Environment & Science

Science of Now: Why does LA always heat up in the fall?

Photo by Bill Gracey via Flickr Creative Commons

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With temperatures expected to reach the hundreds this weekend, many people might be wondering, why does Southern California heat up as the rest of the country cools down?

It turns out, the two phenomena are related, said Eric Boldt of the National Weather Service in Oxnard.

"This happens every year," Boldt said.

As storms travel from the Pacific Northwest to Middle America, Boldt said, they drag a high-pressure system of cool air behind them.

That cool air eventually slides down to an area in the Southwest called the Great Basin. It's just northeast of Southern California, roughly between the Sierra Nevada and the Rocky Mountains.

(The Great Basin Region, photo via Kmusser / Creative Commons)

Boldt said the cool air fills up this region over time, like a bowl filling with water.

"So cold air builds up on the other side of the mountains, say over Nevada and Utah, and then it starts to spill over the mountains and heats up quite rapidly, " he said.

Boldt said this happens because the air is compressed as it flows down the mountains, building heat energy as it speeds up. It's a phenomenon called "compressional heating."

(An image from NWS Oxnard showing how air heats up as it travels from the Great Basin to Southern California)

That newly warmed air flows out to a low pressure system just off the California coast. As it does, it brings baking hot temperatures along with it. It often comes with high winds, the legendary Santa Anas. 

If you are hoping this will be the last stretch of fall heat, Boldt has some bad news.

"Every year I go to buy a pumpkin later in October, we seem to be 85 to 95 degrees," he laughed. "So I think you can plan on another heatwave before we get through October."