Advection Fog

This type of fog forms when warm, moist air moves over a colder surface (such as land, water, or a surface covered by snow or ice) and cools to 100% humidity.

Ingredients

In order for advection fog to form, warm, humid air is moved by the wind (i.e. advected) over a much cooler surface, such as a cold landmass or a body of water, as shown in this diagram of favorable ingredients. The warm air will be cooled from below by being in contact with the cold surface (AKA conductive cooling). This movement of warm air over a cool surface creates a stable layer (a surface inversion), and prevents surface air from mixing up into the atmosphere.

Structure

Advection fog forms (as shown in this image) when warm, moist air advects over a colder surface, causing the air in contact with the surface to cool by conduction. If the air mass at the surface is able to cool to the dew point temperature and reach 100% humidity (i.e. become saturated), there is a possibility that advection fog will form. 

It is possible for fog to form before saturation occurs when the process is taking place over the ocean, due to the presence of abundant salt particles suspended in the air that can act as nuclei that are able to absorb and retain moisture. In the latter case, further cooling is required for low visibilities to occur.

Visibility typically drops rapidly as the air mass moves over a tight gradient in sea surface temperatures.

Land barriers/higher terrain along a coast may prevent fog from advecting inland, known as a wind barrier effect.

Duration

As long as favorable conditions persist, advection fog will remain and even continue to grow if the temperature contrast increases between the air and the colder surface, and additional moisture is added.

At night, the cooling of the land surface may allow fog to spread from the coast further inland. In this case, there are both radiative and advective processes at work.

Due to the broad nature of advection fog, it can persist for days on end, until there is a wind shift.