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Posted at 10:45 AM ET, 06/11/2009

San Francisco's Summertime Stratus Deck

By Ian Livingston

* Flash Flood Watch till 4 AM Friday | Full Forecast | NatCast *

Boaters travel by the Golden Gate Bridge and the bay's opening to the Pacific Ocean on May 30, 2009. Clouds filter through thanks to onshore winds and hilly terrain on both sides.

I spent the end of May in the San Francisco Bay Area and a few days in the city itself. As the heat of summer was taking hold in the D.C. area, as well as the rest of the country, a massive stratus deck parked over the cold waters of the eastern Pacific Ocean had other plans for parts of coastal California. In seeking photo opportunities during the "June Gloom," one can use an understanding of weather and geography much more than just hoping for good luck.

Keep reading to learn more about the stratus deck and to see some photographs of San Francisco.

Summer in San Francisco is typically one of the cloudiest times of the year. While inland locations warm to daily highs between 80 and 100, cold sea surface temperatures average in the mid-and upper-50s along the coastline. These differing temperature regimes help to create pressure gradients that can cause an onshore flow of clouds and cooler marine air.

Daytime heating causes rising air and lower pressure just above the land surface, while cooler offshore air tends to sink and cause higher pressure just above the ocean surface. The resulting pressure gradient creates a surface wind blowing from the ocean toward the land (from high pressure to low pressure).

Higher up in the atmosphere, the warmer and rising inland air is drawn horizontally out to sea to replace the cooler, sinking air. Clouds are created as the warmer air moves over the cooler air. These clouds get pushed inland by the onshore flow when it is present. During summer, onshore winds last well into the night before relaxing prior to dawn when the interior is coolest.

Fishermen and sightseers enjoy some sunshine near the Bay Bridge on May 28.

Unlike spring and fall -- the warmest times in San Francisco -- finding a truly clear day between May and July can be a difficult task. But unless the clouds are unusually thick, there is often at least some blue sky to be found from day to day. You just need to know where to look. My last visit was in spring of 2004, and while watching weather conditions, I tried to focus on different spots this time.

Downtown buildings, including the Transamerica Pyramid (left) on May 28.

The trick is to focus on sections of the city that are on the eastern (and especially southeastern) sections of the peninsula. Fortunately, this includes much of the downtown with its many iconic buildings as well as large portions of the residential districts that contain parks and hills overlooking the city.

San Francisco skyline from a Dolores Park transit stop on May 29.

I stayed in the Mission District while visiting a friend which placed me east of the 900-foot-plus Twin Peaks. This ridge of small hills served to displace the onshore flow -- and thickest clouds -- to my north and south, leaving my location under a gap with daytime sun two of the three days. (As I was leaving, the cloud deck was thickening thanks to an approaching storm and the final day was the cloudiest.) After a day trying to dodge the clouds, I decided to climb the mountain to get a closer look.

San Francisco skyline shrouded in clouds from the top of Twin Peaks on May 29.

After ascending 900-plus feet on steep roads and dirt paths, I found myself at the "summit" of Twin Peaks. Weather conditions were quickly changing, and the cloud deck was just feet above my head (and at times at my level). Winds were gusty from the west -- somewhere around 20mph sustained. It was a worthwhile trip up, but rather cold and misty at the top. I stayed long enough to snap a few mostly-documentary photos and headed back down.

On a clearer day, Twin Peaks provides stellar panoramas of the city.

Downtown skyline behind the Mission District as seen from Bernal Heights on May 29.

San Francisco is know for its hills, so I had to hit at least two of them. Bernal Heights -- peak about 500 feet above sea level, or a bit more than half as tall as Twin Peaks -- was an obvious destination since it was close to where I stayed. I arrived at the heights right before sunset, when the onshore flow was very strong following daytime heating in the interior valleys. Low clouds screamed by from west to east, and the lighting was ever-changing.

In the picture above, a bit of the clear "gap" in the clouds mentioned earlier can be seen (see that image larger). In a picture on Wikipedia from the sky the gap is clearly evident, with a protruding Sutro Tower sitting atop Twin Peaks.

Tree on Bernal Heights as the sun sets behind the hill with low clouds overhead on May 29.

Looking east toward sunnier skies from Angel Island in the San Francisco Bay on May 30.

My last day featured a visit to Angel Island that put me out into the San Francisco Bay to view the fog from yet another angle. Cruising by the Golden Gate Bridge (see large photo), I watched the clouds filtering in as if yearning to journey through the bay. By afternoon the clouds gradually burnt off enough that there was a dividing line of air masses right overhead at Angel Island.

On the return trip to San Francisco, I observed the warm continental air temporarily winning its ever-continuing battle with that of the chilly Pacific. In the afternoon sun Mount "Tam" popped out of the clouds on the horizon. The gray surely returned later that day.

Stratus burning off over parts of the bay with Mount Tamalpais in the distance on May 30.

