It’s been a dreadful couple of weeks for California. Raging wildfires have consumed more than 87,000 hectares (217,000 acres), an area larger than New York City. At least 41 people have died, 75,000 people have had to be evacuated and an estimated 5,700 homes and businesses have been destroyed.

The wine country to the north of San Francisco has been particularly hard hit, with entire towns having to be evacuated. By contrast, Denver, on the other side of the Rocky Mountain chain, received its first snowfall of the year. The co-incidental timing of these two very different weather events was far from a chance occurrence.

The strong high-pressure system that brought the snow over Colorado also fed the dry easterly winds, which rolled over the Rocky Mountains and funnelled through the canyons of the Sierra Nevada. As it tumbled downwards this dry air compressed and warmed, so becoming drier still. By the time the winds reached Sonoma county, California, they exceeded hurricane force gusting more than 77mph (125km/h). All that was needed was a spark and fire was inevitable.

Such winds are known as El Diablo (the devil) in northern California and Santa Ana in southern California, and, combined with a warmer, drier California, these winds are increasingly likely to be associated with wildfires in future.

Research published two years ago in the Proceedings of the National Academy of Sciences showed that human activities have increased the probability that dry years will coincide with warm years. And as these tinder-box conditions become increasingly common the golden state will need to be on high alert each time El Diablo and Santa Ana sweep in.



