Fog, just like clouds, is formed when invisible water vapor in the atmosphere condenses in the air into visible water droplets. You can best think of fog as simply a cloud that reaches the ground. As a pilot you are bound to encounter fog at some point, probably sooner than later as it is one of the most common aviation weather hazards. Fog is broken down into several different types and it’s important to understand the characteristics of each one. This will help you identify the possibility for fog during your preflight.
Three of the more common types of fog, and those you will likely see questioned on your knowledge exam, are:
Radiation fog (ground fog) is formed when terrestrial radiation cools the ground, which in turn cools the air in contact with it. When the air is cooled to its dew point (or within a few degrees), fog will form. This fog will form most readily in warm, moist air over low flatland areas on clear, calm nights.
Advection fog (sea fog) is formed when warm, moist air moves (wind is required) over colder ground or water. An example is an air mass moving inland from the coast in winter.
Upslope fog is formed when moist, stable air is cooled to its dew point as it moves (wind is required) up sloping terrain. Cooling will be at the dry adiabatic lapse rate of approximately 3°C per 1,000 feet.
Additional types of fog include:
Frontal fog (precipitation-induced fog) forms due to the interaction of two air masses. Most commonly associated with a warm front but may also form from a stationary or slow moving cold front. As rain falls from warm air into the cooler air ahead it can become saturated causing prefrontal fog.
Steam fog is formed when cool air blows over a warm moist surface like a lake or warm sea. Water vapor rising from the moist surface is cooled by the cool air above to below its dew point temperature.
Understanding fog and the different types and characteristics will allow you to be a safer pilot. One major area you can look at while conducting your preflight weather briefing is the temperature/dew point spread. This information is a direct correlation to the formation of fog: the smaller this spread, the greater likelihood for fog. Dew point is the temperature at which the air becomes 100% saturated and condenses into visible water droplets. Those water droplets are seen by you and I as fog. A quickly falling temperature and a decreasing temperature/dew point spread is a good indication of fog in the forecast. You can find additional and more in depth information on the subject of fog in The Pilots Manual series textbooks.
