How much fog does your SF neighborhood get? This new map...

Not all San Francisco neighborhoods are equal when it comes to fog — that chilly weather phenomenon for which the city is so well-known.

We all know that the Outer Sunset is prone to gray days, as is the Presidio, but even more centrally located neighborhoods get their fair share of Karl.

A new map developed by the folks at HotPads, the rental listing site, shows just how much fog each San Francisco neighborhood and Bay Area city gets per day, using granular satellite data from the U.S. Geological Service.

The foggiest San Francisco neighborhoods include Ingleside, Mount Davidson Manor, Oceanview and Stonestown, which all receive an average of 12 hours of fog each day. The further southwest one traces the map, the foggier things get.

On the other hand, the neighborhoods located on the northeastern side of the city — like Mission, SoMa, Nob Hill, North Beach and so on — get the least fog per day, about six to seven hours on average.

The Pacific Ocean is the major source of fog for the city, which explains why the more western neighborhoods get such a heavy dose of the cloudy stuff.

Here's a quick summary of how the Bay Area gets its fog: The hot sun heats up the oceanic water, transforming some particles into vapor and forming a thin band known as the marine layer. When the marine layer collides with cold air from the California current — a stream of cold water flowing south along the state's coastline — a thick fog bank forms. Depending on the day's conditions, the fog bank can stretch a hundred miles long to a mile high. Easterly winds then push the fog bank into the city.

There you have it. You can see how much fog your neighborhood gets in a day by toggling the map above and clicking through the slideshow.

Another curious feature of the map is the addition of neighborhoods' median rent. HotPads' economists thought it would be interesting to see if fogginess correlated with rent — and indeed it did. (Note: Correlation does not assume causation in this case. Rent costs are probably not higher simply because of fog, though it may be a contributing factor).

"Our main finding was that in the Bay Area, an hour of sunlight in the summer is equated to $150 more in rent per month," said Joshua Clark, a HotPads economist.

The most expensive parts of the city are also the sunniest, he noted. The pricier homes tend to be built on hills, he explained, which tends to protect them from the low-lying mist.

Susy Guerrero contributed to this report.

Read Michelle Robertson's latest stories and send her news tips at mrobertson@sfchronicle.com.

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