Fog is defined as a cloud having a base of less that 50 feet above the surface. For or low clouds are likely to develop when the temperature/dewpoint spread is close and decreasing. The different types of fog are detailed below.

Radiation Fog

The ground is heated by the sun during the daytime. At night, however, it radiates heat upward. If the skies are overcast, this radiated heat is trapped. The clouds act like a blanket, in a way. If the skies are clear, though, heat will be radiated out into space. The ground will cool and air touching the ground will be cooled by contact. If there is no wind, the ground is able to cool the air below its dewpoint, resulting in radiation fog.

Radiation fog forms on clear nights with calm wind. It is common in flatland areas.

In the morning, the sun heats the ground, which heats the air. As a result, radiation fog usually burns off within a few hours of sunrise. Any wind will hasten its disappearance.

Advection Fog

Advection fog is common is coastal areas and requires wind to form.

When warm moist air moves over a colder surface and is cooled below its dewpoint, advection fog forms. Commonly, air moves from over a body of water over a colder land mass. Advection fog will build and intensify with winds up to 15 knots and usually lifts to form a low overcast layer of clouds.

Upslope Fog

As air pressure decreases, air cools. If wind is blowing against upsloping terrain, such as a mountain range, the air is forced upward. As it moves upward, pressure decreases and the air cools. If it cools to its dewpoint, upslope fog forms.

Upslope fog requires wind to form and may last for several days at a time.

Steam Fog

If cold dry air moves over warmer water, the water will evaporate into the dry air, then immediately condense, forming steam fog. Steam fog occurs under cold conditions. It commonly occurs when a light breeze blows colder air out to sea.

Low level turbulence can occur and aircraft structural icing can become hazardous in steam fog.

Fog is one of the most persistent weather hazards encountered in aviation.

A small temperature/dewpoint spread is essential to fog formation.