The region is buffered from the “dog days” of August and the midsummer heat of July by a natural air conditioning that arrives almost daily on a blanket of coastal fog.

The effect can produce a variety of microclimates with temperatures ranging from the upper 50s at the coast to the 90s and near 100 degrees Fahrenheit just a few miles inland.

Comparable conditions prevailed when the image to the right was taken by NASA's Aqua satellite at 2:25 p.m. Pacific Daylight Time on July 3, 2009.

Western parts of the city of San Francisco can be seen covered in coastal low cloudiness and further chilled by an onshore breeze.

In the close-up image, a narrow sliver of fog is visible streaming through the Golden Gate and over the heart of San Francisco Bay.

Coastal areas of Sonoma and Marin counties, north of San Francisco, were also blanketed with coastal fog.

Temperatures at 2 p.m. local time ranged from 58 degrees at San Francisco’s Ocean Beach to near 90 degrees around Concord, roughly 30 miles away.

The bank of maritime fog and low cloudiness is created along the California coast during summer by a phenomenon known as upwelling. Prevailing westerly winds, combined with the effects of the earth's rotation, drive surface waters of the Pacific in an offshore direction as they are bounced back out to sea when coming in contact with the land.

These waters are replaced by deep, cold currents that ride up over the continental shelf and then flow up to the surface. The strip of colder water then condenses the moist air hovering just above the ocean’s surface into a bank of maritime fog.

The upwelling also brings rich nutrients up to the surface, providing a bounty of food for a variety of marine life.

Image: NASA MODIS Rapid Response System