Advection Fog

Fog is a cloud that is actually on the ground!  There are many different ways that fog forms.  What’s behind advection fog, though, is pretty interesting.  Advection fog is most common along the West Coast of The United States during the spring and first few months of summer.  In fact, most of the fog that affects San Francisco, and the nearby coastal areas, is due to the advection fog process.  Namely the fog that affects the West Coast is known as sea fog,  Advection fog may last for hours, or even days.

Advection Fog From Above

The way advection fog, or more specifically sea fog, is winds blow over warm water that is relatively far out to sea.  There, the air picks up warmth and moisture.  The winds then take that air and drag it over cooler water nearer to the coast.  This process condenses the water vapor out of the air and develops the cloud.  The winds then take the cloud, which is near the surface of the water, and drag it over the near-shore areas of the land.

Advection fog can also occur in interactions between warmer water and cool landmass areas.

In fact, advection fog can occur during other seasons and in other parts of the country.  On a smaller scale, advection sea fog occurs in the Gulf South during the wintertime months.  Here, winds out of the southeast drag air over the warm waters of the central Gulf of Mexico and eventually over much cooler near-shore waters developing fog over places like New Orleans and Southeastern Louisiana.  In fact, sea fog will be socking in Boothville in far southeast Louisiana prompting fog advisories through Friday morning.

US Watches & Warnings, Image: NOAA