Spring fog bank over the Outer Battery, St. John's
This type of fog is known as advection fog. It forms when warm moist air overlies a cooler surface, in this case a cold ocean current (but could also be a lake or snow surface). With the air near the surface cooling below its dew point temperature, fog develops. Once it moves inland, the warmer ground surface raises the temperature of the air above its dew point and the fog dissipates.

Reproduced by permission of Trevor Bell. Photo ©1995.