Advection Fog Formation
Fog is basically just a cloud sitting at the surface, be at that sea level or in mountain areas. Pilots flying early morning on track to their destination this 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 the conditions are right.
It is also 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. When pilots understand this thoroughly, it contributes greatly to their safety.
Previous page talked about radiation fog, here we dive into the other form: advection fog. You will see this type of fog in coastal areas hindering VFR flights any time of the day when conditions are right for it to form.
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 surrounding 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 must to be present too. These are the main factors.
This type forms when moist air moves over an already cold surface and cools from below so that the temperature reaches its dew point the air gets saturated and condensation will take place. There already is a wind moving this air so we have a mixing process taking place at the same time. The air must be stable, else the fog will develop in a stratus/Stratocumulus cloud. And as with radiation fog an inversion must exist so that the fog bank is of limited vertical extend.
They may look the same but there are a few differences between radiation and advection fog:
- Radiation fog forms over land only, where advection fog can form over sea too: cold and warm stream fog.
- Advection fog needs a surface that is already cool (water or land).
- Radiation fog disappears some time after sunrise but advection fog can persist for days, given the right conditions.
- For radiation fog a high pressure area or a col is needed in contrary to advection fog, these are favorable for lights winds.
Both have the same basic principles of formation but under quite different circumstances and events. The location where these types of fog develop can also be related to certain geographic areas, for example, advection fog formed over sea can travel inland and block airports close to the sea or situated in lower parts.
For advection fog to disperse one of the formation factors need to change: thus either less moist air, a change of wind direction or heating of the cool surface, but this might prove difficult if there is a cloud layer above.
On the approach of a warm front, where the warm air will move overhead the heavier cold air, with light winds and any precipitation into the cold sector ahead of the warm front the cool air will cause saturation and fog / stratus development.
Upslope is the same as frontal fog, warm and moist air moving over a cool layer (be that an mountain or hill upslope or a cool pocket of air), its just a different name for the same phenomena.
This type develops when a cold stream is surrounded by warm water and a light wind blows warm moist air over the cold stream, fog will develop but it will be restricted to the cold stream region.
With warm stream fog it is expected that the air above it is warm and moist. When a light wind blows this warm air over a colder surface or body of water, cooling from below will saturate the air and fog may develop.
These are some examples of advection type fog a pilot may see in their career.