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 year. 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%).
Over the course of a year, the temperature typically varies from 44°F to 73°F and is rarely below 37°F or above 82°F.
The warm season lasts from June 13 to October 19 with an average daily high temperature above 69°F. The hottest day of the year is September 2, with an average high of 73°F and low of 56°F.
The cold season lasts from December 4 to February 8 with an average daily high temperature below 58°F. The coldest day of the year is December 26, with an average low of 44°F and high of 55°F.
The length of the day varies significantly over the course of the year. The shortest day is December 21 with 9:34 hours of daylight; the longest day is June 20 with 14:46 hours of daylight.
The earliest sunrise is at 5:47am on June 15 and the latest sunset is at 8:36pm on June 26. The latest sunrise is at 7:37am on November 3 and the earliest sunset is at 4:51pm on December 9.
Daylight savings time (DST) is observed in this location during 2012, starting in the spring on March 11 and ending in the fall on November 4.
The median cloud cover ranges from 33% (mostly clear) to 68% (partly cloudy). The sky is cloudiest on February 1 and clearest on September 21. The clearer part of the year begins around April 22. The cloudier part of the year begins around November 14.
On September 21, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 74% of the time, and overcast or mostly cloudy 26% of the time.
On February 1, the cloudiest day of the year, the sky is overcast, mostly cloudy, or partly cloudy 67% of the time, and clear or mostly clear 33% of the time.
The probability that precipitation will be observed at this location varies throughout the year. Precipitation is most likely around February 4, occurring in 43% of days. Precipitation is least likely around July 9, occurring in 10% of days.
Over the entire year, the most common forms of precipitation are light rain, moderate rain, and drizzle.
Light rain is the most severe precipitation observed during 40% of those days with precipitation. It is most likely around March 10, when it is observed during 17% of all days.
Moderate rain is the most severe precipitation observed during 30% of those days with precipitation. It is most likely around February 4, when it is observed during 17% of all days.
Drizzle is the most severe precipitation observed during 18% of those days with precipitation. It is most likely around August 7, when it is observed during 9% of all days.
During the warm season, which lasts from June 13 to October 19, there is a 13% average chance that precipitation will be observed at some point during a given day. When precipitation does occur it is most often in the form of drizzle (53% of days with precipitation have at worst drizzle), light rain (28%), and moderate rain (13%).
During the cold season, which lasts from December 4 to February 8, there is a 39% average chance that precipitation will be observed at some point during a given day. When precipitation does occur it is most often in the form of light rain (41% of days with precipitation have at worst light rain), moderate rain (36%), heavy rain (13%), and drizzle (6%).
Either snow is exceptionally unlikely to fall at any time during the year at this location or this station does not reliably report precipitation types.
The relative humidity typically ranges from 49% (comfortable) to 94% (very humid) over the course of the year, rarely dropping below 27% (dry) and reaching as high as 100% (very humid).
The air is driest around October 7, at which time the relative humidity drops below 61% (mildly humid) three days out of four; it is most humid around January 4, exceeding 91% (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 year, the dew point typically varies from 38°F (dry) to 57°F (comfortable) and is rarely below 28°F (dry) or above 61°F (comfortable).
The time of the year between April 26 and November 21 is the most comfortable, with dew points that are neither too dry nor too muggy.
Over the course of the year typical wind speeds vary from 0 mph to 24 mph (calm to fresh breeze), rarely exceeding 31 mph (high wind).
The highest average wind speed of 14 mph (moderate breeze) occurs around June 11, at which time the average daily maximum wind speed is 24 mph (fresh breeze).
The lowest average wind speed of 7 mph (light breeze) occurs around January 11, at which time the average daily maximum wind speed is 13 mph (moderate breeze).