San Francisco Fog Explained

by | 14th September 2012



Image of San Francisco taken on August 16th. Image by NASA.

Ever wonder why San Francisco is so foggy?  Well NASA has the answer for you!

  • The foggy trend over this Californian city is due to something called marine layers which are formed in the Pacific Ocean.
  • A marine layer is cool, heavy which occurs when the cold ocean surface meets with the above warm air.
  • Winds from the west have have been pushing these marine layers into the city.  The layer brings with it thick fog that can envelop buildings, bridges, and even people!
  • These marine layers commonly occur in the summer due to overturns in the ocean water.  These overturns bring the cold, frigid waters of the deep to the surface, while sending the warm surface water down to the sea bottom.
  •  It is this cold water that meets with the warm air to form the marine layers which are responsible for San Francisco’s iconic fog.









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