California Fires: Why We're Praying for Onshore Wind

Tony Butt

by on

Updated 35d ago

Cover shot of Huntington Beach with smoke billowing offshore from a few years ago. By Pat Nolan

For most of us, an offshore wind is good news. A warm, offshore breeze in the morning, gently caressing the waves just enough to keep the tubes open, at a surf spot backed by a beautiful mountain range, sounds idyllic to most people. But for others, the offshore wind is starting to become a thing of dread. Especially if you are in California.

The Santa Ana wind in the south, and the Diablo wind in the north, are strong, hot, dry offshore winds that are a major contributing factor to the fires currently raging across California and the Pacific Northwest. These winds blow down the western side of the mountains, accelerating as they constrict through narrow passes, and compressing as they descend, becoming extremely hot and dry when they reach the coast. They are a perfect natural mechanism for fanning the flames and propagating the fire.

Spot guide: California

The Santa Ana and the Diablo winds are a type of wind that occurs in a similar way in many parts of the world. The generic type of wind they belong to is called the Föhn – a term originally used in German-speaking regions of the Alps where the phenomenon was first studied. Föhn winds tend to occur on the leeward side of mountain ranges, and they can last up to several days or weeks. They can cause temperature rises at the bottom of the mountain of over 10°C in the space of a few minutes, and, as well as being one of the main contributors to fires, they can also cause snow to melt very quickly in mountain areas, increasing the risk of avalanches.

Fohn wind.

Fohn wind.

This is basically how it works:
Wind at sea level (not necessarily at the coast) blows towards a mountain or range of mountains. In the case of the Santa Ana and Diablo winds, this initial air flow is around the southern flank of large, well-established high-pressure system in the desert regions to the east. As the air mass hits the eastern slopes of the mountains, it is forced upwards towards the top of the mountain. This process is called orographic lifting.

As the air mass gains altitude, it finds itself surrounded by a lower atmospheric pressure, which causes the air mass to expand. The expansion of the air mass causes its temperature to decrease. This is called adiabatic cooling and is a little tricky to understand unless you know a bit about thermodynamics, so I won’t elaborate. The most important thing to know is that as the air is pushed upwards, its temperature decreases at a more-or-less fixed rate: around 1°C per 100 m elevation.

Now, as the rising air mass cools, it progressively loses its ability to hold moisture. This is another basic principle of physics: the fact that warm air can hold more moisture than cold air. That is why, for example, you get condensation on your windows – when warm air hits a cold surface and the moisture ‘hidden’ inside the air precipitates out as water droplets. Therefore, at some point in its journey up the side of the mountain, the air mass reaches a stage where it can no longer hold the moisture that it originally contained. This moisture precipitates out, forming clouds and perhaps rain or snow.

View this post on Instagram

Over the past few weeks, parts of Washington, Oregon, and California have been devastated by massive wildfires. While many of these fires were started by lightening, and a few by human carelessness, climate change is making these fires larger, more intense, and harder to fight. Drought and heat waves created a volatile environment that have led to a more intense wildfire season. Now, these fires are making climate change worse because of increasing CO2 emissions. Together we need to continue to cut our carbon emissions, move towards renewable energy, and vote for leaders that take climate change seriously. Our thoughts are with all of those currently facing these horrific fires and our hopes are for a future where we can reduce these types of catastrophic events. @fraudfix . #triplit

A post shared by TripLit (@triplit) on

The process of condensation releases heat energy into the air, which partly offsets the process of adiabatic cooling. As a result, during the second part of its ascent up the mountain, the temperature of the air drops at a slower rate than before: this time at a more-or-less fixed rate of about 0.6°C per 100 m elevation.

Now, due to the fact that the air cools at two different rates during its ascent up the mountainside (first at 1°C per 100 m elevation and then at 0.6°C per 100 m elevation), the average rate of cooling would be somewhere between those two values. If, for example, the mountain is 1000 m high and the moisture starts to precipitate out at 500 m, the air cools by a total of 8°C.

Hopefully you are still with me.

Eventually, the air mass reaches the top of the mountain, continues over the top and begins to descend down the other side. All the moisture that the air originally contained has precipitated out in the form of clouds, rain or snow, so the air is now extremely dry. As the air mass descends down the other side of the mountain, it finds itself surrounded by a higher atmospheric pressure, which causes it to compress, which, in turn, causes it to get hotter. This is called adiabatic warming, and is the exact opposite to what happened when the air went up the other side (adiabatic cooling).

Now, here is the really crucial part: On its way down, the air warms at 1°C for every 100 m drop in elevation for its entire journey down the mountain. That means that the average cooling of the air going up was less than the average warming of the air going down.

Assuming that the mountain is 1000 m high, the air now warms by a total of 10°C. Two degrees warmer than when it started at the bottom of the mountain on the other side. It is also much dryer because it lost all that moisture at the top. To add to this, the high atmospheric pressure at the bottom of the lee side of the mountain means clear skies and sunny weather.

Therefore, you have a hot, extremely dry mass of air, accelerating down a mountain and hitting an area where temperatures are already high and the ground is already dry. In these conditions, any small fire will quickly turn into an inferno, and people will be praying for onshore winds and rain.

Read more by forecaster Tony Butt, HERE