To get at what is going on here first we need to recall what the temperature of an object says about that object’s atoms. The atoms of any object are always in motion jiggling around, and the temperature of an object is a measurement of the average jiggling of that object’s atoms: the hotter the object the faster the average jiggling of its atoms. If the temperature of air is dropped below a specific temperature, called the dew point, then the water vapor in that air condenses into a fog as the water molecules jiggle around slower and slower and eventually clump together.