After reading this essay you will learn about:- 1. Definition of Fog 2. Classification of Fogs 3. Effects 4. World Distribution.
Essay # 1. Definition of Fog:
Fog is special type of thin cloud consisting of microscopically small water droplets which are kept in suspension in the air near the ground surface and reduces horizontal visibility. According to Byers fog is defined ‘as almost microscopically small water drops suspended in the atmosphere and reducing the horizontal visibility to less than one kilometre’. It may be pointed out that clouds are formed due to ascent, expansion and cooling of air while fogs are formed due to radiation, conduction and mixing of warm and cold air masses near the earth’s surface.
Fog is formed when the moist air (with relative humidity above 97 percent) becomes saturated, reaches its dew point and further cools so that water vapour is condensed around dust particles, smokes etc. to form tiny water droplets which being very light are suspended in the air.
These microscopically tiny water droplets form smoky clouds which fly with the winds. Invisibility increases as the fog becomes dense and there is darkness when the fog becomes extremely dense. A light fog, called as mist, is that when visibility is restricted to 2 kilometres.
Fog is generally associated with inversion of temperature and occurs in the morning hours but sometimes also continues till noon. Fog occurs during winters in subtropical regions but it occurs in ail seasons in the regions beyond 35° latitudes. Generally, fog looks whitish in colour but over large cities and industrial areas it looks dirty yellow or gray because of mixing of smoke, dust and fly ash.
Essay # 2. Classification of Fogs:
Fogs have been variously classified by different scientists on various bases.
H.C. Willett (1928) classified fogs in two broad categories on the basis of physical processes of their formation e.g.:
(1) Air mass fog, and
(2) Frontal fog.
H.R. Byers (1944) modified the classification of fogs as suggested by Willett and presented the modified form as follows:
(A) Air mass fogs:
(1) Advective Types:
(a) Fogs due to the transport of warm air over a cold surface:
(i) Land and sea breeze fog
(ii) Sea fog
(iii) Tropical fog
(b) Fogs due to transport of cold air over a warm surface:
(i) Steam fog (arctic sea smoke)
(2) Radiation Types:
(a) Ground fog
(b) High inversion fog
(3) Advection-radiation fog
(4) Upslope fog
(B) Frontal fogs:
(1) Pre-frontal (warm front) fog
(2) Post-frontal (cold front) fog
(3) Front passage fog
Critchfield has classified fogs in two broad categories on the basis of evaporation and cooling processes which are responsible for the origin of fogs:
(1) Fogs resulting from evaporation:
(i) Steam fog
(ii) Frontal fog
(2) Fogs resulting from cooling:
(i) Radiation fog
(ii) Advection fog
(iii) Upslope fog
(iv) Mixing fog
(v) Barometric fog
Fogs are classified in 4 types on the basis of visibility:
(i) Light fog (visibility upto 1100 metres)
(ii) Moderate fog (visibility 1100 m-550 m)
(iii) Dense fog (550 m-300 m)
(iv) indtense dense fog (less than 300m)
Radiation fog is formed when warm and moist air lies over cold ground surface. Due to this situation overlying warm and moist air cools and thus dew point is reached, with the result condensation of water vapour around hygroscopic nuclei (dust particles and smokes) forms numerous tiny water droplets and thus fog is originated.
The occurrence of radiation fog requires certain conditions e.g., long and cool winter nights, cloudless sky, sufficient amount of moisture in the air, very weak air motion (light wind having speed of 3 to 5 km per hour) and ground inversion of temperature. Fog is formed at the ground surface during nights and thickness upward. It disappears in the morning with the sunrise because the water droplets are evaporated due to rise in air temperature. Radiation inversion occurs only on land surface.
Radiation fog is more common over large cities and surrounding areas because of abundance of hygroscopic nuclei. When fog is combined with sulphur dioxide it becomes poisonous and causes human deaths. Such fog is called urban smog. Sometimes, fogs also occur at higher elevation due to upper air inversion of temperature. Such fog is called high inversion fog.
Advectional Radiation Fog:
The fog formed due to mixing of warm moist air and cold air due to arrival of warm and moist air over cold ground surface is called advectional radiation fog.
There are certain necessary conditions for the origin of advection radiation fog. e.g.:
(i) Horizontal movement of air,
(ii) Greater contrast between air temperature and the temperature of ground surface,
(iii) Moist air or say high relative humidity in the air, and
(iv) Stable stratification in the atmosphere etc.
The warm and moist air when blows over cold surface (either land surface or sea surface) is cooled from below so that dew point is reached and condensation of water vapour occurs around hygroscopic unclei and thus miscroscopic water droplets are formed to create fogs. Advection fogs are generally originated during winters on land surfaces and during summers on sea surfaces because lands are relatively colder than seas and oceans during winters.
