San Francisco Weather - 7 Day Forecast

"The coldest winter I ever spent was a summer in San Francisco."  - Unknown

Mark Twain once said, "If you don’t like the weather in New England, wait a few minutes."  In San Francisco, if you don’t like the weather, just drive a few miles. The temperature at both ends of the mile-long Golden Gate Bridge can be 20°F different. Since a city’s average temperature for the year is not helpful to travelers, here is a general breakdown of what to expect:

For the most part, San Francisco’s weather is temperate and, within limits, predictable although it has its own micro climate.  San Francisco temperatures seldom ever dip below 40°F, or reach or exceed 90°F.  Its summers are famously foggy; but for the summer visitor who hasn’t heard, it’s important to know about The City’s "natural air conditioner" - the fog. But it gives tourists the excuse to buy a souvenir SF sweatshirt or scramble to buy a windbreaker.  It can be in the 80s downtown, and then the fog rolls in in the afternoon, shaving 30 degrees off the top.  It can be in the 50s after the fog comes into The City, even as it’s still in the 90s in the surrounding Marin, Sonoma, Solano, Contra Costa, Alameda, Santa Clara and San Mateo Counties.  In winter, the surrounding waters tend to keep The City from getting as cold as other parts of the Bay Area.  It frequently can go below freezing in the surrounding counties, and be 50F in San Francisco. Even within  the city limits, the temperature can change from one block to the next. The Mission and Potrero Hill are rumored to be a few degrees warmer than the rest of the city year round, while the Sunset and Richmond districts (surrounding Golden Gate Park) are arguably the coldest, windiest and rainiest.

If you're packing for summer... 

The typical summer weather often catches the visitor off guard.  They are either wearing shorts in 50 degree cold and wind or a jacket and sweater on a beautiful day.  Dress in layers and be have a way to carry the layers with you when you peel them off.  A typcial summer day starts with fog over the City.  It can cover the whole city or only a portion of it.  Most days, the sun will burn the fog back from the east toward the ocean.  Sometimes the fog will burn all the way back to the ocean and sometimes the western portion of the city will remain covered with fog all day while the eastern part is sunny or sometimes the whole city never sees the sun all day.  When the fog does burn back toward the ocean, by 4:00-5:00pm the winds kick up and the fog starts rolling back in over the City and it gets chilly.  When you dress in layers, you are bundled up in the morning, peel off the outer layers for the afternoon sun, and put them back on as the late afternoon wind kicks up, and the fog rolls in.

The Temps, the precip, year 'round: 

Summer

In the summer months, in the 50s overnight, going up to 75°F in the afternoon until the fog comes in, dropping it to the 50s.  But the fog doesn’t always come in, and it often only reaches to the western half of The City.  Nearly everyday, by 3:00pm, there is a west wind (from the ocean) of 25-30mph.  The wind calms by dark.  In August, there might be a few showers, even one thunderstorm, as the edge of the Southwest’s monsoons reach the Bay Area.

Autumn

Autumn weather is the most dependable.  Warm (80°F in The City, 100°F in the East, South or North bay cities) and dry "Indian Summer" days prevail in October, when there’s least likely to be any fog or rain.  Still, it can be in the 50s at night.  Some say this is the best time of year to visit The City, and that the real summer here happens in September & October. 

Winter

The rains can be heaviest in November-December.  These are cool winter storms from the Pacific, not monsoons.  December also usually brings another type of fog: ground fog or "Tule (tu-lee, as in marsh reeds) Fog," primarily throughout northern and central California, and it can occur in San Francisco.  This fog doesn’t roll in, as during the summer.  It just "becomes," rising from ground level at night.  It may burn off in the morning, or linger all day.  It can make driving dangerous at night, and treacherous on Interstates 580 and 5 anytime it's present.  Tule fog can linger into January and February, but usually these months are usually just cold - by San Franciscans’ standards - and clear.  It may not get above 50F in San Francisco, or above 40F in the suburbs.  There can be a week of 20s in the surrounding counties.  Snow is very rare, except for a single annual 1-3" dusting atop the nearby peaks of Mt. Tamalpais, Mt. Diablo, and Mt. Hamilton.  The Santa Cruz mountains may share the snow at these times.  There may be enough in patches for an hour or so of demonstration skiing, time for the TV news crews to get up the hill, and then it’s gone.  Once every few years, snow flurries may reach San Francisco proper for a few minutes, but seeing snow on the ground here is such a rare event, you'd be hard pressed to find a local who's seen it happen.

Spring

The rains usually return heavily in March.  Then April weather is the most uncertain of the year.  There can be a freak dry heat wave in the 80s for a few days, and a week later it can rain as in March. By now, the surrounding hills are a vibrant green, and the wind makes them appear to ripple with waves.  May can be sunny and mild, or there can be sustained overcast with temperatures usually in the 60s and 70s. The summer pattern and golden hills return in May or June.

The word "usually" is used liberally here.  Two major factors can exaggerate or upend the above conditions, El Niño and La Niña, bringing either year-round drought or profuse winter rains, respectively.  A drought can leave hills grey, no snowpack in the Sierras, and severe restrictions on discretionary water use as reservoirs are drawn down.  In La Niña, the profuse rains can wash houses down hills, and flood rivers in the surrounding counties.

All in all, in comparison with most places in the United States, San Francisco's climate is one of the most even and predictable. This quality is also what makes it one of the most abnormal climates in the entire country.   

The city averages a little over 300 days of sunshine per year.  San Francisco has a reputation of being windy, due to its location near cold ocean waters and its topography which funnels the winds through the city, so you'll want to have at least a jacket.  Layers of clothes will ultimately serve most practical. Ocean water temperatures are typically in the 50s and 60s.

Air conditioning in San Francisco?

In San Francisco, air conditioning is not present in many of the residences, offices, theaters as well as older, boutique style hotels. According to weather.com, the warmest month on average is September with an average high of 71°F and low of 56°F. San Francisco gets about 15 days per year with a temp over 90°F. (5 times per year, 3 days per time) April can see some of those days, May can see some of those days and September, October and even November can see some of those days. They can also occur during any of the months inbetween. Typically, there are not more than 3 over 90 degree days in a row. By the 4th day of high temps, the fog gets sucked into the Bay Area and things cool down with the natural air conditioning provided by the ocean. The odds of you selecting one of the "rare" periods of high temps is actually pretty low. Most importantly, even in the heat waves, it always cools down at night.  People will find that the lack of A/C is not an issue 99.9% of the time.  If air conditioning is important to you, double check the hotel, because not everyone has it. Also, do a search on the forum for "air conditioning" and you will see a lot of discussion on this topic.