Average Weather in May in San Francisco California, United States
Daily high temperatures are around 67°F, rarely falling below 60°F or exceeding 77°F.
Daily low temperatures are around 54°F, rarely falling below 50°F or exceeding 59°F.
For reference, on September 20, the hottest day of the year, temperatures in San Francisco typically range from 58°F to 72°F, while on January 3, the coldest day of the year, they range from 45°F to 56°F.
Average High and Low Temperature in May
The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on May. The horizontal axis is the day, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.
Average Hourly Temperature in May
La Serena, Chile (5,732 miles away); Cilaos, Réunion (11,292 miles); and Albany, Australia (9,147 miles) are the far-away foreign places with temperatures most similar to San Francisco (view comparison).
The month of May in San Francisco experiences decreasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy decreasing from 38% to 27%.
The clearest day of the month is May 31, with clear, mostly clear, or partly cloudy conditions 73% of the time.
For reference, on January 11, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 55%, while on July 20, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 92%.
Cloud Cover Categories in May
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In San Francisco, the chance of a wet day over the course of May is decreasing, starting the month at 9% and ending it at 5%.
For reference, the year's highest daily chance of a wet day is 31% on February 20, and its lowest chance is -0% on July 25.
Probability of Precipitation in May
To show variation within the month and not just the monthly total, we show the rainfall accumulated over a sliding 31-day period centered around each day.
The average sliding 31-day rainfall during May in San Francisco is gradually decreasing, starting the month at 0.7 inches, when it rarely exceeds 2.0 inches, and ending the month at 0.3 inches, when it rarely exceeds 1.2 inches.
Average Monthly Rainfall in May
Over the course of May in San Francisco, the length of the day is increasing. From the start to the end of the month, the length of the day increases by 49 minutes, implying an average daily increase of 1 minute, 38 seconds, and weekly increase of 11 minutes, 28 seconds.
The shortest day of the month is May 1, with 13 hours, 46 minutes of daylight and the longest day is May 31, with 14 hours, 36 minutes of daylight.
Hours of Daylight and Twilight in May
The latest sunrise of the month in San Francisco is 6:13 AM on May 1 and the earliest sunrise is 24 minutes earlier at 5:49 AM on May 31.
The earliest sunset is 8:00 PM on May 1 and the latest sunset is 25 minutes later at 8:25 PM on May 31.
Daylight saving time is observed in San Francisco during 2018, but it neither starts nor ends during May, so the entire month is in daylight saving time.
For reference, on June 21, the longest day of the year, the Sun rises at 5:48 AM and sets 14 hours, 47 minutes later, at 8:35 PM, while on December 21, the shortest day of the year, it rises at 7:21 AM and sets 9 hours, 33 minutes later, at 4:54 PM.
Sunrise & Sunset with Twilight in May
We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.
The chance that a given day will be muggy in San Francisco is essentially constant during May, remaining around 0% throughout.
For reference, on August 22, the muggiest day of the year, there are muggy conditions 0% of the time, while on January 1, the least muggy day of the year, there are muggy conditions 0% of the time.
Humidity Comfort Levels in May
This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.
The average hourly wind speed in San Francisco is essentially constant during May, remaining within 0.1 miles per hour of 9.5 miles per hour throughout.
For reference, on May 30, the windiest day of the year, the daily average wind speed is 9.7 miles per hour, while on October 21, the calmest day of the year, the daily average wind speed is 7.2 miles per hour.
The highest daily average wind speed during May is 9.7 miles per hour on May 31.
Average Wind Speed in May
Wind Direction in May
San Francisco is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water.
The average surface water temperature in San Francisco is essentially constant during May, remaining around 53°F throughout.
The lowest average surface water temperature during May is 53°F on May 4.
Average Water Temperature in May
Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).
Temperatures in San Francisco are sufficiently warm year round that it is not entirely meaningful to discuss the growing season in these terms. We nevertheless include the chart below as an illustration of the distribution of temperatures experienced throughout the year.
Time Spent in Various Temperature Bands and the Growing Season in May
Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.
The average accumulated growing degree days in San Francisco are increasing during May, increasing by 272°F, from 609°F to 881°F, over the course of the month.
Growing Degree Days in May
This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.
The average daily incident shortwave solar energy in San Francisco is gradually increasing during May, rising by 0.9 kWh, from 6.9 kWh to 7.8 kWh, over the course of the month.
Average Daily Incident Shortwave Solar Energy in May
For the purposes of this report, the geographical coordinates of San Francisco are 37.775 deg latitude, -122.419 deg longitude, and 59 ft elevation.
The topography within 2 miles of San Francisco contains significant variations in elevation, with a maximum elevation change of 597 feet and an average elevation above sea level of 128 feet. Within 10 miles contains significant variations in elevation (1,270 feet). Within 50 miles contains very significant variations in elevation (3,894 feet).
The area within 2 miles of San Francisco is covered by artificial surfaces (82%) and water (15%), within 10 miles by water (62%) and artificial surfaces (29%), and within 50 miles by water (41%) and grassland (20%).
This report illustrates the typical weather in San Francisco year round, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016.
Temperature and Dew Point
There are 2 weather stations near enough to contribute to our estimation of the temperature and dew point in San Francisco.
For each station, the records are corrected for the elevation difference between that station and San Francisco according to the International Standard Atmosphere , and by the relative change present in the MERRA-2 satellite-era reanalysis between the two locations.
The estimated value at San Francisco is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between San Francisco and a given station.
All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Tables of the Sun, Moon and Planets , by Jean Meeus.
All other weather data, including cloud cover, precipitation, wind speed and direction, and solar flux, come from NASA's MERRA-2 Modern-Era Retrospective Analysis . This reanalysis combines a variety of wide-area measurements in a state-of-the-art global meteorological model to reconstruct the hourly history of weather throughout the world on a 50-kilometer grid.
Land Use data comes from the Global Land Cover SHARE database , published by the Food and Agriculture Organization of the United Nations.
Elevation data comes from the Shuttle Radar Topography Mission (SRTM) , published by NASA's Jet Propulsion Laboratory.
Names, locations, and time zones of places and some airports come from the GeoNames Geographical Database .
Time zones for aiports and weather stations are provided by AskGeo.com .
Maps are © Esri, with data from National Geographic, Esri, DeLorme, NAVTEQ, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, and iPC.
The information on this site is provided as is, without any assurances as to its accuracy or suitability for any purpose. Weather data is prone to errors, outages, and other defects. We assume no responsibility for any decisions made on the basis of the content presented on this site.
We draw particular cautious attention to our reliance on the MERRA-2 model-based reconstructions for a number of important data series. While having the tremendous advantages of temporal and spatial completeness, these reconstructions: (1) are based on computer models that may have model-based errors, (2) are coarsely sampled on a 50 km grid and are therefore unable to reconstruct the local variations of many microclimates, and (3) have particular difficulty with the weather in some coastal areas, especially small islands.
We further caution that our travel scores are only as good as the data that underpin them, that weather conditions at any given location and time are unpredictable and variable, and that the definition of the scores reflects a particular set of preferences that may not agree with those of any particular reader.