A type of fog which forms on cool, clear, calm nights near the ground, and usually disappears by noon.
Radiation fog, also known as ground fog, occurs as the ground cools off at night by radiating energy (in the infrared) out into space. For optimal cooling, skies should be clear without any shielding clouds. If the air is calm near the surface and close to its dew point, it can easily cool to the point of saturation, leading to the formation of fog.
The fog should still be around at sunrise, but as the sun's radiation heats the earth the ground-level temperature once again rises above the dew point. Hence the fog disappears; this is known as "burning off."
Driving through mountains as the sun comes up is probably the best way to experience this fog. It sits in the valleys, not reaching up to the peaks, and usually below the level of the road. But if the road dips below the top of the fog, you'll suddenly find your visibility reduced to practically nil until regaining clear sight just as suddenly when you climb up to clear air once again. Though driving through fog isn't pleasant, traveling above it adds a fantastic element to the scenery. It can be breathtaking. In my experience, the best mountain range in North America for morning fog is the Appalachians; the air in the Rockies is usually too dry.
Despite its positive aesthetic effects, radiation fog can be a particularly insidious hazard to airplane pilots. It occurrs under precisely those conditions usually considered ideal for flying, so pilots must take precautions even when the weather seems perfect.