This report describes the typical weather at the San Francisco International Airport (San Francisco, California, United States) weather station over the course of an average December. It is based on the historical records from 1974 to 2012. Earlier records are either unavailable or unreliable.
San Francisco, California has a mediterranean climate with dry warm summers and mild winters. The area within 25 mi of this station is covered by oceans and seas (48%), built-up areas (16%), grasslands (14%), forests (13%), and shrublands (8%).
The month of December is characterized by gradually falling daily high temperatures, with daily highs ranging from 58°F to 54°F over the course of the month, exceeding 63°F or dropping below 49°F only one day in ten.
Over the course of December, the length of the day is essentially constant. The shortest day of the month is December 27 with 9:34 hours of daylight; the longest day is December 1 with 9:45 hours of daylight.
The earliest sunrise is at 7:06am on December 1; the latest sunset is at 5:01pm on December 31; the latest sunrise is at 7:25am on December 31; the earliest sunset is at 4:51pm on December 9.
Daylight savings time is observed in 2012 but it neither starts nor ends in December, so the entire month is in standard time.
The median cloud cover ranges from 57% (partly cloudy) to 64% (partly cloudy). The trend is for the sky to be becoming cloudier with the median cloud cover increasing by 7% from the start to the end of the month.
On December 16, a representative day, the sky is clear or mostly clear 38% of the time, partly cloudy 20% of the time, and mostly cloudy or overcast 43% of the time.
The probability that precipitation will be observed at this location varies throughout the month. Precipitation is most likely around December 31, occurring in 39% of days. Precipitation is least likely around December 1, occurring in 34% of days.
Throughout December, the most common forms of precipitation are light rain, moderate rain, and heavy rain.
Light rain is the most severe precipitation observed during 43% of those days with precipitation. It is most likely around December 14, when it is observed during 16% of all days.
Moderate rain is the most severe precipitation observed during 34% of those days with precipitation. It is most likely around December 31, when it is observed during 13% of all days.
Heavy rain is the most severe precipitation observed during 13% of those days with precipitation. It is most likely around December 29, when it is observed during 5% of all days.
Either snow is exceptionally unlikely to fall at this location in December or this station does not reliably report precipitation types.
The relative humidity typically ranges from 58% (mildly humid) to 93% (very humid) over the course of a typical December, rarely dropping below 36% (comfortable) and reaching as high as 100% (very humid).
The air is driest around December 1, at which time the relative humidity drops below 70% (humid) three days out of four; it is most humid around December 31, rising above 90% (very humid) three days out of four.
Dew point is often a better measure of how comfortable a person will find the weather than relative humidity because it more directly relates to whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid.
Over the course of a typical December, the dew point typically varies from 38°F (dry) to 49°F (dry) and is rarely below 28°F (dry) or above 55°F (very comfortable).
Over the course of December typical wind speeds vary from 0 mph to 14 mph (calm to moderate breeze), rarely exceeding 26 mph (strong breeze).
The highest average wind speed of 8 mph (light breeze) occurs around December 4, at which time the average daily maximum wind speed is 14 mph (moderate breeze).
The lowest average wind speed of 7 mph (light breeze) occurs around December 31, at which time the average daily maximum wind speed is 13 mph (moderate breeze).