Why is September the hottest month in San Francisco?

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Why is September the hottest month in San Francisco?

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San Francisco’s position, on a peninsula with mountains to every side, makes the weather primarily influenced by the ocean and, critically, *wind*.

The summer months see an interesting phenomenon, where interior portions of the bay warm up, which causes constant winds inland, which brings in cold, damp air from the ocean. In September, there’s a period where the inland areas are cooler than the ocean surface, which causes “warm”, dry air to blow over San Francisco from the east.

I don‘t know about San Francisco specifically, but areas next to the sea have a special climate (maritime climate, roughly translated from german). It works like this: the water works like a battery, it takes long to charge, and takes long to discharge (charge is warmth), so in spring and summer the water‘s only being heated up. Winds blowing from the sea towards the land are cold because of the sea. So late in the year the water is fully heated up and can slowly give away its warmth, which results in a cold summer and warm winter.

Going further into a landmass will cause the opposite effect. Our ground is like a small battery, it can‘t hold „charge“ for long, but doesn‘t have to be charged for long. Result is a warm summer and cold winter.