September weather forecastSan Francisco, CA
September
Weather in September
San Francisco takes on a lovely hue in September as summer begins to make way for fall. This month marks a transition from the mild temperatures of August to slightly cooler ones, with temperatures turning a bit nippy, especially after the sun goes down. September signals the start of Indian summer, a period of warm, dry weather often experienced in the city. The city’s skyline is often strikingly clear, offering unobstructed views of its architectural charm. The climatic conditions transition further into fall as October arrives.Temperature
San Francisco's temperatures peak on average in September, reaching 73.2°F (22.9°C) at their highest and 56.7°F (13.7°C) at their lowest.Humidity
The average relative humidity in September in San Francisco is 69%.Rainfall
July through September are months with the least rain in San Francisco when the rain falls for 2.5 days and typically aggregates up to 0.08" (2mm) of rain.Ocean temperature
With an average ocean temperature of 57.4°F (14.1°C), September is the month with the warmest seawater in San Francisco.Note: Taking a dip in 57.4°F (14.1°C) can be perilous. Submersion in temperatures under 50°F (10°C) triggers severe cold shock accompanied by hyperventilation.
Daylight
In San Francisco, the average length of the day in September is 12h and 25min.On the first day of the month, sunrise is at 6:40 am and sunset at 7:38 pm. On the last day of September, in San Francisco, sunrise is at 7:05 am and sunset at 6:53 pm PDT.
Sunshine
In San Francisco, California, the average sunshine in September is 10.9h.UV index
The months with the highest UV index are April, June through September, with an average maximum UV index of 5. A UV Index of 3 to 5 symbolizes a moderate threat to health from exposure to the Sun's UV radiation for ordinary individuals.Note: The maximum UV index of 5 during September leads to these instructions:
Ensure safety - Imperative it is to shield skin and eyes from damage. Solar radiation is at its peak intensity during midday, so it's advisable to stay indoors or in the shade. In the presence of strong sunlight, it's crucial to wear sunglasses that ward off UVA and UVB rays.
Published by: Weather U.S. | About Us
Data Sources | Weather Forecasting & Climate
Frequently asked questions
Average temperature in September
San Francisco, CA
- Average high temperature in September: 73.2°F
The warmest month (with the highest average high temperature) is September (73.2°F).
The month with the lowest average high temperature is December (55.8°F).
- Average low temperature in September: 56.7°F
The month with the highest average low temperature is September (56.7°F).
The coldest months (with the lowest average low temperature) are January and December (45.9°F).
Average pressure in September
San Francisco, CA
- Average pressure in September: 29.92"Hg
The month with the highest atmospheric pressure is January (30.13"Hg).
The months with the lowest atmospheric pressure are June, August and September (29.92"Hg).
Average wind speed in September
San Francisco, CA
- Average wind speed in September: 5.8mph
The windiest month (with the highest average wind speed) is May (7.6mph).
The calmest month (with the lowest average wind speed) is November (5.5mph).
Average humidity in September
San Francisco, CA
- Average humidity in September: 69%
The month with the highest relative humidity is March (76%).
The month with the lowest relative humidity is October (67%).
Average rainfall in September
San Francisco, CA
- Average rainfall in September: 0.08"
The wettest month (with the highest rainfall) is February (1.38").
The driest months (with the least rainfall) are July, August and September (0.08").
Average rainfall days in September
San Francisco, CA
- Average rainfall days in September: 2.5 days
The month with the highest number of rainy days is March (9.3 days).
The month with the least rainy days is June (1.9 days).
Average sea temperature in September
San Francisco, CA
- Average sea temperature in September: 57.3°F
The best month for swimming (with the highest average sea temperature) is September (57.3°F).
The coldest months (with the lowest average sea temperature) are April and June (53°F).
Average daylight in September /
Average sunshine in September
San Francisco, CA
- Average daylight in September: 12h and 24min
The month with the longest days is June (Average daylight: 14h and 42min).
The month with the shortest days is December (Average daylight: 9h and 36min).
- Average sunshine in September: 10h and 54min
The month with the most sunshine is July (Average sunshine: 11h and 48min).
The month with the least sunshine is December (Average sunshine: 5h and 48min).
Average sunshine days in September
San Francisco, CA
- Average sunshine days in September: 24.3 days
The month with the most sunshine days is October (25.8 days).
The month with the least sunshine days is March (20.8 days).
Average UV index in September
San Francisco, CA
- Average UV index in September: 5
The months with the highest UV index are April, June, July, August and September (UV index 5).
The months with the lowest UV index are January, February, November and December (UV index 3).
Average cloud cover in September
San Francisco, CA
- Average cloud cover in September: 14
The month with the most cloud cover is March (Cloud cover 36).
The months with the least cloud cover are July and September (Cloud cover 14).
Average visibility in September
San Francisco, CA
- Average visibility in September: 6mi
The months with the highest visibility are January, February, March, April, May, June, July, August, September, October, November and December (6mi).
The months with the lowest visibility are January, February, March, April, May, June, July, August, September, October, November and December (6mi).
