Winter Solstice and Current Weather

The winter solstice occurs at 9:30 PST this evening.  At that time, the sun will be directly over the Tropic of Capricorn, or 23.5 degrees S, in the Southern Hemisphere, the southernmost location on Earth at which the sun is directly overhead.  This represents the longest day of daylight for Southern Hemisphere locations and the shortest daylight period here in the Northern Hemisphere.  Starting tomorrow, the daylight period will become incrementally larger for us until the Summer Solstice next June.

It certainly hasn't felt much like winter here in the Bay Area - we have received very little rain this month and are quickly building up a precipitation deficit.  A persistent upper-level ridge of high pressure located over the eastern Pacific has been responsible for the dry but occasionally cool and windy weather.  This pattern will likely last into early January.
Right now, a strong offshore pressure gradient is developing across California, something we have seen repeatedly this month.  Just of the past couple hours, these offshore winds have produced major temperature and humidity differences across the Bay Area.  In the North Bay, temps rose several degrees and humidities dropped - after the sun went down.  This was due to northeasterly flow descending the Sierra and then the Coastal Range.  As air descends, it is compressed, causing it to warm and dry.  The graph attached shows the temperature, dew point, and relative humidity at the Santa Rosa airport for the past three days.  Notice the final few hours - temperature jump and dew point (and relative humidity) decrease.  The temperature jumped from 42 to 58 between 6 and 7 p.m., a pretty remarkable increase with the sun already down.  Similar conditions were experienced in Napa and some locations in the Central Valley.  Oakland, meanwhile, is already down to 40 degrees with 86% relative humidity.

Happy Winter!