In the still, cool, clear mornings of autumn when the damp earthy smell is almost a sweet scent, it reminds us of why we choose to live here. The beauty that autumn brings is almost beyond comparison. The skies are often a brilliant blue and the quickly moving puffs of cumulous are often a stark white. But with all that beauty there is another element of autumn that while fine to look at when sitting on the ground, can be serious and sometimes deadly for unaware or unprepared pilots. That element is fog.
Fog, in its simplest definition is like a cloud that forms at or very near the ground. There are different types of fog that form under different atmospheric conditions and because of unique topography. It will most likely begin to develop when the temperature-dew point spread is less than 5 F and is decreasing. When the temperature-dew point spread begins to increase fog would begin lifting.
The two main types of fog that most people will experience are radiation fog, and advection fog. One might also experience upslope fog near mountains, and sea fog near large bodies of water, and it isn’t uncommon in winter to see freezing fog that coats trees and fences with a prickly, white frosting.
Radiation fog, commonly referred to as ground fog, develops as the daytime heating of the ground is released back into the atmosphere at night. As the ground cools, and assuming there is a sufficient layer of moist air very near the ground, the air becomes saturated and fog develops. Radiation fog will always be found at ground level and that fog layer can vary in thickness from 3 to 1,000 feet. This type of fog is stationary and is noted for reducing forward visibility to near zero.
Advection fog may appear to be like radiation fog as it also develops as a direct result of condensation. But rather than being caused by surface temperature variation, it is caused by the horizontal movement of warm, moist air over the colder surface. This type of fog differs from radiation fog by its horizontal movement along the ground.
All pilots need to remain aware of the conditions which cause radiation fog and advection fog. Remember, radiation fog is generally more localized and doesn’t last as long once daytime heating begins. Thus when flying at lower altitudes, pilots may encounter areas of it as it begins to lift until it dissipates in the now warmed air.
By getting a good weather briefing before takeoff and maintaining weather awareness throughout the flight, you will be able to enjoy the beauty of the season that can only be seen from above.