As we go through late summer and fall, fog becomes a part of our daily weather report for the early morning hours for most areas in the valley. John Lang from Greenup, Kentucky asks "what causes fog to form at night? It seems to be most common this time of year."
As the night in a valley becomes cool, clear, and calm, the ground radiates (releases) energy gained from the sun during the day in to space. Because the ground radiates energy in to space, it begins to cool. As it cools, the air interacting with the ground begins to warm the ground through conduction, trying to offset the change in temperature of the ground. The ground continues to cool as it still radiates energy, including the energy transferred to it from the air. The air then begins to cool near the ground in and around the valley. This leads to relatively warm air being higher away from the ground and the sides of the valley. Since cold air is denser than warm air, the cold air sinks to the lowest point in the valley, along the valley sides, as shown in the valley fog diagram. As the cold air sinks, warm air aloft replaces that air. This new warm air transfers more energy to the ground. The ground continues to radiate energy in to space and, through conduction, the air temperature of the warm air lowers. This air becomes relatively cool, sinks into the valley and adds more cool air to the lowest part of the valley. This cycle repeats and cool air continues to sink into the valley. This process is known as cold air drainage and it is just that; cool air draining in to a valley. The valley begins to fill from the bottom with cold layers of air. If there is sufficient moisture in the air, fog will begin to form in layers as the air temperature approaches the dew point. The point at which dew forms.
There's an old saying, for that every foggy morning you have in August equals a snowfall in winter! We shall see...
Thanks for reading, keep those great questions coming and be sure to post you comments!