Dense advectional fogs are also formed where cold and warm ocean currents coverage. For example, dense fogs are formed near New- foundland due to convergence of cool Labrador and warm Gulf Stream ocean currents. Similarly, dense fogs are formed near Japanese coast due to convergence of cool Kurile and warm Kuroshio currents.
The fogs occurring over sea surfaces are called sea fogs which are generally formed near the coastal areas frequented by cold ocean currents. For example, cool California current (along the Californian coast), cool Peru Current (along the Peruvian coast), cool-Benguela current (along the western coast of South Africa) and cool Western Australian current cause sea fogs. The sea breezes transport sea fogs inland up to the distance of 400 km.
Steam fogs are in fact advectional fogs which are formed when cold air moves from land over oceanic surface and there is evaporation of large quantity of moisture from water surface to saturate the overlying cold air. It is interesting to note that in such situation overlying cold air is warmed from below and thus water vapour due to evaporation of warm water surface rises upward and condenses after meeting cold air above to form fog.
The vapour particles above the water surface look as if steam is coming up from the water. This is why such fogs are called steam fogs. They are also called evaporation fogs.
Upslope or hill fogs originate when continental warm and moist air rises upslope along the hillslopes because the rising air is saturated due to cooling and condensation of moisture around hygroscopic nueclei and forms fogs which cover the lower segments of hillslopes. It may be mentioned that such fogs are formed due to adiabatic expansion and cooling of moist air. Such fogs are very common on the hillslopes in temperate regions. They may occur in any season.
Fronts are formed when two contrasting airmasses (warm and cold airmasses) converge along a line. Warm air is pushed upward by cold air and hence overlying warm air is cooled from below due to underlying cold air and fogs originate after condensation. Such fogs are formed in temperate regions. In fact, the saturation and condensation of overlying warm and moist air causes rainfall which saturates the underlying cold surface air and thus fog is produced.
Essay # 3. Effects of Fogs:
Fogs effectively hinder sea navigation, land and air transport systems. Dense fogs cause severe accidents on high-speed highways involving collision of trucks, buses, cars etc. Landing and take-off of aircrafts are delayed by dense fogs causing economic loss to airlines and inconvenience to stranded passangers.
Dense fogs are navigational hazards in the seas as ships and huge supertankers carrying oil sometimes collide resulting into spilling of huge quantity of oil causing enormous oil slicks on sea water which cause ecological disaster. Vehicular traffic comes to grinding halt in the event of dense fogs.
Sometimes, fogs are so dense and visibility is so reduced that walking by human beings becomes impossible. When fogs are polluted through sulphur coming out of the chimneys of the mills, they become poisonous and health hazards.
For example, killer poisonous smog was formed due to mixing of dense fog with smokes and sulfide fumes coming out from the chimneys in the Donora Valley of Pennsylvania (USA) on 26 October, 1948.
This toxic fog (smog) caused 20 human deaths and 43 per cent inhabitants fell ill due to cough and respiratory problem. Similar poisonous fog was produced due to mixing of sulphur dioxide coming out of the factories of zinc smelters and sulphuric acid in the Meuse Valley of Belgium in the month of December, 1930. This killer fog claimed 63 human lives due to obstruction in respiratory system. The poisonous fog of December, 1952 claimed 4000 human lives in London.
Sometimes, fogs are also economically beneficial to human society. For example, they protect tea and coffee plants from scorching sunlight on the hillslopes. Mocha coffee on the Yemen hills (Arabia) is highly benefitted from fogs.
Essay # 4. World Distribution of Fogs:
If the world distribution of fogs is considered it becomes apparent that they are more widespread over oceans. The oceanic fogs are generally advectional in origin because relatively warm air coming from over the land surfaces produces fog over relatively colder oceanic surfaces. Fog areas are positively related to cool ocean currents.
The eastern portions of the oceans and western continental margins are characterized by most fogs because these areas are frequented by cool ocean currents (e.g. California current, Peru current, Benguela current, Canary current, Western Australia current etc.).
Frontal fogs are produced in the high latitudes due to convergence of cold polar air mass and warm westerlies mainly in the eastern North Atlantic Ocean and western coasts of north-west Europe and in the eastern North Pacific Ocean near Aleutian Islands and Alaska coast. Radiation fogs are produced in low latitudes on continental areas during winter season.
Dense fogs are formed at the places where cold and Warm Ocean currents converge e.g., near Newfoundland and Grand Brnks (due to convergence of cold Labrador Current and warm Gulf Stream) and near Japanese coasts (due to convergence of cold Kurile current and warm Kuroshio Current).
Important foggy areas of the USA are California coast. New England outer coast, Northern Pacific coast line, Applachian valleys, Pacific coast valleys, Middle Atlantic coast, Great Lakes, Southern Atlantic and Gulf coastal waters, Ohio, Missouri, Great Plains and upper Mississippi valley.