Is the Bay's summer fog disappearing?

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clear low cloud all day

One row per summer (June to August), one cell per day. A cell's shade is how many of the day's eight 3-hourly readings had the cloud ceiling at or below 400 m. The bar at the right is that summer's share of all readings. Blank cells are days with too few readings.

How this was measured

Source: . Each airport's hourly reports are read on the eight synoptic hours only (00, 03 … 21 UTC), because 1969–1972 survive only at 3-hourly resolution and an hourly year would otherwise get three times the chances to catch fog. NOAA's older database stops in August 2025, so 2025 and 2026 come from its successor, GHCNh, which gives the same answer as the older one to a tenth of a point on the summers both cover. "Low cloud" is a ceiling at or below 400 m (about 1,300 ft), the marine layer the Bay calls fog; fog measured by visibility is not counted. SFO's record switches from the old SAO format to METAR reports in July ; the summers either side of it differ by points.

What this cannot tell you: whether the drop between the 1950s and the 1960s is weather or a change in how observers reported ceilings. Both airports show it, which rules out one station's move but not a nationwide practice change. Built .