You know the old rule: Temperature declines as you gain elevation. Here’s an adjunct to that: Except when it doesn’t. It’s fairly common in the San Francisco Bay Area to have cool marine air trapped under a layer of much warmer air. It can be a startling experience to start a walk in the cool damp air in our flatlands neighborhood and cross suddenly–in the matter of just a few feet–into very warm, much drier air.
There’s a beautiful case in point this morning. The lower elevations around the Bay are cocooned in a blanket of cool, moist air. Here in Berkeley, one station has the temperature as 58 degrees Fahrenheit and 91 percent relative humidity. That’s at an elevation of 361 feet–probably up on campus. At the 1,300-foot level in the hills, less than five miles away as the crow flies, it’s 78 F. with 29 percent humidity. Further afield, atop Mount Diablo (about 20 miles to the east; elevation 3,849), it’s 82 F. and 8 percent while at the base of the mountain (in Clayton, 518 feet) it’s 67 F. and 44 percent.