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jawad_babar
Dec 25, 2005, 10:17 PM
Hi, can anybody explain me how fog is made?

PrettyLady
Dec 25, 2005, 10:29 PM
Fog is the condensation of moisture in the atmosphere near the surface of the earth. And fog is formed in an analogous way through the rapid cooling and condensation of water vapor in the air near the earth's surface.

jawad_babar
Dec 25, 2005, 10:42 PM
Thanks for the reply.
I understand that fog is actually water vapours that are condensed. Everywhere air has water vapours and has a certain level of humidity but the fog is produced only in specific regions.
I live a city which is on plain and coming days will be extremely foggy but if I travel towards north, they are more colder but there is no fog in those northern areas. In fact these northern areas are even more humid.
I need to know what are the conditions under which fog is produced in certain areas and is not produced in other areas.
Will appreciate comments on this.
Thanks.

PrettyLady
Dec 25, 2005, 10:50 PM
Fog can be considered a cloud at ground level. The processes forming it, however, are usually different from those that form clouds. Like clouds, fog is made up of condensed water droplets which is the result of the air being cooled to the point actually, the dewpoint where it can no longer hold all of the water vapor it contains. For clouds, that cooling is from the rising of air parcels, which cools from expansion. For fog, which occurs next to the ground, there are usually other reasons for this cooling. For instance, rain can cool and moisten the air near the surface until fog forms. Also, infrared cooling of a cloud-free, humid air mass at night can lead to fog formation - this is called "radiation fog". Radiation fog is most common in the fall, when nights get longer, and land and water surfaces that have warmed up during the summer are still evaporating a lot of water into the atmosphere. Finally, a warm moist air mass blowing over a cold surface (usually snow or ice) can also cause fog to form this is called "advection fog.

In the spring, snowcover can melt rapidly if a warm, humid air mass overrides the snow and the snow then lowers the temperature of this humid air to the point that fog forms. The fog droplets forming at the snow surface release the latent heat of condensation, which helps to melt the snow.

fredg
Dec 26, 2005, 03:38 AM
Hi,
Here is a link that might answer your last questions:

http://www.newton.dep.anl.gov/askasci/gen99/gen99301.htm

Best wishes.