Fog is essentially a dense cloud of water droplets, or cloud, which is in contact with the ground.
When night conditions are cold, clear, and calm, the ground releases the heat it absorbed during the day.
As the temperature of the ground decreases, it cools the air above it to the dew point (the point at which water vapor condenses into droplets of liquid water), forming a cloud of water droplets known as radiation fog. This is the kind of fog one sometimes sees settling in a valley.
Fog also forms when warm, moist air travels over a cold surface. The moisture in the air condenses and forms advection fog, or "land fog."
There is also another type of fog known as sea fog, which is carried from place to place on air currents. This type, which often occurs around San Francisco in the United States, is difficult to dissipate because it continuously forms.
Water droplets are only about 0.01 millimeter in diameter. A dense fog contains about 1200 visible drops per cubic centimeter of empty space - barely enough water to wet an object's surface.
Fog is distinguished from mist only by its density, as expressed in the resulting decrease in visibility, fog reduces visibility to less than 1 km, whereas mist reduces visibility to no less than 1 km but less than 2 km.
The foggiest place on Earth appears to be the Grand Banks, an area off the coast of Newfoundland.
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