How fog forms and lifts.

Fog differs from clouds only in that the base of fog is at the Earth’s surface while clouds are above the surface.

Fog forms when the temperature and dewpoint of the air become identical (or nearly so). This may occur through cooling of the air to its dewpoint (producing radiation fog, advection fog, or upslope fog), or by adding moisture and thereby elevating the dewpoint (producing frontal fog or steam fog). Fog seldom forms when the temperature-dewpoint spread is greater than 2°C (4°F).

When the temperature of the air is reduced to the dew point, the air is completely saturated and moisture begins to condense out of the air in the form of fog, dew, frost, clouds, rain, or snow.

Surface temperature-dewpoint spread is important in anticipating fog but has little bearing on precipitation.

Radiation fog (see Figure 18-1 and Figure 18-2) is a common type of fog, produced over a land area when radiational cooling reduces the air temperature to or below its dewpoint. Thus, radiation fog is generally a nighttime occurrence and often does not dissipate until after sunrise.

Factors favoring the formation of radiation fog are: 1) a shallow surface layer of relatively moist air beneath a dry layer, 2) clear skies, and 3) light surface winds. Terrestrial radiation cools the ground; in turn, the ground cools the air in contact with it. When the air is cooled to its dewpoint, fog forms.

Greenhouse warming is enhanced during nights when the sky is overcast (see Figure 7-2). Heat energy from the Earth can be trapped by clouds, leading to higher temperatures as compared to nights with clear skies. The air is not allowed to cool as much with overcast skies. Under partly cloudy skies, some heat is allowed to escape, and some remains trapped. Clear skies allow for the most cooling to take place.

The magnitude of diurnal temperature variation is primarily influenced by surface type, latitude, sky cover (e.g., clouds or pollutants), water vapor content of the air, and wind speed. Temperature variation is maximized over land, at low latitudes, with a clear sky, dry air, and light wind. Conversely, temperature variation is minimized over water, at high latitudes, with a cloudy sky, moist air, and strong wind.

Shortly after sunrise, incoming solar radiation received at the Earth’s surface (insolation) becomes greater than outgoing terrestrial radiation and the Earth’s surface warms.

As the sun rises and the temperature increases, radiation fog lifts and eventually burns off.

Fog will often lift into a layer of stratus by an increase in wind or a rise in temperature.

Ground fog usually burns off rather rapidly after sunrise. Other radiation fog generally clears before noon unless clouds move in over the fog.

Radiation fog is restricted to land because water surfaces cool little from nighttime radiation. It is shallow when wind is calm. Winds up to about 5 kt mix the air slightly and tend to deepen the fog by spreading the cooling through a deeper layer. Stronger winds disperse the fog or mix the air through a still deeper layer, with stratus clouds forming at the top of the mixing layer.

Advection fog (see Figure 18-5 and Figure 18-6) forms when moist air moves over a colder surface and the subsequent cooling of that air to below its dewpoint. It is most common along coastal areas but often moves deep into continental areas. At sea, it is called sea fog. Advection fog deepens as wind speed increases up to about 15 kt. Wind that is generally stronger than 15 kt lifts the fog into a layer of low stratus or stratocumulus clouds.

Unlike radiation fog, wind is required to form advection fog.

On clear nights, with relatively little to no wind present, radiation fog may develop.

Any increase in wind also speeds the dissipation of radiation fog.

A pilot will notice little difference between flying over advection fog and over radiation fog. Also, advection fog is usually more extensive and much more persistent than radiation fog. Advection fog can move in rapidly regardless of the time of day or night.

Upslope and advection fog, unlike radiation fog, may not burn off with the morning sun but instead can persist for days.

A surface-based inversion typically develops over land on clear nights when wind is light. The ground radiates and cools much faster than the overlying air. Air in contact with the ground becomes cool, while the temperature a few hundred feet above changes very little. Thus, temperature increases with height.

The principal characteristic of an inversion layer is its marked stability, so that very little turbulence can occur within it.