by David Murphy
Fog forms when moist air near the earth's surface is cooled, or when moisture is added to cool surface air.
For example, you'll often see fog develop at night after a rain, when there's plenty of moisture in the ground and the air is cooling off overnight. Cool air gently moving into a warm, moist region can also trigger fog.
An important property of air comes into play here. Air's ability to store moisture changes as its temperature changes. Specifically, cooler air cannot store as much moisture as warmer air. As a result, when air is cooled, its moisture (which is held in the air as invisible water vapor) condenses into visible water droplets. When this happens above the ground, you get a cloud. When it happens at ground level, you get fog! Again, clouds and fog are the same thing---they're both made-up of tiny water droplets. The only difference is their location above the surface.
But fog doesn't form easily...
Special circumstances must be present. It usually helps to have moist soil---a source of additional moisture right at ground level. You also want the air at the surface to cool. The best time for this to happen is just before dawn, because the sun has been absent for a long period of time and the ground has had all night to give up the heat it stored during the previous day. Two additional factors also help with fog formation: clear skies and light wind. Why? Clouds and wind both prevent heat from escaping from the surface. The water droplets in clouds absorb some of the rising heat and radiate it back to the ground. Wind carries rising heat horizontally along the surface, rather than allowing it to rise. Indeed, the most common type of fog usually forms in the pre-dawn hours when the sky is clear and winds are light. As the temperature drops, the air at the surface (which has absorbed some of the evaporated moisture from the damp soil) can no longer hold onto the invisible water vapor. The water vapor condenses into water droplets---and you have fog!