SFO against normal, September 2026
September 2026 at SFO ran 0.8°F above normal. Normal here is the 1991-2020 average for that same month, which is the standard climate baseline — so this is a comparison with the recent past, not with the start of the record.
Across the whole series, September has averaged -0.6°F against that baseline, over 79 years. The warmest was +4.6 (2017) and the coldest -5.0 (1948). A single month is weather; the reason to post one every month is that the band they fall in has moved — which is what the 12-month average on the chart is there to show.
The station is the caveat. One weather station stands for the whole metro: San Francisco International Airport (NOAA USW00023234), whose record starts in 1945. The ground around a thermometer changes over decades, so some of the drift in this line is the site and not the climate. That is why the anomaly is measured against a fixed 1991-2020 baseline rather than against the early record.
Source: NOAA Global Historical Climatology Network daily records for San Francisco International Airport (USW00023234), via RCC-ACIS, aggregated to monthly means and differenced against the 1991-2020 normal for each calendar month. Auto-generated each build.