Wind, Humidity, and the California Fires


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Water is growing scarce in California. Image courtesy of USGS

Fire and water have been on the weather menu this week in California. The state’s multi-year drought has left half of California in what’s called an exceptional drought, the highest level on the drought scale.

While fires often occur in sparsely-populated areas that contain dry brush and forested land, one of this week’s fires was a definite exception.

A brush fire near the I-15 moved onto the highway itself, burning at least 20 vehicles.

The Cajon Pass Interstate Fire

The fire that swept across the Interstate began on Friday afternoon in the Cajon Pass area 55 miles northeast of Los Angeles. It moved quickly up a hill, pushed by the desert winds. Once on the highway, vehicles became fuel for the flames, and some cars exploded as the drivers and passengers ran down the highway. No one was injured.

Traffic was busy as the highway is a main connector between Las Vegas and southern California. The flames then moved into the community of Baldy Mesa, scorching an additinal 44 vehicles and burning 3 homes. Over the weekend, light rain and higher levels of moisture in the air helped firefighters contain the blaze.

What Role Does Wind Play in a Wildfire’s Spread?

Anyone who’s blown on a campfire knows that wind can play different roles in fueling and spreading a fire. Oxygen is essential to a fire, and without oxygen a fire will gradually die. Wind not only provides more oxygen, it moves as well.

A strong wind can push a fire into new areas, and the stronger the wind near the fire, the faster the fire moves. Wind also makes the fire flatter, which pre-heats the brush or forest. This well-heated fuel catches fire easily, especially when the wind moves sparks in its direction.

Wind moves into the area of least resistance, just as water does. With its varied terrain of mountains, valleys, and canyons, California’s landscape means that fires may rush along hills or through canyons, pushed by the wind.

California’s long drought has led to ideal fire conditions. Photo: USGS

What Role Does Humidity Play During a Wildfire?

Over the weekend, the Cajon Pass fire diminished due to light rain and humidity. During the fire season, any rain is a welcome relief.

While rain doesn’t automatically quench a fire, it can help firefighters contain the fire as it dampens potential fuel. However, in an environment that’s very short on rain, firefighters have another ally as well: relative humidity.

While pouring rain or even light rain can dampen a fire, what role does the overall humidity in the air play in fire management? The moisture in the air is called Relative Humidity, which is abbreviated as RH. This is the ratio of moisture in the air compared to the amount of moisture that would saturate the air, and it’s measured as a percentage.

When you see an RH number, think about it as a number that you can compare to previous days or years to show you how “wet” the air is at the moment. It doesn’t need to be raining to be humid.

Materials in the environment constantly exchange moisture with the air around them. Think about a silk scarf on a clothesline. This lightweight fabric will dry very quickly on a hot, dry day, sending moisture into the air around it. On a humid day when the dew falls in the morning and the scarf is still on the clothesline, the scarf will be damp, even though there has been no rain. It receives moisture from the air.

The same holds true for materials in our forests. If the air is very dry, moisture will move from plant material into the air, making those plant materials drier. They become even better fuel for forest fires. When the air is damp, those plant materials become less dry and will not catch fire as easily. This reduces the risk of forest fires. This is particularly true for lighter fuels such as needles. Like the silk scarf, these gain and lose moisture easily. Even though it is not rain, the relative humidity can impact the likelihood that a specific plant will become fuel for a fire.

California Wildfires in July

What’s in store for California for the rest of July? While unusual seasonal rainfall has dampened wildfires and the fire risk in the southern part of the state, the prevailing conditions are still those of exceptional drought. Between January and July of this year, firefighters responded to more than 3,300 wildfires, 1000 more than in previous years. As of July 20th, 19 notable fires of varying sizes were still burning in the state. With California’s drought in its 4th year, the rains in the south are bringing relief but not a complete reprieve from the state’s wildfire season.

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