Dense fog shrouded much of the Midwest this morning; the visibility in Columbia was less than a third of a mile. Extensive fog is most common during the early spring and late autumn; in the spring, it is generally due to warm, moist air settling over cold ground while, in the fall, it is usually caused by cold air interacting with warm, moist soil. In either case, the air near the ground reaches its dew point, i.e. it becomes fully saturated with water vapor, and fog develops; calm conditions or light winds favor this process while stronger winds mix the air and disrupt the saturation.
The fog will last until the surface air temperature rises above or falls below its dew point. Should the air cool, precipitation occurs and the fog dissipates; if the air warms, it expands and becomes less saturated, in which case we say that the fog has "burned off."
Today, the dense, early morning fog was associated with an air temperature of 38 degrees F. Five hours later, the temperature has risen into the mid forties and the ground haze has given way to a low overcast (the fog has lifted to an elevation where the air is at its dew point). Should the day continue to warm, the cloud cover will continue to rise and may clear altogether.
