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 are 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 almost always the result of rising of air, which cools from expansion. But 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.
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