The Decline of San Francisco Fog

The Bay Area experienced an unusually hot summer due to the lack of the iconic San Francisco Fog; but the fog does more than just cool things down around town.


Fog is inextricably linked to images of San Francisco. A white blanket that sweeps past the Golden Gate Bridge, across the city, over the hills and down the streets. It strikes during summer, preying on ill-prepared tourists in t-shirts. The fog moves in slowly at first, then spreads faster and further as the wind increases throughout the day. When Karl The Fog wins (personification dubbed by a mysterious San Francisco resident), as he so often does, the city starts to vanish beneath his ethereal blanket, moving from northwest to the southeast.

San Francisco Fog
Fog moves from the coast across the city of San Francisco. Photo courtesy of Wikipedia.


It’s a battle of wills and physics – the high-angle summer sun versus Karl’s chilly and steadfast devotion to his city. The Presidio, the Marina, Lower Pac Heights- he swallows neighborhoods one by one. The Mission, Potrero Hill and SoMa often left to fend for themselves as a few of the city’s lone sunny outposts. Twin Peaks might as well be The Matterhorn as the thick cloud rips up the hill then tumbles back down the other side. And abandon all hope if you live in the Richmond or the almost rhetorically titled “Sunset”.

However the summer of 2015 was different. Heat waves seemed to be the norm. The traditionally overcast and cool summer days only occasionally broken up long stretches of above average temperatures. Weeks in a row without the fog allowing residents to enjoy Ocean Beach on a daily basis, an area typically overcast and windy.

This bizarre summer made Bay Area residents ask – what exactly causes the San Francisco fog, and what were the missing elements this summer?


How Fog Is Born

Fog is very simply a low lying cloud. As air cools, moisture contained within it is pushed out and condenses on microscopic particles, creating an equally microscopic water droplet suspended in mid-air. If enough of those droplets accumulate in a certain space the result is the beginning of a cloud. If the cloud is low enough to the ground and it limits visibility to less than 3000 feet, it’s referred to as fog.

The fog banks that reside off the coast of Northern California result from this same process occurring on a large scale. The warm air that sits above the expanse of the Pacific Ocean is humid, a result of moisture evaporating from long unobstructed exposure to the tropical sun. As the air moves from the warmer climates of the western Pacific toward the continental United States, it’s cooled by the progressively colder water. The newly cooled air pushes that moisture content out to condense on whatever particle is suspended in the air – often salt or other minerals kicked up by ocean spray. This forms the thick fog offshore, what is referred to as the ‘marine layer’.


CA coastal upwelling
Sea surface temperatures show cold water near-shore pushing warm water away from the coast as a result of upwelling

The marine layer alone isn’t enough to explain the density of fog that can consume the Northern California coast. As the offshore fog layer reaches the coastline it grows even denser as a result of the relatively colder near-shore water, a result of a process called upwelling. Water sitting at the surface along the California coast is pushed westward offshore by currents and winds. Deep water – frigid, but rich with nutrients – rise up to replace the departing surface waters. Upwelling off the California and Oregon coast occurs almost exclusively during the spring and summer, and it causes water near shore to be colder than surface water further offshore (see picture). Although it makes a wetsuit necessary during summer months, the wealth of nutrients brought up with the cold water is the reason that the West Coast has such abundant sea life.


Cold Ocean, Hot Valley

Fog squeezes through between San Francisco to the south and the Marin headlands to the north
Fog pushes through the opening between San Francisco to the south and the Marin headlands to the north. Photo courtesy NASA.

The chilly Pacific and the blistering summer heat of the Central Valley creates a dramatic temperature difference, referred to as a gradient. As the hot air over Sacramento and the rest of the Central Valley rises, the cooler air from the west moves in to take its place. Like a ball rolling down a steep hill- the larger the temperature gradient, the stronger the winds blowing in from the coast. This strong contrast puts San Francisco in an uncommon climate zone.

The wall of fog has developed, now it just needs an open door. While the steep cliffs that symbolize the California, Oregon and Washington Coasts act as a wall to keep the fog from penetrating beyond the immediate coastline, the natural harbor of San Francisco Bay is the fog’s easy way in. The fog moves through the opening between San Francisco and the Marin Headlands like almost nowhere else on the west coast. A geologic bottleneck that creates a powerful fog and wind tunnel; something to which anyone who has driven, walked or biked the Golden Gate Bridge can attest.


