Why is San Francisco so Foggy?

Many of us have heard about foggy London town but people who live in San Francisco know that this great city on the west coast of the United States can also’ be quite foggy and cool, especially in the summer. It’s tempting to think of San Francisco as a unique location and it may just be that.

It seems that the unusual atmospheric conditions are the result of a collision of large air masses, one hot and the other much cooler. Moisture in the masses will condense and produce clouds, as is usually the case when these types of air masses meet. But it seems that the hills and valleys of San Francisco also play a part in the foggy conditions.
Condense water vapor that might otherwise hang around at high altitudes dips to ground level, creating foggy conditions in The City. Accompanying this foggy atmosphere are temperate conditions, with few cold days and few hot days. If you haven’t experienced San Francisco weather, expect crisp days, cool days and at the best of times, clear air. But be prepared for fog!

Water, Water!

If that isn’t enough to satisfy your curiosity about fog and San Francisco, consider this: The city is surrounded by water, with the Pacific Ocean and San Francisco Bay supplying plenty of moisture. Sometimes the surrounding areas get very warm. This combines with the cool, moist air over the bay and ocean. When the winds are just right, the moist air moves into San Francisco as fog, which is just a ground-level cloud.

Naturally, if you move inland, into the main part of the state of California, you find less fog because the air is less moist. In some areas, there are no foggy days to speak of. But the warmth of these areas contributes to more summer time fog in San Francisco.

In some parts of San Francisco you may spend an entire day in foggy conditions. This is especially true near the marina and the ocean beaches. Fog might clear toward evening, depending on the amount and direction of wind. So, if you are going to central California and plan to make The City part of your holiday trip, be prepared for something other than sunshine and heat.

Though we often hear about sunny California, this won’t necessarily be the conditions you’ll find in the Bay area. You’ll have to stay inland or head south to experience the sun-and-fun. Oh, you might have a rare day for outdoor pleasure but the temperature may not be high enough for the swim suit.

All things considered, clearest weather you’ll find will come with cool temperatures inland. This produces less fog. Sunshine is more common in late September and in October. Contrary to popular belief, these may be the warmest months in the San Francisco area. You might plan to visit the Golden Gate and other attractions when the leaves are changing color and there is no fog to block the sun.