An English country girl in California: San Francisco: the air-conditioned city

Mark Twain's assertion that 'the coldest winter I ever spent was a summer in San Francisco' is oft-quoted around the Bay and is recorded on hundreds of fridge magnets and t-shirts. He was nothing if not accurate - the fogs that roll in from the Golden Gate lower the temperature far below that experienced inland. Right now, the western states are baking in a heatwave so severe it is being reported in the British papers, yet it is cool, sunny and breezy (blowing a gale) in San Francisco. It is known as the air-conditioned city - sunshine and wind together is the order of the day.

The Golden Gate Bridge veiled in fog, with the rest of the city in sunshine

Until now. Today was my first experience of the true San Franciscan fog, which blows in from the sea and sweeps inexorably from Ocean Beach to the Embarcadero, making the tourists shudder and the inhabitants turn up their heating. Yet it has an ethereal beauty, cloaking the streets in mystery and bathing the street lights in a ghostly halo. I have just walked home through Pacific Heights, one of the most beautiful districts of San Francisco, and could see wisps of vapour drifting past the imposing porches and neo-Classical facades of the city's grandest mansions. There was no one around, and, from the high ridge of Pacific street, both the Bay itself and the inland areas of the city, Buena Vista and Castro, were hidden in waves of shimmering fog.

Pacific Heights in fog

I went to university at St Andrews, on the west coast of Scotland, where the view was frequently hidden by the damp, clammy Scottish Haar. Its Californian cousin is cold, certainly, and when especially thick can bedew everything in glittering pinpricks of vapour, but it is gentle and mysterious, beguiling, not threatening. Even in daytime, one can see its folds sweeping by inexorably, as if driven by ancient purpose, unconcerned by the tiny human figures going about their insignificant lives.

Fog beyond the Palace of Fine Arts and an Americas Cup competitor

The Golden Gate Bridge veiled in grey, seen from Fort Point

The Golden Gate Bridge emerges

I had seen the Californian fogs before, in the guise of the marine layer that lies along the coast like a scarf dropped by one of Neptune's daughters. Driving up Highway 1 in a Ford Mustang convertible, the fog would periodically cast a chill over the road that made me shiver, before I climbed beyond its folds to brilliant sunshine. Hiding the horizon and lying thick and impenetrable wherever the cold sea met the hot land, the marine layer can cloak whole areas in grey cloud.

Evening mist off the beach at San Simeon

The marine layer from Highway 1

Sometimes, it burns off within hours of sunrise; other days, it firmly denies any glimpse of the magnificent ocean views that lie just beyond the phenomenon. I have never seen a weather system like it - on some days, the Golden Gate Bridge can be completely hidden, as if some giant, in a fit of pique, had emptied a sack of cotton wool balls over the entrance to the Bay, when the rest of San Francisco is basking in unbroken sunshine. If you cycle or drive through it, it is freezing, and turns your skin clammy and damp, yet within minutes of leaving its chilly embrace, you are as warm and comfortable as if you had spent the day on the beach. It moves faster than you would believe possible, sweeping in to swallow up the view, yet can dissipate in moments. It is capricious, magical, beguiling, strange and beautiful, as much part of San Francisco as the cable cars and, so long you have a warm coat to hand, one of the thousands of things that make this city one of the most wonderful on Earth.