Courtesy of Newsbound, "De-mist-ifying The San Francisco Fog"
Courtesy of Newsbound, “De-mist-ifying The San Francisco Fog”


“The Coldest I’ve Experienced Was A Summer In San Francisco”

Famous hyperbole aside, the San Francisco fog can turn comfortable summer days to a damp winter-like chill in the matter of hours. When the full-force of the Pacific Ocean marine layer is brought to bear it can produce a fog so thick it appears to be raining. In its most delicate form the fog keeps temperatures in San Francisco comfortably mild, but the lack of fog through much of July and August of 2015 led to an uncharacteristically hot summer. Based on temperatures taken at San Francisco International Airport, the number of days in July and August (2015) that were above the normal high temperature mark were 22 and 23 days, respectively. August had two record high temperature days and two more that almost were. July didn’t break any temperature records this year but there were three days that were only a few degrees shy of record highs. As of this writing, September has seen 11 days with temperatures above the average high, one day that tied for record highs and three near-misses. Karl The Fog has clearly been on extended vacation this year.


The range of conditions that can occur during the summer as a result of the fog. Pictures taken from a BloomSky weather station located near Fisherman’s Wharf; the Golden Gate Bridge is on the far left and Alcatraz Island on the far right.



Water When We Need It

The fog does more than cool things down. The vast amount of moisture transported inland from the coast is crucial to vegetation, much of which would all but dry out during the dusty California summers. The giant Redwood trees, unique to the Central and Northern California coast, are direct beneficiaries of this special method of irrigation. Despite the virtually rainless summer climate in California, these impressive trees get about one third of their necessary moisture during the summer, derived directly from the fog.

Fog helps to block the strong summer sun, reducing evaporation of the limited amount of moisture in the soil. It also provides much-needed water all along the coast during the driest time of the year. The fog hits the impassable wall of Redwood trees that are hundreds of feet tall. Moisture collecting on leaves and branches falls like rain to the forest floor; it’s quite literally a rainforest microclimate surrounded by an arid region.

However as the trees can grow to over 350 feet tall, the Redwoods need a special method to overcome gravity in order to pull water all the way up from their base. The water remaining on the leaf that doesn’t fall to the forest floor can be absorbed through the leaf surface and moved down the tree from above. This top-down process helps pull water up the extreme heights of the trunk from below like a siphon; but if the water ceases from above then an air pocket forms in the trunk, killing the tree.

In addition to just receiving water from the fog, biologists who study the trees believe they collect nutrients trapped in the water as well. Remember: the fog forms around particles in the air, including the minerals kicked up out at sea from the nutrient-rich ocean. It takes a lot of nutrients to keep a thousand-year-old tree growing.



This summer has seen a very large and stagnant body of water in the Pacific; the lack of circulation has warmed the water even more and further reduced the northerly winds that would ordinarily churn up and cool down the water. The resulting weaker winds over the months of June, July and August have led to less upwelling to feed the fog. This “feedback loop” is a difficult cycle to break and will likely persist until a large event, such as a storm, instigates a shift in the pattern.

The noticeable absence of the San Francisco fog this summer seems to be indicative of a larger trend. As with all airports, weather is pivotal for daily operations at San Francisco Airport. The airport has been diligently tracking visibility on an hourly basis since the 1950’s, as a Berkeley professor studying the area’s fog had discovered. Their data shows that the fog is thinning. The number of recorded foggy hours, as well as the intensity of the fog, has decreased by over 30% in the last 60 years. This pattern means that San Francisco’s iconic fog, not to mention the Redwood forests themselves, are being threatened.

An abnormally warm Pacific Ocean is a large part of why the weather this summer has been so unusual in the Bay Area, but only explains the recent lack of fog. Only time will tell what next summer will bring, and if the fog will continue to decline over the next decade and beyond.







“The Deep, Cold Secret Behind Summer Fog.” QUEST. August 8, 2011. Accessed August 30, 2015., Accessed August 30, 2015

“Upwelling.” NOAA Ocean Explorer Podcast RSS. Accessed August 30, 2015.

“Another Foggy Day in San Francisco : Image of the Day.” Another Foggy Day in San Francisco : Image of the Day. Accessed August 30, 2015.

Tennesen, Michael. “Clearing and Present Danger? Fog That Nourishes California Redwoods Is Declining.” Scientific American Global RSS. December 9, 2010. Accessed August 30, 2015.

“National Weather Service – Western Region Headquarters.” NOAANWSWestern Region GeoRSS News Headline. Accessed September 21, 2015. Accessed August 30, 2015.

File:San Francisco Downtown, and Golden Gate Bridge early morning panorama.jpg. Accessed August 30, 2015.,_and_Golden_Gate_Bridge_early_morning_panorama.jpg Accessed September 18, 2015.

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