Whether you travel by car, bus, airplane or boat, fog can be your worst nightmare. The greatest problem in fog is visibility, which can at times go down to zero in very dense fog situations. As a matter of fact, visibility is part of the definition of fog. Fog is a cloud form at the surface of the earth consisting of a multitude of minute water droplets suspended in the atmosphere. According to international definition, fog is present when horizontal visibility drops below .62 miles or one kilometer.
Heavy fog is defined as visibility below one quarter of a mile.
Fogs of all types originate when the dew point and the air temperature become identical, or nearly so, provided that sufficient condensation nuclei are available in the air. This generally occurs two ways around this planet, the first by cooling the air to its dew point, such as advective, radiation and upslope fog. The second cause is elevating the dew point by adding moisture to the air mass, like steam fog or frontal fog. Fog seldom tends to form when the dew point temperature differs from the air temperature by more than 4 degrees Fahrenheit, and wind conditions over 10 m.p.h. are present. Read more about fog , ground fog and the different types of fog .