphere, winds flow clockwise around high pressure and that creates the aforementioned flow.


With such a flow regime, the winds are forced over and descend down the elevated terrain and mountains on the western edge of the basin and in California. Since Mt. Diablo is in the region east of the Bay Area, these particular winds get the name Diablo Winds. Here is where the physics comes in. As these winds descend, they are compressed and warmed. These winds can reach tropical storm (39 mph) to hurricane force (74 mph). The warming is caused by something called adiabatic compression. To really explain this, I have to mention something called the First Law of Thermodynamics. It is an equation that relates internal energy changes in a system to the work done by the system and changes in heat added to a system. If a process is adiabatic, there is no heat added to or lost from the system, which means that all changes in internal energy would be related to work done.

Curt Kaplan/NWS

Now let’s apply that to the atmosphere associated with Diablo Winds. KidsGeo.com actually does a nice job of simplifying these concepts,

As air is heated it expands becoming less dense, and as a result, lighter. Because it is lighter, it rises upwards above the cooler air. As it does so, this air continues to expand. This is because there is less pressure higher in the atmosphere, allowing the air molecules to spread out more. In order to spread out, these molecules require energy. As they do so, they become less agitated and vibrate slower. As a result, the temperature of these air molecules drops, despite the fact that no heat has been removed from them. This process is referred to as adiabatic cooling……As the air cools down, it again begins to fall towards the surface of the Earth. As it sinks deeper into the atmosphere, the pressure from the weight of the air above it pushes air molecules closer together, causing them to become more agitated and heating them up again. As a result, their temperature rises, even though no heat has been added. This process is referred to as adiabatic warming.

The compression of gas in a cylinder with a piston in combustion engine is a good “real world” example of adiabatic compression (though even it is not a perfect example).

Dr. John Monteverdi is a meteorology professor in the Department of Earth and Climate Sciences at San Francisco State University. He points out another interesting meteorological-physics process at play,

The hydraulic jump phenomenon is also at play. There is “ducting” under the inversion layer if the inversion is just at the top of the mountains, or near a canyon in which the mountain peaks are above it. This explains the sudden increase in winds in certain areas around Santa Rosa.

An inversion is a layer in the atmosphere in which temperature increases with altitude. It serves as a cap of stability. The ducting is essentially a forced channeling of the winds beneath that “cap.”

The Santa Ana Winds in southern California may be more familiar to people. It is a similar process and can often fan fires in that region. National Weather Service-Oxnard Meteorologist Curt Kaplan points out that these wind systems can be a bit more complex than just subsidence. He messaged,

There are several factors that can generate a Santa Ana event in Southern California. The majority of our weak to moderate Santa Ana events are driven by pressure gradients (difference in pressure over a given distance). When the high pressure is to the east of the region, the winds are impacted by the terrain-induced friction.

Kaplan notes that stronger Santa Ana events also have cold air at higher levels, typically associated with an upper-level low-pressure system. Kaplan goes on to say “I do not know the local effects of the northern California offshore events, but I do know the Southern California events.”

As with the Diablo Winds, the Santa Ana Winds fan the fires if there is ample “fuel” and dryness. October is the time of year for Santa Ana Winds so alert levels are particularly high for fires in other parts of California too. Ironically, a wet 2016 has provided ample fuels for this season as that vegetation dries.


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