It happens every summer. Somebody arrives at Pier 40 in shorts, because it is July, and July means shorts almost everywhere else in the country. Forty minutes later they are wearing every layer they brought, and wishing they had brought one more.
Then the summer visitors go home, and San Francisco has the warmest, clearest weeks of its whole year, in September and October. It sounds backwards. It is not, and once you know why, it changes when you book.
Why July is cold
The ocean off the Golden Gate stays cold all year. In summer, California's Central Valley bakes, the hot air over it rises, and cool air off the Pacific is pulled inland to take its place. The Golden Gate is the one gap in the coastal hills at sea level, so that cold, damp air pours straight through it, and it brings the fog with it.
The hotter the valley gets, the harder the pull. That is why the city can sit under grey sky in July while the valley runs twenty or thirty degrees warmer. Our guide to how the wind and weather work on the Bay goes through the whole engine, and why it makes the summer afternoons such good sailing.
Why September and October are warm
By September the valley starts to cool off. The gap between the hot inland air and the cold ocean narrows, the pull weakens, and the fog stops pouring in every evening. The sun is still high and the afternoons are still long, so with less fog in the way, the city finally warms up.
Every so often in autumn the wind turns right around and blows from the land out to sea instead of the other way. Those offshore days bring some of the warmest weather San Francisco gets all year, and the clear air and long views the Bay is famous for in the fall.
The Bay, month by month
Here is the year at a glance. The temperatures are for downtown. Out on the water, once the breeze is up, it can feel a good ten degrees colder.
| Month | Average high | Rain | On the water |
|---|---|---|---|
| January | 58°F | 4.4 in | Winter storms come and go, with clear, often calm days between them. |
| February | 60°F | 4.4 in | Still the rainy season. Crisp, clear days after each front. |
| March | 62°F | 3.2 in | The afternoon westerly starts to come back. |
| April | 63°F | 1.6 in | The breeze builds most afternoons, and the crowds have not arrived. |
| May | 64°F | 0.7 in | The summer pattern returns, and with it the first fog. |
| June | 67°F | 0.2 in | Long days. Fog near the Gate on many mornings and evenings. |
| July | 66°F | Almost none | Peak fog. Grey mornings, a strong afternoon breeze. Dress for cold. |
| August | 68°F | 0.1 in | Still foggy, still windy, still colder than people expect. |
| September | 70°F | 0.1 in | The warmest month. The fog thins and the views open up. |
| October | 70°F | 0.9 in | Warm and clear. The breeze still comes in, often softer. |
| November | 64°F | 2.6 in | Cooler and quieter. The first storms arrive. |
| December | 58°F | 4.8 in | Short days, quiet water, clear spells between storms. |
Average high and rainfall for downtown San Francisco, from NOAA's 1991 to 2020 climate normals.
What it means for your charter
If you want warmth and long views, book September or October. The breeze still comes in most afternoons, usually softer than in summer, and it is many sailors' favorite time of year. October also brings Fleet Week.
If you want wind, book July or August, and come dressed for it. Summer afternoons are the Bay at its most exciting, and almost always colder than you expect.
Winter is quieter. The westerly mostly switches off, and the clear, calm days between storms can be beautiful. We watch the forecast closely and reschedule if a front makes it unsafe. Whatever the month, the current through the Gate matters as much as the wind: tides and currents on the Bay explains how captains plan around it.
About that Mark Twain line
The coldest winter I ever spent was a summer in San Francisco.
It is probably the best-known line about the city's weather, and Mark Twain almost certainly never said it. Researchers have found no trace of it in his papers or speeches, and the joke itself is older than he is. Twain did write about San Francisco's climate, in Roughing It, and he called it mild. Quote Investigator traces the whole story.
Plan your sail
If you remember three things:
- September and October are the warm months.
- July is foggy and windy, and colder than it sounds.
- Whatever the month, bring one more layer than you think you need.
If you can choose, choose September or October. If you cannot, July is still a great day on the water, as long as you dress for it. Look over the fleet and the current rates, then ask us for a quote with your date and headcount.