San Francisco in September

How September 2027 scores in San Francisco

• Overall score

  8*/10 (provisional) 8*/10 Excellent

  The main score for San Francisco in September 2027, from 1 to 10, where 10 is best for the visitor. Weather counts for 40%, crowds 25%, daylight 10% and uniqueness 25%. Air quality is shown on its own. How we score

  Overall 8*/10: (0.4×10 + 0.25×5 + 0.1×8) ÷ 0.75 = 6.05 ÷ 0.75 = 8.07 → 8. *From three of the four aspects: uniqueness is not measured for this month, so the other three aspects' weights are scaled to add up to 1.

• Weather

  10/10 10/10 Ideal

  Weather 10/10: the average high is 74.8 °F with 1.8 mm of rain (1991–2020 normals, NOAA National Centers for Environmental Information (NCEI)). That high is inside the 70–86 °F comfort band, so only rain lowers the score.

  How this was scored

  • NOAA National Centers for Environmental Information (NCEI), SAN FRANCISCO INTL AP, CA US (USW00023234), 1991–2020 normals (www.ncei.noaa.gov, checked 2026-10-19)
  • The data file read (www.ncei.noaa.gov, checked 2026-10-19)

  high 74.8 °F: no deduction; rain 1.778 mm: −0.04; 10 − 0.04 = 9.96 → 10

• Crowds

  5/10 5/10 Moderate

  Crowds 5/10, Moderate: in September, passengers arriving at the airport ran at 1.018 times their year's monthly average (median of 2023–2025), compared with the rest of San Francisco's year. Based on passengers arriving at San Francisco International (BTS T-100 Domestic and International Segment, all carriers), US Bureau of Transportation Statistics (BTS). Airport passenger counts include residents and connecting passengers.

  How this was scored

  • passengers arriving at San Francisco International (BTS T-100 Domestic and International Segment, all carriers), US Bureau of Transportation Statistics (BTS) (www.transtats.bts.gov, checked 2026-10-19)
  • The data read (www.transtats.bts.gov, checked 2026-10-19)
  • The data read (www.transtats.bts.gov, checked 2026-10-19)

  2023: 1.031; 2024: 1.018; 2025: 0.978; median 1.018; 10 − 9 × (1.018 − 0.60) / 0.80 = 5.30 → 5

• Daylight

  8/10 8/10 Long days

  Daylight 8/10: in September 2027 the sun is up for 12 h 25 min a day on average, from sunrise to sunset at 37.62° N, 122.36° W (NOAA solar equations).

  How this was scored

  • Destination point 37.62° N, 122.36° W, the weather station's published coordinates (www.ncei.noaa.gov, checked 2026-10-19)
  • Sunrise and sunset by the NOAA Solar Calculator equations, checked against NOAA's own Solar Calculator code (the US Naval Observatory's service didn't answer us) (gml.noaa.gov, checked 2026-10-17)

  mean of 30 days from sunrise to sunset: 12 h 25 min (12.41 h); 1 + 1.5 × (12.41 − 8) = 7.62 → 8

• Uniqueness

  Not measured

  Uniqueness: not measured. The San Francisco Travel festivals and events calendar didn't cover this month when it was checked. See How we score.

• Air quality

  8/10 8/10 Mostly good air

  Air quality 8/10: 46 of 60 measured days in September (2023–2024) were in EPA's Good range for fine particles (PM2.5 at 9.0 µg/m³ or less), at 4 monitors within 25 miles. It isn't part of the overall score. Source: US Environmental Protection Agency, AirData (monitors operated by Bay Area Air Quality Management District).

  How this was scored

  • US Environmental Protection Agency, AirData (monitors operated by Bay Area Air Quality Management District) (aqs.epa.gov, checked 2026-10-19)
  • Oakland monitor (06-001-0009-3), 13.4 miles from the destination point, Population exposure (aqs.epa.gov, checked 2026-10-19)
  • San Pablo monitor (06-013-1004-3), 23.5 miles from the destination point, Population exposure (aqs.epa.gov, checked 2026-10-19)
  • San Francisco monitor (06-075-0005-3), 10.3 miles from the destination point, Population exposure (aqs.epa.gov, checked 2026-10-19)
  • Redwood City monitor (06-081-1001-3), 12.8 miles from the destination point, Population exposure (aqs.epa.gov, checked 2026-10-19)
  • The data read (aqs.epa.gov, checked 2026-10-19)
  • The data read (aqs.epa.gov, checked 2026-10-19)
  • The data read (aqs.epa.gov, checked 2026-10-19)

  2023: 22 of 30 days Good; 2024: 24 of 30 days Good; pooled 46 of 60 = 0.767; 1 + 9 × 0.767 = 7.90 → 8

  This is a summary of past measurements, not health advice. For today's air, see AirNow or the local agency.

