San Francisco Weather

And Why It's So Nutty

fog.jpgSeptember is the warmest month in San Francisco, with highs of about 70°F/21°C and lows of 56°F/13°C. Nice.

But be prepared for it to change.

San Francisco weather can be surprising. Mornings are often cool and overcast. Then the sun comes out. So now it's 80°F, and you're enjoying warm sunshine and clear skies, when all of a sudden the temperature drops 30 degrees, a crazy wind starts to blow and fog rolls in.

San Franciscans find no end of humor in a sight so common it's become cliché: the summer—vacationing family wearing tank-tops, shorts, and flip-flops, all huddled together for warmth, dashing for the souvenir shop to get a matching set of San Francisco-themed hoodies.

HAR! We've all seen it.

It's not like they haven't been warned. Most travel-related Web sites and guidebooks issue the same warning: Beware San Francisco's summer fog … dress in layers. Now you too have been warned.

And to satisfy the junior meteorologist in all of us, we looked for an explanation. For that, we turned to our friend, meteorologist Michael Kelting, the weather anchor at KRON4 TV news.

He explained that the air in California's central valley gets really hot during the summer, often well over 100°F. The hot air expands and rises, creating low pressure. To relieve the low pressure, ocean air is drawn through the Golden Gate. Lots of air.

Meanwhile, ocean currents outside the Golden Gate draw cold water from the North, which mixes with the warmer air, creating fog. Thus it's fog—cold, damp fog—that gets drawn through the Golden Gate and settles right over San Francisco.

The result: a sudden drop in temperature and corresponding spike in hoodie sales.

Fortunately, the sun is not as strong in September as it is earlier in the summer, and the central valley doesn't get as hot. Also, pressure systems ride the jet stream north and cancel out the sea breeze. For you at Oracle OpenWorld 2008, it means excellent weather.

But bring a sweater just in case.


The Fog and cold Ocean air comes in from the North West of the city, so the South East corner of the city is the warmest and gets fog a lot less often. The Moscone Conference Center is located is in the very warm and usually fog free South of Market (or SOMA) area, so just pack a light sweater and bring the sun block.

Posted by David Haimes on August 14, 2008 at 05:13 PM PDT #

Yep always freezing in the summer and living east bay now , I find myself acting like a tourist sometimes and getting off the BART in tank top..freezing! September to November...Perfect!

Posted by on August 15, 2008 at 04:21 AM PDT #

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