Fog Formation

Fog is basically just a cloud sitting at the surface, be at that sea level or in mountain areas. For pilots flying early morning on track to their destination it can be a very limiting factor. Airports located in a valley or in a lower part of the country are also favorite places for fog to form when conditions are right.

For pilots it is important to understand the processes leading to the formation of fog, it might save the day and flight. Fog or cloud develops by either cooling a parcel of air to its dewpoint or by adding enough moisture to reach saturation.

Types of Fog

There are two types of fog formation processes, cooling the surface by radiation of heat and by advection of moist air over an already cool surface. In both cases air above the surface will cool to its dewpoint (RH 100%) but the way it does that is different as are the circumstances around it.

For fog to form the temperature/dewpoint (T/Td) spread must be small enough or a RH of 100% will never be reached and an abundance of condensation nuclei are the main factors.

Dew and radiation fog

After sunset, when the earth does not receive anymore solar radiation (insolation) it will start to cool and radiate heat into outer space when there is no cloud layer. Conduction will cause the air layers adjacent to the earth to cool also. Provided the relative humidity is high enough, only a little cooling is needed and the air will become saturated and condensation takes place. As air is a very bad conductor the result will be a shallow layer of water on the surface. Dew has formed.

Fog formation

For fog to form there is a need for a very light wind. This makes sure that cool surface air will mix with the layers above and the layer within which water drops are present so that fog will form. If there is too much wind fog will not form but low stratus is likely to develop and it will be detached from the surface.

Thus we need the following conditions for fog to form: a clear sky, very light winds, relatively high humidity and a stable atmosphere. Conditions wich we easily find in high pressure areas and in autumn and beginning of winter when the surface is already starting to cool and warm moist air is still present. In spring, the same can happen when warm moist air moves in over a cold surface, low stratus and fog are common then.

Fog becomes cloud

After sunrise, the sun will start to heat up the friction layer at the top of the fog bank and this will start to mix the air even more and droplets at the top of the bank will begin to evaporate (demanding more latent heat) so that the fog initially will intensify in the first hour or so. After some time and when the heat of the sun has a chance to warm up the earth the fog bank will detach and will start to lift off in a few places and low stratus and or stratocumulus is formed. In absense of sunshine (high altitude cloud layer) the fog can persist for many hours.

Radiation fog occurs very often during night time, but as said above, a light wind or mixing of air layers is reguired to form it. After the sun rises this mixing will help the formation of fog at or just after that time. Sometimes this type of fog can be very persistent, especially near water bodies or in lower areas as valleys where katabatic winds will help the formation.

Valley fog

Usually forms in a valley where a river or lake supplies the warm moist air and a katabatic airflow from the valley walls supplies the cool air thus encouraging condensation and fog formation in the valley. This can be persistent in winter time when the sun is unable to warm the valley walls, depending on orientation, due to the low angle of the sun over the horizon. This type of fog starts as a radiation type but when the cool air decends into the valley it turns into advection.

Upslope fog

Sometimes, when a light wind blows moist air uphill over a snow covered mountains, upslope fog may develop. It has then become advection fog.

Steaming fog

There are a number of causes for the formation of steam fog: addition of moisture from a warm wet surface to a layer of colder air above so that this air becomes saturated or by very cold land air over a relatively warm sea of lake surface.
An example of steaming fog is when in winter time (Alaska) cold air flows offshore over a (relatively) warm lake or sea and if the RH is high, fog develops easily over the water surface when the layer of air is stable, else this fog will lift off and form stratus.