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If you’ve visited San Francisco, or lived there in the past, then you are well acquainted with the fog that blankets its hills, sometimes obscuring from sight all but the tallest buildings. Despite the fog, the city's climate is quite temperate. Though it will boast a few warm days, particularly in early fall, and a few cold ones, temperature change in “the city” is not significant, bordering on the cool and crisp for most days.
The explanation for the fog is actually quite simple. The city is bordered on three sides by water, framed like a jewel between the Pacific Ocean and the San Francisco Bay. Meanwhile, the summer temperatures of surrounding cities, like those in the East and North Bay can cook in the summer months, with temperatures easily reaching into the low 90s to low 100s F (32.22-37.78 C). The heat produced by inland temperatures, combined with the cool water of the Bay and Pacific Ocean, and the winds coming in from the water, turn the city into a paradise for lovers of fog. North Bay and South Bay residents also benefit from fog creeping in at night to cool down hot temperatures.
Generally, the farther east you are from the coast, the less fog you get. Areas in the East Bay like Pleasanton and Walnut Creek receive very little cool down from what some weathermen in San Francisco call the “fair weather maker” of fog. Areas can be more or less foggy depending upon the location. If you’re near the SF Marina, or near Ocean Beach, the fog can envelop the area for most of the day during the summer months. The Sunset District tends to see fog clear just in time for sunset and then re-gather with the approach of night. Areas downtown may clear up sooner, depending upon proximity to the water.
Because the cities around San Francisco are warmest during the summer months, this is generally when the fog is deepest. Tourists often make the mistake of traveling to “sunny California” packing only shorts and tank tops, and find that stays in San Francisco can be very cold indeed. It’s a good idea to bring sweaters, long pants and sweatshirts if you plan to visit the city during summer.
The mildest weather begins to occur as inland temperatures cool, producing less fog. City residents and smart travelers relish the sunshine of late September and October. These are frequently the warmest months, when the Golden Gate is truly gold in the color of turning leaves and plenty of sunshine.