The warmest month in the city, September is definitely a good time to visit San Francisco. Just bring your layers.
What is the weather like in San Francisco in September?
Expect temperatures around high 60s (19-22'C) by day to mid-50s (11-14'C) by night. Pleasant weather, coupled with diminishing crowds (tourists love summer), it's a good time to visit the city.
Mind the famous San Francisco fog in early mornings and late afternoons as you would anywhere along California coast but more so in San Francisco. Feels cooler, like on cloudy day elsewhere, until mid-morning when 'Karl' bows out of heavens. It happens to start our tour at 10 am at Union Square in slightly soupy conditions and basking in glorious Embarcadero sunshine at the end of the tour (just in time for the pictures).
What to wear in September in San Francisco?
Bring layers. If the question is 'shorts or jeans', the answer is consider both but definitely bring your jacket as it feels cold when the sun goes down.
The umbrella is useless. September is pretty dry and even when it rains it's just feeble sprinkles for half an hour or so.
Apply sunscreen. September is not so windy in San Francisco, but the city is by the ocean so it breezes all the time and you might not notice the sun's impact on your skin. Wear sunscreen as a precaution.
How weird is the weather in San Francisco in September?
San Francisco lives by its own weather standards and even different neighbourhoods subscribe to specific microclimates.
Financial District and Outer Richmond lie just 5 miles apart but could register a weird 10 degrees in difference any given time. Heck, you may even turn around the corner and find yourself in a different climate zone!
West of the Golden Gate Bridge tends to be breezier and colder than the Eastern part of the city but its hilly nature makes it hard to set in stone any rule. Weather in SF can change by the hour so bring your layers and you'll be just fine.