San Francisco during this time of year may require some patience from a photography standpoint, but there are also numerous rewards. In a city that is in such a geographically fascinating spot, it is not hard to get some spectacular views in, even when coastal low clouds and fog are in the forecast.

See many more photos at my San Francisco photo page on Flickr.

By Ian Livingston  | June 11, 2009; 10:45 AM ET
Categories:  Education, Photography  
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Next: Juicy June Primes Area for Flash Flooding


Next trip go to Buena Vista Park in the Haight for even more spectacular views and much sunnier skies in the summer.

Posted by: Tess6 | June 11, 2009 11:26 AM | Report abuse

Out west there seems to be much more microclimatic influence than around here. San Francisco provides an excellent example. The closest analog around here involves localities along Chesapeake Bay, especially some of the islands.

Other western microclimates involve mountain/valley locales such as Jackson Hole, WY and Missoula, MT. The Grand Tetons are one of the most spectacular mountain/valley microclimatic locales in the Western Hemisphere. Mountain/valley contrasts in the Appalachians, while somewhat pronounced, pale by comparison.

Posted by: Bombo47jea | June 11, 2009 11:31 AM | Report abuse

Tess6, you can see Buena Vista Park in the photo from Twin Peaks (upper left). Clouds were pretty consistent over that whole area much of the time. It is a little closer to downtown but lower in elevation. I got some sunnier views but did not include them here as to focus on the stratus intrusion etc. I will be posting more to the Flickr page linked at bottom of the post.

Bombo, the microclimates are certainly something. I spent much of the week in the east bay where temperatures climbed to around 80 with much more sun most of the time. In SF it was easy to walk through temperature swings of 10-20 degrees based mostly on where the clouds/wind were most extensive. I wish the east had more geographical similarities to the west. It's more exciting out there on that front in my opinion.

Posted by: Ian-CapitalWeatherGang | June 11, 2009 12:10 PM | Report abuse

The fog usually "breaks" at Buena Vista Park, however it has been an unusually foggy and somewhat damp late May and June in SF which your photos reflect.

Posted by: Tess6 | June 11, 2009 12:21 PM | Report abuse

Ian: you said "I wish the east had more geographical similarities to the west. It's more exciting out there on that front in my opinion."

Why not move to the west!

Posted by: hurlisano | June 11, 2009 12:57 PM | Report abuse

More weather notes: While we are getting all this rain, some areas of Australia are getting cold winter weather. The Canberra area dropped to -5 Celsius [low 20's], making for heavy frost, and some burned-over areas from the past summer's fires are now snow-covered. There was a major power failure in Canberra, with no electricity in some areas. In addition to the winter weather, Australia is experiencing a swine [H1N1] flu outbreak, perhaps a factor in the WHO pandemic declaration. One Australian Rules football team, the Brisbane Broncos, threatened to cancel a scheduled match due to the flu epidemic down under.

Posted by: Bombo47jea | June 11, 2009 1:41 PM | Report abuse

The stratus you refer to, is typically refered to by the locals as the marine layer.

Posted by: johnnyd2 | June 11, 2009 2:33 PM | Report abuse

hurlisano, not meant to be a hit on the east or anything. I think many photographers would agree the west has some really awesome scenery -- thanks in large part to the more “youthful” geologic formations. I grew up in So Cal, but enjoy being an East Coaster.

johnnyd2, that is true, though the marine layer is made up of stratus (or stratocumulus) and actual lower-lying fog (which I did not see much of this trip), so both are correct terms I think. SF also gets radiation fog, but that’s more a cool-month phenomenon and has different origins.

Posted by: Ian-CapitalWeatherGang | June 11, 2009 2:46 PM | Report abuse

I lived in the City for many years and learned to appreciate the fog, microclimates, wind and the like that was the weather there. I live in a (relatively) sunny portion of the city - The Mission - and a could look from my apartment as the clouds would literally pour over Twin Peaks and cover the City. It would just be clouds where we were, but to my friends further West, it was soupy fog.

Posted by: oldtimehockey | June 11, 2009 5:10 PM | Report abuse

The one day I got to spend on a boat on the Bay in San Francisco was clear, hot, sunny...and still. No wind at all. Even so, it was spectacular to motor a 52' wooden-hull replica up and down on the water. October of 2004, that was.

Posted by: --sg | June 11, 2009 10:47 PM | Report abuse

I lived in San Francisco for a year in an apartment across the street from the panhandle of Golden Gate Park and every day during the summer the clouds/fog would recede to the west around 11:00 AM and we would have brilliant sunshine for a couple of hours. And then the soup would creep back overhead and hang there until the next day. Despite having grown up in North Dakota, I have to say that I have never been colder than on a summer evening in San Francisco. That cold damp wind comes roaring over the hills and rips into your inevitably underclothed self, as it's hard to talk yourself in to wearing adequate clothing. "It's August, for God's sake. It doesn't feel right to be putting on your winter coat!"

Posted by: Middle-agedF | June 12, 2009 11:16 AM | Report abuse

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