![]() Half Moon Bay, on the San Mateo coast, gets about 29 inches of rain a year Palo Alto, 15 miles to the southeast, and across ridges that rise to more than 2,000 feet, gets about 16 inches. ![]() Similar, if less startling differences, can be found throughout the region. That's a 4-to-1 ratio in about 15 miles as the crow flies." "You get up around the Ben Lomond area in the Santa Cruz Mountains, you get around 60 inches. "At San Jose airport, you get 15 inches of rain a year," Null says. Perhaps the best-known example of a local contrast between a wet site and a nearby dry site is in the South Bay. Bay Area meteorologist Jan Null points out in his narrative summary of San Francisco's climate that the city's average annual rainfall varies from more than 22 inches in the hills in south-central neighborhoods to 18 inches in the city's northeastern corner. Hese effects - enhanced orographic rainfall and corresponding rain shadows - can lead to surprisingly dramatic differences in rainfall over short distances. Since rain will start much sooner in such orographically favored areas, he says, "those places get a huge amount of rain, both in terms of intensity and duration." "Technically, this is called a warm-rain process - you can picture it as really, really intense drizzle that gets added to the storm system." and they're adding a whole bunch of water to clouds at a low level as the air runs up the mountain slopes," Lareau says. The southwest-facing mountains "are lifting that really moist layer of marine air. ![]() Neil Lareau, a professor of atmospheric sciences at the University of Nevada, Reno, says the orientation of the mountains and ridges in the Bay Area - they generally face southwest - is also important in squeezing out more rainfall from storm winds blowing from that direction. On the east side, then you get sinking air, which is detrimental to the formation of precipitation." So you get vertical motion, you get more air rising up, you get more precipitation out of it. "That process lifts that air up, and our better precipitation-producing clouds are those that have stronger vertical motions. that air flow that's below two or three thousand feet, or in the case of San Francisco below a thousand feet, it can't run through the mountains, so it gets forced up over the top," says veteran Bay Area meteorologist Jan Null. ![]() When moisture-laden air is driven across coastal terrain by vigorous winter storm winds, that process of orographic lifting leads to significantly higher amounts of rainfall over the mountains and hills.Īt the same time, areas to the east of the high terrain are in "rain shadows" that receive dramatically lower amounts of precipitation than sites in or to the west of the mountains. ![]()
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