H.C. Willett, for the first time, classified fogs on the basis of important fog producing processes. His classification not only gave an insight into the physical processes bringing about the fog, but it also provided the basis of fog forecasting.
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According to him, air should be cooled below the dew point to produce fog, or it should be provided with additional moisture to such an extent that its actual temperature should equal the dew point.
On this basis, he classified fog into two major categories: (a) Air mass fog, and (B) Frontal fog which is formed by the evaporation of frontal precipitation in dry and cold air mass.
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Air mass fog:
This type of fog forms within air masses and is not affected by the frontal activity. However, certain types of air mass fog may be related to frontal activity. Three main types of air mass fogs may be recognised according to the chief meteorological fog forming processes. They are radiation fog, advection fog and upslope fog.
Radiation fog:
It is also called the ground fog. It forms when calm moist air cools in place by nocturnal radiation. If the air is completely calm, instead of fog only dew or frost is produced.
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A light wind is a necessary condition for the formation of fog, because it produces slight turbulence in the air near the ground.
A certain amount of mixing is necessary, which produces a thick layer of fog. If the wind happens to be strong, it will distribute the excess water vapour into upper warmer layers and prevent fog formation.
This type of fog requires long winter nights and cloudless skies for maximum cooling of the ground and the adjacent air at night. Relative humidity at sunset should be high, so that even a small amount of cooling will lower the temperature to the dew point.
A light wind of 3 to 5 km per hour velocity is ideal, because it stirs the cold air in contact with the ground and scatters it sufficiently so that a solid fog layer 10 to 30 m thick is produced.
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Since cold air flows to the lowest point, the radiation fog is thickest in the low areas or valleys. As soon as the sun comes up the horizon and has warmed the earth’s surface, the radiation fog disappears.
The fog is then said to ‘lift’ or ‘burn off. Actually the fog particles consisting of microscopic water droplets evaporate as the temperature of the air is increased.’
Since after sunrise the surface air warms first, so the fog evaporates from the bottom up, giving the impression of lifting. The remnant part of the fog may appear to be a low white cloud layer.
It is to be noted that radiation fogs form only over the land. On the oceans, the diurnal range of temperature being negligible such fogs are always absent. In the continental interiors, when anticyclones are formed, ideal conditions exist for the formation of radiation fogs.
High-inversion fog is essentially a winter phenomenon. This fog is produced when a real temperature inversion exists at an altitude of 100 to 600 meters above the ground.
This fog does not form as a result of a single night of radiation cooling; rather it forms due to long-continued cooling by radiation. The most favourable locations for the high-inversion fogs are provided by the continental regions of high latitudes where winter-radiation cooling takes place.
However, when polar-maritime air mass becomes stationary over a continent, moisture and temperature conditions are most favourable for the formation of radiation fog.
Autumn and winter are the most favourable seasons for this type of fog. Similarly the centre of an anticyclone, where winds are light and skies clear, is an ideal place for it. During the winter season, it persists for several days in the western part of Europe.
A dense fog of this type persists throughout the day in the San Joaquin Valley of California. Even in some tropical inland valleys, radiation fogs are so thick that they may not clear away until midday. Coastal areas, because of the high relative humidity, experience radiation fogs.
Advection fog:
‘Advection’ implies the horizontal movement of wind. The advection fogs form when warm, moist air blows over a cool surface and is chilled below its dew point. Motion is implied in the formation of this type of fogs.
This type of fog may be produced by any of the two processes given below: (a) Cold air may pass across a warmer air prevailing over there; (b) Warm moist air may pass over a cold surface and become chilled by contact and by mixing with the cold air lying over the cold surface beneath.
The necessary conditions which favour the formation of advection fogs are the following: (a) A greater contrast between the air temperature and the temperature of the surface beneath it, (b) a moderate wind velocity, (c) initially high relative humidity in the air and (d) stable stratification in the atmosphere.
There are three sub-types of advection fogs: sea fog, tropical air fog, and steam fog. A brief discussion of these sub-types follows:
Sea fog is formed by the cooling of sea air over a cold current. Sea fog is not confined to coastal areas only, but can occur anywhere over the ocean where there is contrast in water temperatures.
Since most of the cold currents are found near the coasts, sea fogs are fully developed in the coastal areas. Light wind is necessary for this type of fog as well. Generally sea fogs are favoured by wind velocities ranging from 4 to 13 knots.
However, they are seen even when the wind velocity is about 26 knots. But when the wind velocities are high, the mechanical turbulence transfers heat towards the ground and the inversion is diminished. Under such conditions, a stratus cloud instead of fog is formed at the ground.
Fogs of the Grand Banks off Newfoundland are the typical example of sea fog. They originate because here the warm Gulf Stream comes into contact with the cold Labrador Current.
There is a great contrast in the temperatures of these currents and those of the overlying air masses associated with them. The Grand Bank region is covered with thick fog throughout the year, the fog frequency being greater in winter than in summer.
Tropical-air fogs are different from sea fogs, because here the cooling is produced by the passage of air from low latitudes pole-ward over the oceans. Such fogs are generally found over the open seas in their pole-ward parts.
The advancing tropical air may produce this type of fog over land also in winter. This is so because the latitudinal temperature gradient is greater over the continents than over the oceans.
Because of the roughness of land surface and greater atmospheric turbulence produced over there, the formation of surface fog is rather difficult.
On the contrary, relatively stronger winds over the oceans favour the fog formation. The tropical-air fogs, therefore, are more common over the oceans.
Steam fogs have the appearance of steam. They are produced by evaporation from a warm water surface into the cool air above. This type of fog is also called evaporation fog.
When cool air moves over warm water surface, a large quantity of moisture may evaporate from the water surface to saturate the overlying air.
The rising water vapour, after meeting the cold air condenses and rises with the air that is being warmed from below. It commonly occurs over water bodies, large or small, in the fall and early winter when the water may still be warm and the air is getting cold.
Arctic sea-smoke is the name used to indicate a dense and often extensive steam fog that occurs over the high latitude ocean areas in winter. This type of fog is quite common in the Arctic region, especially where both ice and open water are present.
In such areas, the air moving from the continents into the open ocean may be 20°C to 30° colder than water. Intense steaming from the water surface saturates a large volume of water. The fog thus produced may be dense.
Upslope fog:
This type of fog is produced because of the cooling of air by adiabatic expansion as it flows up the slope. This is one of the few kinds of fog that becomes denser, or is maintained in the presence of relatively high wind velocities.
This is because of the fact that the downward transport of heat by turbulence is offset by the rapid cooling accompanying the fast rise of air up the slope. Fog of this type is a common feature in the interior plains of the United States.
It generally occurs when moisture-laden winds of the Gulf of Mexico follow the slope of the rolling plains west of the Mississippi. This type of fog is locally known as the ‘Cheynne fog’ in the south- eastern part of Wyoming State in the United States of America.
Frontal fog:
Fogs produced along the front of two different air masses are called the frontal fogs. They are caused by saturation of the cold surface layer of air by rain falling from the ascending warm air mass.
Evaporation from the falling warm rain saturates the cold air, if the temperature of raindrops is higher than the dew point of air. If the air is marked by stability and the wind is light, the condensed moisture produces fog near the earth’s surface.
This type of fog is sometimes distinguished as pre-frontal and post-frontal fogs. Frontal fogs may form rather quickly, in one to three hours, and may be widespread.
When the wind velocity is relatively high, the base of the fog may lift to form a low stratus cloud. Cold front fogs form behind the slowly moving cold fronts by the same process as in the case of warm front fogs.