San Francisco Fog

San Francisco is dubbed as the “Land of Summer Fog”. Every summer, a fog envelops the San Francisco Bridge. Many visitors are amazed to find out that San Francisco is cold in summer. Every guidebook warns about our summer fog and advises to dress in layers. Of course, the many sweatshirt vendors are most pleased that tourists fail to prepare for our nippy summers.  As Herb Caen wrote,

The miraculous thing about the annual summer miracle is that the miracle continues… The fog is on schedule, morning and night, billowing through the Golden Gate despite the indignant protests of the horns under the great bridge. The tourists shiver and wrap themselves in their own arms but continue to walk around in their undershorts. I believe they are shivering in delight, actually. I don’t know who started the fad, now custom, of traveling in shorts rather than on the shorts but it is probably the same oaf who first removed his suit jacket upon being seated at a restaurant table.”[1]

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Summer fog is more specifically known as radiation fog. This phenomenon is most frequent during the long, clear nights of autumn, though it may occur at any season, particularly in rather stagnant air conditions near the center of a high-pressure area. The fog sometimes appears to rise from the ground; but actually, the only thing that rises is a zone of temperatures lower than the dewpoint of the air in question. As the ground and the moist air immediately above the ground slowly radiate their warmth away into space, the temperature of this lower air falls, approaching its dewpoint. When the two coincide, fog appears in visible form. As the top of the fog layer itself radiates away heat much faster than dry air, the fog usually continues to deepen until sunrise. According to San Francisco Government Website, San Francisco’s summer fog forms when a warm air mass flows across a cooler ocean. The surface waters near the coast are much colder than offshore Pacific surface waters.[2]

Another San Francisco resident, Harold Gilliam, informed that the San Francisco Bay Region:

the characteristic summer fog is regarded as a boon and a blessing, bring the clean, salty aroma of the Pacific and keeping the days refreshingly cool as it advances from the ocean in fantastic flowing forms that from day to day are unique and unpredictable. Shaped by the slant of the California coastline and the oceanic currents of wind and water, by the presence of a strait and estuary penetrating the coastal mountain ranges, and by the hill-and-valley contours of San Francisco and the Bay Region, the advance of the summer fog inland from the Pacific is one of the planet’s most awesome natural spectacles. The Golden Gate is the only complete breach in the Coast Range, which borders the Paci?c for most of California’s length. As a result, the Bay Region is the meeting place of continental and oceanic air masses. Through the funnel of the Golden Gate and San Francisco Bay, the immense aerial forces of sea and land wage a continual war, and the tide of battle often flows back and forth with regularity. The line between the two types of air masses, particularly in summer, may zigzag through the streets of San Francisco and extend in similarly erratic fashion across the entire region.[3]

Usually, summer fog is intensified if the breeze is somewhat stronger since the surface coldness is mixed by eddying and turbulence through a considerable layer of the lower air, causing true fog that blots out hillocks and hollows alike. The summer fog usually begins at or near the surface, working gradually upward, though to an observer on the ground it seems to materialize rather suddenly out of nothing more tangible than a few stray wisps of vapor. The maximum height of the fog depends on many aerological factors, but it is seldom more than a few hundred feet. Temperature in the fog layer usually decreases with altitude at something like the moist adiabatic rate (-2½°/1000 ft); and just above the top of the fog there is usually, if not always, a temperature inversion.[4]

As the summer fog creeps though the San Francisco Bay, this phenomenon has a propensity to deepen through the hours of darkness and, curiously enough, may increase for a brief time after sunrise. This last effect may be due to either of two factors: the power of sunlight to make floating particles attract water; or the increase of lower-level turbulence and mixing caused by solar heating at the surface. It is a rather frightening experience to take off from a clear airport at dawn, and see it fog in under you a few minutes later.

Moreover, the worst radiation fogs are found around great cities, where innumerable smoke, oil, and soot particles impair visibility on their own account, serve as ready nuclei for condensation, and hinder the evaporation of fog once formed. Thus the infamous San Francisco fogs, which greatly increased during the late nineties with the growth of the great city, may hang on in yellowish ‘pea-soup’ form, perhaps for days, until the advent of a new air mass and freshening winds. Though less tenacious, radiation fogs are nearly as common in many regions of the open country. They can also occur over limited areas of calm ocean, but the usual sea fog arises out of different causes.[5]

As summer fog is a frontal fog, water is added to air until it becomes saturated and water droplets form. This usually happens ahead of an approaching warm front which produces a warm gentle rain that saturates the air and results in widespread fog. Summer fog poses a danger to people. When summer fog occurs, it is advised that people should be alert and get a bearing on a safe refuge before an approaching fog gets so thick you cannot see your way. Nevertheless, the beauty, joy and wonder people behold when seeing the summer fog definitely outweighs the dangers. As summer is not as harsh, the fog makes the city cooler, an ideal tourist destination and a great place to live.