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 alot 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”.
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