The Karl the Fog Tracker

San Francisco is well known for its frequent fog. In fact San Franciscans are so familiar with this weather phenomenon that they are on first name terms with their local atmospheric weather anomaly.

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. When this warm air cools the moisture condenses - creating fog. In the mornings the sun begins to heat the land. This 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 (usually) burned off during the course of the morning and afternoon.

You can view the latest fog conditions using the Bay Area Fog Tracker. The San Francisco Chronicle's Bay Area Fog Tracker is an interactive map which visualizes the current fog and cloud conditions in San Francisco. The interactive map allows you to view a 12 hour loop of cloud and fog conditions in the Bay Area. It also includes details on the current amount of cloud cover, the height of the cloud ceiling and the current visibility (in miles).

You can see the daily pattern of San Francisco's fog very clearly on Fogust, an interactive map visualizing San Francisco's fog by time of day (and by month). 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. According to the map July and August are 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.