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 April. 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 April is characterized by gradually rising daily high temperatures, with daily highs around 63°F throughout the month, exceeding 74°F or dropping below 56°F only one day in ten.
Over the course of April, the length of the day is increasing. From the start to the end of the month, the length of the day increases by 1:06 hours, implying an average day-over-day increase of 2.2 minutes.
The shortest day of the month is April 1 with 12:39 hours of daylight; the longest day is April 30 with 13:45 hours of daylight.
The earliest sunrise is at 6:15am on April 30; the latest sunset is at 7:59pm on April 30; the latest sunrise is at 6:54am on April 1; the earliest sunset is at 7:33pm on April 1.
Daylight savings time is observed in 2012 but it neither starts nor ends in April, so the entire month is in daylight time.
The median cloud cover ranges from 49% (partly cloudy) to 56% (partly cloudy). The trend is for the sky to be clearing with the median cloud cover decreasing by 7% from the start to the end of the month.
On April 15, a representative day, the sky is clear or mostly clear 41% of the time, partly cloudy 22% of the time, and mostly cloudy or overcast 37% of the time.
The probability that precipitation will be observed at this location varies throughout the month. Precipitation is most likely around April 1, occurring in 34% of days. Precipitation is least likely around April 30, occurring in 24% of days.
Throughout April, the most common forms of precipitation are light rain, moderate rain, and drizzle.
Light rain is the most severe precipitation observed during 46% of those days with precipitation. It is most likely around April 1, when it is observed during 15% of all days.
Moderate rain is the most severe precipitation observed during 31% of those days with precipitation. It is most likely around April 1, when it is observed during 11% of all days.
Drizzle is the most severe precipitation observed during 14% of those days with precipitation. It is most likely around April 30, when it is observed during 5% of all days.
Either snow is exceptionally unlikely to fall at this location in April or this station does not reliably report precipitation types.
The relative humidity typically ranges from 51% (mildly humid) to 89% (very humid) over the course of a typical April, rarely dropping below 34% (comfortable) and reaching as high as 99% (very humid).
The air is driest around April 28, at which time the relative humidity drops below 60% (mildly humid) three days out of four; it is most humid around April 1, rising above 85% (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 April, the dew point typically varies from 40°F (dry) to 50°F (very comfortable) and is rarely below 32°F (dry) or above 55°F (comfortable).
Over the course of April typical wind speeds vary from 3 mph to 23 mph (light air to fresh breeze), rarely exceeding 30 mph (strong breeze).
The highest average wind speed of 13 mph (moderate breeze) occurs around April 30, at which time the average daily maximum wind speed is 23 mph (fresh breeze).
The lowest average wind speed of 12 mph (gentle breeze) occurs around April 1, at which time the average daily maximum wind speed is 21 mph (fresh breeze).