Fog is actually tiny water droplets in the air. When the air temperature
and the dew point (point at which water condenses) become the same,
the water in the air condenses creating water droplets as fine as
a mist. The type of fog that hugs the ground and is the greatest
menace to drivers is called "radiation fog". It is called
radiation fog because the process begins when heat is radiated from
the Earth into space at an unusually high rate, such as on clear
nights (meteorologists call this radiation cooling). As the ground
cools, it cools the layer of air near the earth's surface. If there
is enough moisture in the air, it condenses into tiny water droplets
that float in the air - fog. This kind of fog hugs the ground, and
is usually less than a couple of hundred feet thick.
Fog usually occurs in weather that
seems "perfect", usually on still evenings or mornings.
Fog often forms in valleys and low spots because as the air cools
it becomes heavier and flows downhill. A patch of dense fog may
extend for only a few feet or for miles. Wind can prevent fog from
forming because turbulence caused by blowing over hills, trees,
houses, etc. causes the cool air to mix with warmer air from higher
elevations preventing conditions that cause fog. In the morning,
the sun's heat warms the ground and air and causes the water vapor
to evaporate or "burn off" the fog.