San Francisco through the year: scores out of 10, where 10 is best for the visitor; * marks a provisional score, explained on that month's page

Scores summarize official data with the method on How we score. They aren't a recommendation, and any given year can differ.

When to go

September is usually among the warmest months in San Francisco, according to the National Park Service, and NOAA's normals for the airport station, listed in the facts, show warm afternoons and almost no rain. The Park Service still says the city can be foggy and windy at any time of year and that microclimates change from neighborhood to neighborhood, so bring a layer. The Folsom Street Fair is listed for late in the month, and Salesforce's Dreamforce conference is scheduled for a few days before it, so check those dates against your trip, especially when you compare lodging.

Trip facts

• NOAA NCEI's monthly normals (1991-2020) list an average daily high of 74.8 and an average daily low of 55.9 degrees Fahrenheit for the San Francisco International Airport station in September. (www.ncei.noaa.gov, checked 2026-10-17)
• NOAA NCEI's monthly normals (1991-2020) list average monthly precipitation of 0.07 inches for the San Francisco International Airport station in September, and an average of 1.6 days with a maximum temperature at or above 90 degrees Fahrenheit. (www.ncei.noaa.gov, checked 2026-10-17)
• The National Park Service's San Francisco Maritime National Historical Park weather page says the warmest months are usually September and October, that the city can be foggy and windy any time of the year, and that the Bay Area's microclimates can change dramatically from neighborhood to neighborhood. (www.nps.gov, checked 2026-10-17)
• Folsom Street's site lists the Folsom Street Fair for September 26, 2027 (shown on the site as 9/26/2027). (www.folsomstreet.org, checked 2026-10-17)
• Salesforce's Dreamforce page says Dreamforce is back September 21-23, 2027 in San Francisco. (www.salesforce.com, checked 2026-10-17)
• San Francisco International Airport's ground transportation page says BART is the fastest way to San Francisco and the East Bay, that the SFO BART Station is in the International Terminal, that BART connects to the AirTrain for access to all domestic and international terminals, and that pick-up for Lyft and Uber is on Level 5 of the Domestic Garage. (www.flysfo.com, checked 2026-10-17)
• BART's airport connections page says trains depart from the SFO International Terminal and it's just 30 minutes to downtown San Francisco, with weekday airport service from 4:58 a.m. (first train departing SFO) to midnight. (www.bart.gov, checked 2026-10-17)
• SFMTA's Visitor Passport page says a Visitor Passport gives unlimited rides on Muni, Muni Metro, historic streetcars and cable cars for 1, 3 or 7 consecutive days, and is not valid on BART, other transit systems, tour buses, or for transportation to or from San Francisco International Airport (SFO). (www.sfmta.com, checked 2026-10-17)
• SFMTA's cable car fare page says that to board cable cars at Powell & Market, Bay & Taylor and Hyde & Beach Streets, you must purchase your fare in advance, during daily hours that SFMTA's fare page and its cable car page state differently. (www.sfmta.com, checked 2026-10-17)
• The National Park Service's Alcatraz FAQ says the ferry is operated by Alcatraz City Cruises, that advance ticket sales are strongly recommended, and that tickets to Alcatraz may sell out several weeks ahead during summer months and around holiday weekends. (www.nps.gov, checked 2026-10-17)
• The San Francisco Treasurer & Tax Collector's page says San Francisco imposes a 14% transient occupancy tax on the rental of accommodations for stays of less than 30 days. (sftreasurer.org, checked 2026-10-17)
• The San Francisco Police Department's auto break-in guidance says to place all loose items in the trunk of your car before you arrive at your destination and, for travelers from out of town, to check your luggage at the hotel instead of keeping it in your car. (www.sanfranciscopolice.org, checked 2026-10-17)
• The Transportation Security Administration's REAL ID page says that starting May 2025, U.S. travelers must be REAL ID compliant to board domestic flights and access certain federal facilities. (www.tsa.gov, checked 2026-10-17)