Fog is simply a cloud that touches the ground. It forms when air cools to its dew point - the temperature at which the air can no longer hold moisture as a gas. This causes invisible water vapor to condense into tiny, floating water droplets. Depending on the local weather and geography, fog is created in a few specific ways.
Radiation Fog: Forms on clear, calm nights when the ground radiates heat into the atmosphere. The cooling ground chills the air above it to the dew point.
Advection Fog: Created when warm, humid air moves over a colder surface (like cold ocean water or snow). This is very common along coastlines.
Evaporation (Steam) Fog: Happens when cold air passes over warmer water. The warmer water evaporates moisture into the cold air until it saturates and condenses.
Upslope Fog: Occurs when moist air is pushed up a hill or mountain by the wind. As it rises, the air expands and cools, forcing condensation.
Valley Fog: Cold, heavy air sinks into a valley, trapping moisture. It often forms during the winter and can last for days.
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