Understanding The California Fires


The numerous wildfires that have broken out across California over the last few days have made national headlines. While wildfires aren’t uncommon for this area, the weather plays a huge factor in how quickly the fires can spread. Here I outline some of the factors that allowed these fires to grow so rapidly.


Obviously dry conditions are a major factor in allowing fires to develop and spread but how does a wet spring do the same? From January 1st 2017 through June 11th 2017, the Sacramento International Airport received 22.14 inches of rain, a record breaking amount. This allowed plants and other vegetation to flourish and grow more than they had in decades. 

- Advertisement -

Once June came and they stopped receiving rain, all of this vegetation rapidly dried up, becoming the perfect tinder for wildfires.

In addition to the lack of precipitation, temperatures in Northern California were well above average for the period stretching from July through September and many stations set daily record high temperatures for many days in a row. Several other stations recorded all time record highs, where records go back as far as 150 years.


For the last month, there has been a large area of high pressure, commonly called a ridge, over areas of the Western Rockies and the Eastern Sierra-Nevadas. Air will typically flow away from these high pressure areas and towards low pressure areas in an effort to “equalize” the pressure. In this particular situation, this caused winds to flow from east to west over the Sierra Nevada mountains.

As the winds push over the mountains the air compresses and three things happen. The air warms, it speeds up, and the humidity drops significantly.

Photo Credit: The Washington Post

Let’s put this into perspective. Imagine that to the east of the mountains, there is some cool, moist air that has a temperature of 39° and a dew point of 34°. This air has a relative humidity of 82% which is considered pretty humid for a winter air mass. As that air moves over the mountain top, it will sink down to the lower surface elevation where the temperature is 80°. The dew point of this air doesn’t chance, but the relative humidity does. The new relative humidity is 19%, which is very, very dry. This enhances the already dry conditions in these drought stricken areas.

As this air rushes down the mountain, it is also being compressed. You may have experienced this phenomena in your ears while driving down a mountain. The air has nowhere to go when being compressed, so it compromises by speeding up. That is why winds on the down-slopes of mountain ranges tend to be much faster than those on the up-slope sides. This same phenomena often occurs near Denver and other eastern Colorado cities.

This past few days, the Sacramento area saw wind gusts upwards of 30 mph and a few areas saw wind gusts over 45 mph. This just allowed the already burning fires to spread quickly and efficiently across thousands of acres. In addition, these strong winds also can bring down trees and other structures that may have already been compromised by fire.

Unfortunately, the conditions that lead to these fires don’t seem to be changing anytime soon. These fires have been relentless causing air quality issues across the West Coast. In fact, smoke from these fires can easily be seen on visible satellite imagery. As of 10 PM on Wednesday October 11th, Red Flag warnings are in effect for much of the California Bay Area