Dense fog shrouds Columbia this morning, the combined effect of yesterday's heavy rain and the presence of a cold front across central Missouri. Fog is the result of air that has reached its dew point; in other words, the air is fully saturated with water vapor. Further cooling would lead to precipitation while solar radiation (which will intensify by later in the morning) heats the air above its dew point and dissipates the fog.
In summer and fall, fog generally develops when cool air settles over warm, moist ground; conversely, in winter or spring, it typically occurs when warm, moist air spreads above cold ground. In either case, calm conditions favor fog development while the presence of wind tends to mix the air, disrupting its saturation. Since cool air, having a greater density than warm air, settles in topographic depressions, summer and autumn fog is most common in river valleys.
Fog is especially common in coastal areas, where large bodies of water moderate the air temperature while the adjacent land warms or cools with the seasons. In the Pacific Northwest, cold ocean water (courtesy of the Japan Current) is a chronic source of coastal fog while, in the British Isles, the Gulf Stream bathes cold, rocky landscapes with warm, moist air, producing the mild but foggy climate of that region.
