"The coldest winter I ever spent was a summer in San Francisco." This famous (although apocryphal) remark from Mark Twain was as true then as it is still today, and if you've ever wondered why our summer climate feels so bizarre, you're definitely not alone. Many first-time visitors to our unique city between June to August expect a sunny Californian escape, only to be overwhelmingly greeted by dense fog and blustering cold winds.
This unrelenting fog, or Karl as affectionately personified, has become such an iconic character of San Francisco for the summer, separating us apart from other American cities otherwise sweltering during this season. Surprising to some and pleasant to others, we've got the clear breakdown to answer all your questions on how this chilly blanket of gray occurs, which actually arrives through an astounding scientific phenomenon.
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The answer is equally intuitive as it is fascinating- as the California weather heats up, a low-pressure atmosphere is created due to heat rising. This in turn invites in cold air from the city's surrounding Pacific Ocean that gets sucked in through the easiest access point- our Golden Gate.
Air always wants to move from areas of high pressure to low. Our ideal low-pressure zone acts as a powerful vacuum to siphon in this moist seawater air that condenses from the heat, an upwelling that creates Karl and the famous marine layer that remains due to this temperature inversion.
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A temperature inversion is when the air continues to warm as it rises, which is the complete opposite of what normally happens.
San Francisco's prime location by the water, its terrain, elevation, and pressure gradients all interact to enact this intriguing anomaly, with the warm air above keeping the cold marine layer and fog (specifically advection fog) below trapped, like a lid over the city.
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When it comes to explaining the rush of breeze that accompanies Karl, when all the air gets sucked through a bottleneck like the Golden Gate, the pressure to squeeze through comes out into our city in greater force, something called the "Bernoulli Effect."
Also recognizing how air moves from high to low pressure zones, greater heat from the rest of the state on certain summer days means a greater pressure difference, welcoming in bigger amounts of wind. Colder water cycling from Alaska, known as the California Current, also further adds to this catalytic disparity.
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But how often does Karl make an extended appearance? San Francisco averages about about 108 foggy days in a year, concentrated most during summer, playfully known as "June Gloom" or "Fogust."
This amount definitely plays a critical role in our local ecosystem, as many wildlife and plants have adapted to this environment. San Francisco's human residents have also grown to appreciate this wonderous weather, making the most of it through taking advantage of the chilliness in recreation, themed events, and captivatingly moody photography.
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While this chilly veil may not be the ideal summer accessory for some, you cannot deny its striking beauty and serene ambience. The fog acts as a cooling reprieve from other hot and humid environments for visitors, offering an exclusive must-witness marvel that continues to astound and sustain the city's nature, aesthetic, and magic.
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Photography by: Ian Simmonds/Unsplash
