The Sacramento and San Joaquin Valleys are areas of variable relative humidity.  During the warm season, humidities are characteristically low and occasionally, under the influence of the “norther”, readings may drop to below 10 percent.  In the delta area, at the confluence of the Sacramento and San Joaquin Rivers, a strong inflow of marine air during the summer creates a transition zone between the high humidities of the coast and the low readings of the interior.  Winter values are usually moderate to high.  A shallow layer of ground fog, known locally as “tule fog”, frequently forms at night and can persist for as long as two or three weeks.

Many California thunderstorms produce so little precipitation that range and forest fires often result from the lightning strikes, although heavy precipitation occasionally results. Some flash flooding has been reported as a result of thunderstorms.  Hail up to one-half inch in diameter is sometimes reported, but serious hail damage is infrequent.

FLOODS – In southern California most flooding is the result of heavy precipitation over periods of one or two days.  The short streams and steep watersheds emptying onto lowlands that may be heavily populated, produce large volumes of water within short periods and damage is often severe.  The problem is sometimes compounded by the denuding of large areas of watershed by fire during the previous season.