NEWTON:Highest Altitude for Fog
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Name: Sandra
Status: educator
Grade: 6-8
Location: Outside U.S.
Country: USA
Date: Spring 2014

What is the highest altitude of fog?


Fog can occur at any altitude, even at greater elevations of mountains.

Fog occurs when the air temperature drops below the dew point temperature. This commonly happens at night as the Earth's surface cools, or when a cold air mass moves into an area where there is sufficient evaporation taking place from the ground, snow, or a water body to add extra water vapor to the air.

David R. Cook Meteorologist / Team Lead Atmospheric and Climate Research Program Environmental Science Division

Fog is droplets of liquid water that form when concentration of water in the atmosphere exceeds the concentration of water-saturated air at the existing temperature. In some cases the water droplets vapor pressure even exceeds the equilibrium vapor pressure of water at the existing temperature. This common condition is thermodynamically unstable. However, because the fog droplets as so small, the surface tension prevents their growth of larger droplets. Fog droplet formation and dissipation is a complicated atmospheric condition. The web site gives a good explanation of this complicated atmospheric phenomenon.

Vince Calder


The factor distinguishing a fog from a low cloud is the source of the water vapor that forms the fog as stated in this Wikipedia article:

"The term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated locally (such as from a nearby body of water, like a lake or the ocean, or from nearby moist ground or marshes).

There does not seem to be an agreed max altitude for something called fog, but this aircraft/helicopter pilot would call the max altitude of a fog to be 750 feet above ground level (AGL) which is a number that is arguable, but is in the ball park. Certainly any moisture obscuration at 1000 feet above ground level would be called a cloud.

Sincere regards, Mike Stewart

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