Half Moon Bay weather in September

San Francisco-forum threads on September weather are consistently upbeat, calling it one of the better months of the year here, mild days in the 70s once morning fog burns off by late morning most days. Posters describe the fog itself as a near-daily fixture along the waterfront and bridge in the early hours, thinning out by 11am before a clear, mild afternoon takes over. The summer fog pattern that keeps downtown cool in June and July has largely broken by this point, and the heavier weather systems that arrive later in autumn haven't started yet, leaving September genuinely dependable. Regulars still recommend layers over shorts, since spots right on the water - the Wharf, Ocean Beach, the bridge itself - run noticeably cooler than the rest of the city regardless of the season. The record: 21.6C days feeling like 21C, 14C nights, just 2.5mm across 0.5 days, a 16.8C sea, 75 percent humidity and 11.2 hours of sun.

September and October are the actual summer — the fog retreats and it's 22°C and clear. June and July are the grey ones, whatever the latitude suggests. Always carry a layer; crossing from the Mission to Ocean Beach can cost you ten degrees.