Dome of Warm Air Turning Up the Heat

Unless you spent all of Monday in a mall or the frozen food section of your local market, you don’t need a weather person to tell you it was unseasonably warm in the San Fernando Valley. But here are the stats.

It hit 106 degrees in both Van Nuys and Woodland Hills at approximately 3 p.m., making them the hottest spots in the Valley, according to Rob Krohn of the National Weather Service office in Oxnard.

Northridge hit 105 degrees and it got up to 102 in Burbank, Krohn said. And the Santa Ana wind season has yet to begin.

“This was not a surface condition, it had nothing to do with winds,” said meteorologist Wes Etheredge of WeatherData Inc., which provides weather information to The Times.

The culprit, rather, was an upper-level ridge sitting over California. Etheredge described it as a “dome of warm air” that is sinking, making temperatures near the ground soar.

“We call it compressional warming,” he said.

“It’s like when you put air in your tires,” Krohn explained. “The stem gets warm because the air is compressing. When you let the air out, it feels cool, because it’s expanding.”

The upper level ridge is, unfortunately, moving only slowly to the east. “We probably won’t get a slight cooling until Thursday or Friday,” Krohn said.