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!
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It’s a few degrees warmer in the city thanks to all of the hard surfaces and lack of shade, but you need to dress in layers regardless because the weather changes throughout the day. It’s much more a function of weather and going indoors vs outdoors than it is the few degree difference between the city and the suburbs.
The bay area microclimates are more pronounced than most places. Usually entire regions share a similar basic climate. The difference between SF, san jose, and sacramento is pretty extreme even though they are not that far away from each other geographically.
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.
It’s complex, but primarily it’s the result of having many different types of surface combined with changes in elevation (hills & tall buildings) that affect airflow.
Asphalt, grass, woods, and especially water heat up at different rates when hit by the sun, and cool down differently as well; consistent shading, like from hills, affects this, too. This leads to somewhat predictable airflows – hot air rises, cold air rushes to fill in, etc. This changes temperatures of the air, which interacts with humidity to create or destroy fog. Fog itself scatters sunlight so it can keep its own area cooler even if nearby, where fog never formed, it’s a bit warmer. San Francisco temperatures also tend to be around some inflection points (none of this would matter much if it were consistently 95F) so the differences appear more drastic.
So, basically, complex interaction of the many different things present in SF that aren’t there in, like, Houston.
The landscape around you has a huge influence on the climate of an area.
For example an area primarily consisting of open, concrete areas will typically heat up quicker and feel hotter.
An area with a lot of plants and vegetation will be cooler, as the plants will help create shade and diffuse the heat of the sun.
An area near a body of water may be more humid or slightly cooler as the water acts like a heat sink absorbing the warmth of the sun and evaporating.
Wind patterns will be affected by the shape of the surrounding landscape and the way the air flows around it.
Add together all of these factors and more, and an area will develop a specific signature to the weather patterns it typically experiences.
Often these patterns will gradually change and it won’t really be too noticeable, but in certain locations you will find a much more abrupt line – where a certain geographical feature like a hill divides two areas, or a specific wind pattern flowing down a valley is suddenly cut off or redirected there can be quite a sudden change.
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