Two years of hourly satellite imagery over 210 Bay Area zones: Lakeshore sits under the marine layer 53.2% of summer hours, while Candlestick Point SRA manages 25.6%.
This is the fog map San Francisco arguments need. It reads NOAA GOES-18 satellite radiances straight from the public archive, one scan every hour from 2024-08-08 to 2026-08-07, and asks of every pixel whether a low cloud deck was sitting on top of it. That is 3.7M zone-hours across 210 zones: 117 San Francisco neighbourhoods plus towns through Marin, the East Bay, the Peninsula, the South Bay, the North Bay and the Santa Cruz coast. Fog is not identified by eyeballing a threshold: a gradient-boosted classifier was trained against 16 airport weather stations and scored on stations it never saw.
Any place, or six side by side
Every chart on this page shows a chosen handful of the 210 measured zones. Use the selector pinned at the top to pull any of them through all of it: pick one place and this panel gives its own daily and seasonal curve against its region, while every section below gains its outline, its row, its line and its own numbers. Add a second, or up to six, and they are compared directly instead, each holding one colour from the map through the rankings to the charts. The selection is written into the address bar, so a chosen set survives a reload and can be sent to someone.
Nothing selected yet. Pick a place from the bar at the top of the page and every section below switches to it: its own daily and seasonal curves here, its outline on the map, its row in the rankings, its line on each chart, and its numbers under each heading. Add up to six and they are compared side by side, each in its own colour.
The picker covers all 210 zones: 117 San Francisco neighbourhoods and 93 towns across Marin, the East Bay, the Peninsula, the South Bay, the North Bay and the Santa Cruz coast, and it holds six of them at a time. Every number it shows is read from the same hourly record as the charts below and carries the same caveat: the ordering is reliable, a single percentage is approximate, and a gap of a point or two between two selected places sits inside the noise.
Right now live
Everything else on this page is the two-year record. This is today. Sixteen airport weather stations report what the sky is doing right now, and the same 210 zones are read off them: not by copying the nearest airport's reading, but by measuring how far the Bay currently sits from its own normal and moving every zone by that much from its own. Ask it where the sun is and it answers with a place and a distance.
A station reading is a point, and the marine layer has an edge that can run between two of them, so a single zone's percentage here is softer than the two-year figures below. What survives that is the ordering and the size of the anomaly. Fog means the same thing it means everywhere else on this page: an opaque ceiling at or below 2,000 feet. Updated hourly; if the pull fails the panel says so rather than showing you the climatology with a live label on it.
The next two days
The next 48 hours, hour by hour. The first three are what the stations can actually see; after that it is the National Weather Service gridded forecast, read as an anomaly against each grid cell's own two-year record and then applied to the zones inside it. That last step is why the coast and the valley diverge here the way they do in the climatology: the forecast supplies the day, the record supplies the geography.
A forecast is not a measurement. NWS publishes a ceiling height and a sky cover, not a probability of a marine layer, so translating one into the other is the single hand-set step in this pipeline, and it is the part most likely to be wrong. Treat the shape of the next two days as the useful output and any one hour's percentage as indicative.
Where the fog sits in San Francisco
Every San Francisco neighbourhood shaded by how often a low cloud deck sat over it. The first frame is summer (May to September), when the west-side gradient everybody argues about is real and steep: the ocean-facing avenues run roughly 2.1 times foggier than the sheltered east side. Click any season or month below the map to page through the year and watch the pattern build and collapse. Hover any neighbourhood.
All 16 frames share one fixed colour scale (5.5% to 66.1%), so seasons and months are directly comparable to each other; a frame looking paler means less fog, not a rescaled legend. The four seasons tile the year around the marine-layer season this report is built on, so summer is the five months May to September and the remaining seven split into spring, fall and winter. 2024-08-08 to 2026-08-07. Daytime hours are classified on the 0.5 km visible band; night on 2 km infrared. Cloud positions are parallax-corrected (GOES-18 views the Bay at about 46 degrees off vertical, so a 400 m cloud top is displaced roughly 420 m before correction).
The same city, four times a day
The same map, drawn four times over: one panel per block of the clock, all four on one colour scale so they can be read against each other. Fog is a night and morning animal. Across the city in summer the deck covers 52.3% of night hours and 42.9% of the morning, then falls to 15.8% between noon and 4pm before creeping back to 22.5% in the evening. Switch season above the panels to see the whole pattern shrink, and watch which side of the city lets go of it last.
Blocks are local time and tile the clock: night is the twelve hours from 8pm, then three four-hour daylight blocks. All 4 panels across all 5 season settings share one fixed scale (1.4% to 64.1%). Night panels rest on 2 km infrared, where San Francisco spans about five pixels, so read the night maps as a citywide level rather than a neighbourhood ranking; the three daylight panels are on the 0.5 km visible band and do resolve the west-to-east gradient. Outlines are simplified for these small panels; the full-resolution polygons are in the map above.
The foggiest neighbourhoods, and the clearest
The ten foggiest San Francisco neighbourhoods (blue) against the ten clearest (gold), by summer fog frequency. Lakeshore leads at 53.2%, Candlestick Point SRA trails at 25.6%. Note how tightly the top ten cluster: the west side is uniformly fogbound, and the real variation is east of Twin Peaks.
Percentages are the mean calibrated fog probability over all summer hours, not a count over a cutoff, because a cutoff overpredicted rare inland fog by up to three times. Neighbourhoods closer together than about 700 m share satellite pixels and will read near-identically; that is a resolution limit, not a measurement.
Fog by the hour
Fog frequency by hour of day for a spread of San Francisco neighbourhoods, foggiest at the top. Reading left to right is a summer day: the deck thickens overnight, peaks around dawn, and burns back toward the coast through the afternoon. All times Pacific.
Summer hours only. Night rows rest on 2 km infrared, where San Francisco spans only about five pixels, so neighbourhood contrast is genuinely weaker after dark than during daylight.
The daily cycle, five places
The same daily cycle for five reference points, from the open coast to the inland valley. The gap between them is the whole story of Bay Area microclimate: the coast barely clears at all, while the South Bay is essentially fog-free by mid-morning.
Summer months, local time. Each series is the mean calibrated fog probability at that hour across all summer days in the record.
How the rest of the Bay compares
Sub-regions ranked by mean summer fog frequency across their zones. San Francisco leads at 39.7%; South Bay trails at 13.2%.
Each bar is the mean across that sub-region's zones, so it reflects the towns chosen rather than land area. Coordinates come from the US Census 2023 gazetteer.
The season, month by month
The seasonal cycle for the same five places. Bay Area fog is a summer phenomenon driven by the marine layer, which is the opposite of most of the country.
Monthly means across both years of record. Note that inland North Bay sites show a second, smaller winter signal, which is valley fog forming by a different mechanism than the summer marine layer.
How this was measured, and what it cannot tell you
An honest account of the method and its limits. The short version: trust the ordering, treat single percentages as approximate, and expect less neighbourhood detail at night than during the day.