Your SF sun chasing guide.

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Your SF sun chasing guide.

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The fog

Karl is what San Francisco calls its fog. It comes in through the Golden Gate and the low gap over the Sunset.

The ridge

It meets the ridge at Twin Peaks and Mount Sutro and stops. East of that line is the fog shadow, which is how Dolores Park can be twenty degrees warmer than Ocean Beach in the same hour.

The hills

The marine layer is shallow, usually a thousand to sixteen hundred feet, so hilltops stand above it while the streets below stay grey. Going up is a different move from going east.

The forecast

Eleven places, from Ocean Beach in the west to Dogpatch in the east, twenty-four hours ahead, so you can see which side of the fog line each one is on.

The picture

The cross section is drawn from the same reading the list is ranked on, place by place. Drag the hours and you are watching the layer come in off the Pacific and pull back, which is the thing the numbers are describing.

Clear: Cloud overhead under 45 per cent and visibility over 5 km. Both, or it is not clear.
Order: Calmest first inside each group, so exposed spots sink to the bottom.
Quiet rule: When nothing sits above the line, no place is named. Silence is the honest answer.
Height: Cloud is read at four altitudes and compared against each spot's own elevation, so a hilltop standing above the layer is not counted as fogged in.
The line: Each spot has its own sunlight line: the height you would have to climb to get out from under the cloud. Stand at or above it and you are clear. It is the same curve the band is drawn from, at that spot's own place in the city, so the picture and the list cannot disagree.
The fog line: The body's top edge, drawn where cloud overhead crosses 45 per cent. Its softness is the model's own uncertainty about where the layer ends, so a vague edge means a vague reading.
Height: Drawn to true scale up to 300 m, which is above every place on the chart, and compressed above that. The marine layer often sits at 700 m, and drawing that literally would push the cloud off the top of the picture and leave a third of it empty, since nowhere in the city is that high.
The front: The model runs on a 3 km grid across a city 11 km wide, so it cannot place the fog's edge more precisely than about a quarter of the chart. Each place is judged on its own grid cell and nothing else, so where two neighbours disagree the front falls between them. Add ?debug to the address to see what the model returned for every place.
Underneath: The body is densest at the layer's top and thins toward the ground, which is what a deck sitting above you looks like: you can see across the city beneath it. On the hours the fog is genuinely at street level it stays solid all the way down, because then you cannot.
Moving: Drag the hours and the layer grows and recedes rather than jumping. What you are watching is the same reading at a different hour, eased between the two.
Between hours: The forecast is hourly. The scrubber moves in quarters and reads between two hours, which is where the layer is seen rolling in and out.
Refresh: Hourly forecast, twenty-four hours ahead, all eleven spots in one request.

Source

Weather by Open-Meteo, CC BY 4.0, non-commercial use.

Measured, and not

Cloud, visibility, wind and temperature are forecast model output, not station observations. The year-round clear-sky figure used to break ties is a local estimate rather than a measurement.

Privacy

No accounts, no tracking, nothing stored. The page talks to one weather API and to nobody else.

Resolution

The model runs on a 3 km grid and the city is about 11 km across, so several places share a grid square and read the same air. That number is what the cross section actually has to draw with.

Why

It started as a personal question: where in this city is it actually clear right now, and is it worth the trip.

What it is

A prototype rather than a product. One rule about where the fog stops, applied to live data, resolved to a single place.

Who

Made in San Francisco by Anto, somewhere east of the ridge.

Built with by Anto