Ask A Scientist Weather Archive

name         Vincent
status       other
age          40s

Question -   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.
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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
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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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