San Francisco's Seasons of Fog

San Francisco is well known for its frequent fog. In fact San Franciscans are so familiar with this weather phenomenon that they are now on first name terms. The reason that San Francisco sees so much fog, especially in the summer, is that big expanse of water called the Pacific. The cold ocean waters of the Pacific cools the warm air above. Cool air doesn't hold as much moisture as warm air. The moisture therefore condenses as the warm air is cooled, creating fog.

In the mornings the sun begins to heat the land. Hot air rises and the cooled foggy air over the Pacific is sucked inland. As the day progresses the sun heats the air and San Francisco's fog is therefore (usually) burned off during the afternoon.

You can see this process very clearly on Fogust, an interactive map visualizing San Francisco's fog by month and time of day. The map uses historical data from NOAA's GOES-15 to provide a visual guide to the historical levels of fog experienced during different months and over the course of a typical day.

The map has three buttons for each month of the year. Judging by the map July and August seem to be the foggiest months. If you switch between the 10 am, 12 pm and 4 pm buttons in July then you can observe the process described above, as the the fog forms over the Pacific, rolls inland and then gets burned off in the afternoon.