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Name: Vincent
Status: other
Age: 40s
Location: N/A
Country: N/A
Sunday, December 01, 2002

I have lived my entire life in California. Why does it seem that the vast majority of the time it rains a lot only at night, and during the daytime the rain is only light or intermittent? Thank you for your answer.

Dear Mr. Vincent-

California has a great variety of weather patterns, due to the presence of the Pacific Ocean on the west, and the topographical effects of the Sierra Mountains on the east. Climate regimes can vary greatly in just a few miles because of those factors.

Some types of precipitation are diurnally affected, such as thunderstorms that result from surface heating. These storms usually form in the afternoon, and may extend well into the nighttime, before dissipating in the early morning. Other thunderstorms associated with frontal movements may occur at any time, when conditions are favorable.

The heavy rains you mention usually occur in California as a result of storms moving onshore from the Pacific ocean. But time of day usually is not a factor in these precipitation events. These storms may seem to occur during the evening and night, because people tend to be at home more at those times, and are more aware of adverse weather conditions. During the daytime, when they are at work, these rain storms may be less noticed.

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO

The rainfall pattern is very geography-dependent, even in California the Salinas Valley, and the Redwood forests have very different rainfall pattern -- and Death Valley gets precious little. On the ocean side of the mountains it rains periodically due to the influx of moisture off the ocean during the daytime which rises up the westward slopes and then when the sun goes down the air cools and compresses until it falls below the dew point and rain results. On the eastern slopes of the same mountains it may not rain very much at all unless the prevailing winds are strong enough to carry the moisture over the mountain peaks. So you can see it is pretty complicated.

Vince Calder

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