The Bay Area is warmest in September near the coast and the bay, and in July inland. September is the warmest month in San Francisco (average high 70.2°F), Oakland, Napa and Santa Rosa. San Jose peaks in August, and the inland valleys around Livermore and Concord peak in July, with average highs near 89°F.
That surprises most visitors, who expect July and August to be warmest everywhere. In San Francisco, July is one of the coolest summer months because of fog. The city's real summer comes in September and October, a stretch locals call Indian summer.
What Is the Warmest Month in San Francisco?
September is the warmest month in San Francisco, with an average high of 70.2°F downtown, based on NOAA 1991-2020 normals. October is close behind at 69.8°F. July averages only 66.3°F and August 67.9°F, because the summer marine layer, the cool, damp ocean air, keeps the city cool and gray.
September and October also feel warmer than the numbers suggest. The fog retreats, the wind eases and the sun shines through the afternoon, so a 68°F September day feels far warmer than a 66°F foggy July day.
Why Are September and October Warmer Than July?
September and October are warmer than July along the coast because the North Pacific High weakens in fall and the marine layer pulls back. In July and August, that high is at full strength offshore, driving strong onshore wind, deep fog and cool afternoons across the coast.
In September, the high weakens and drifts south, the sea breeze eases, and afternoons that were foggy in July turn sunny. For the full explanation, see why September is hotter than July in the Bay Area.
Average high (1991-2020 normal)
- July
- 66.3°F
- October
- 69.8°F
Ocean water offshore (monthly average)
- July
- 55.6°F
- October
- 57.4°F
Mornings with fog (2025)
- July
- 28 of 31
- October
- 14 of 31
Average 2 p.m. wind (2025)
- July
- 10.1 mph
- October
- 7.0 mph
| Measure | July | October |
|---|---|---|
| Average high (1991-2020 normal) | 66.3°F | 69.8°F |
| Ocean water offshore (monthly average) | 55.6°F | 57.4°F |
| Mornings with fog (2025) | 28 of 31 | 14 of 31 |
| Average 2 p.m. wind (2025) | 10.1 mph | 7.0 mph |
Sources: average highs from NOAA 1991-2020 station normals via RCC-ACIS; ocean water at NOAA buoy 46026, 18 nautical miles west of San Francisco, from the NOAA NCEI Coastal Water Temperature Guide; fog and wind from the NOAA HRRR analysis for downtown San Francisco in July and October 2025, one year only.
What Is the Warmest Month in Each Part of the Bay Area?
The warmest month in the Bay Area depends on how close a place is to the ocean. Coastal and bayside cities peak in September, the South Bay peaks in August, and the inland East Bay valleys peak in July. Wine country valleys like Napa and Santa Rosa peak in September, during harvest.
| Location | Warmest month | Average high | July average high |
|---|---|---|---|
| San Francisco (downtown) | September | 70.2°F | 66.3°F |
| San Francisco Airport | September | 74.8°F | 72.6°F |
| Oakland Airport | September | 74.1°F | 71.6°F |
| Santa Rosa | September | 83.9°F | 81.6°F |
| Napa | September | 84.4°F | 83.3°F |
| San Jose | August | 81.4°F | 80.8°F |
| Concord | July | 88.0°F | 88.0°F |
| Livermore | July | 89.0°F | 89.0°F |
NOAA 1991-2020 normal daily highs. Source: NOAA station records via RCC-ACIS.
What Is the Hottest Month in the Inland Bay Area?
July is the hottest month in the inland Bay Area. Livermore averages a July high of 89.0°F and Concord 88.0°F, with August nearly as hot. Inland valleys heat strongly during the long days of midsummer, and the afternoon sea breeze cools them only partway.
By September, inland highs slip a few degrees, and by October they average in the upper 70s. Wine country is the exception: Napa and Sonoma stay hot into September, which is harvest season.
When Is the Best Time for Warm-Weather Activities?
The best time for warm-weather activities in most of the Bay Area is September and October. The fog has retreated, temperatures are warm without being extreme, and the days are still fairly long. For San Francisco, plan warm-weather visits for September or October, not July.
For the most heat, such as lake days at Lake Berryessa or trips to the Delta, July and August bring the highest temperatures and the longest days. Fall's warmth comes with a catch, because October brings Diablo wind events. These hot, dry offshore winds have driven Northern California's worst wildfires, so check fire weather before heading into the hills.
