I grew up right outside of San Francisco and was conditioned to dress in layers as the weather is so dependent on where in the city you are. I never understood why though. Thanks!
In: 8
So let’s start with fog (a.k.a. the marine layer).
Fog happens when the air temperature is the same as the dew point; it’s also, technically, a cloud. The Pacific Coast is the more fog-prone coast in the U.S., since cold water is coming down from Alaska and the difference in air temperature will be less, on average, than the Gulf Coast.
Most of the time in these conditions, the wind is blowing in from the ocean, and taking the cold air and fog with it.
Sometimes the fog is shallow, and the sun is strong enough to raise the ground temperature well above the local dew point, and the fog can burn off. Other times, the marine layer is thick and locked firmly in place, and there isn’t enough solar energy to raise the temperature. Now it’s cool and grey all day.
SF’s hills can also come into play. In general, air that’s being forced up a slope will saturate, and air coming down the other side will dry out. If the overall conditions are *just* in balance, this will mean fog on the windward side of the mountain, and clear conditions on the leeward side.
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