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Name: Angela
Status: student
Grade: 6-8
Location: MI
Country: N/A
Date: 2/7/2005


Question:
Where do the water molecules in the fog come from?


Replies:
Angela,

The water that becomes fog is always in the air in the form of water vapor, which is a gas. To get fog, you have to cool the air to a low enough temperature (called the Dew Point Temperature) that the water vapor can no longer remain just a gas. Below this temperature the water vapor forms into water droplets, which, when plentiful enough, become visible as fog. The same process creates clouds.

David R. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory


The water molecules in fog come from the atmosphere. However, the formation of fog is much more complicated than just the simple condensation of water vapor. I suggest you do a "Google" search on the terms: "mechanisms of fog formation" and "mechanisms of cloud formation". You will find sites like:

http://www.physicalgeography.net/fundamentals/8f.html and
http://www.atmosphere.mpg.de/enid/t9.html

The mechanisms of cloud formation and fog formation have a lot of factors in common because roughly speaking fog is 'just' a cloud on the ground, but the details of just "how" they occur can get more involved.

Vince Calder


Angela,

Scientifically fog is a cloud with its base at or very near the Earth's surface. Air is saturated with water vapor when sufficient water vapor is added to the air or, more commonly, when the air is cooled to its dew point. Near Earth's surface heat is readily exchanged between the ground and the air above. During evening hours, the surface radiates heat away, and the surface and adjacent air cool rapidly. This radiation cooling accounts for the formation of dew and some types of fog. The water vapor is in the air. I hope that this helps.

Sincerely,

Bob Trach


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