Define Fog?Classify fog & explain their formation

The obscurity by the surface layers of the atmosphere owing to the presence of water droplets formed by condensation, amplified by suspended particles of smoke & dust which occur.

                    Fogs are classified as follows:-

a)Advection Fog:- A fog formed when a relatively warm, moist & stable air mass moves laterally over a cooler surface thereby reducing the temperature of the lower layer of the air mass causing inversion of temperature until it reaches the dew point when condensation takes place. This may occur when warm maritime air moves over the cold landmass or when warm continental air mass moves of-shore across a cold ocean current.Eg.-Gulf stream meeting air above cold Labrador current near the grand banks of new foundland.

                           It sometimes develops over a frozen, snow covered land mass in conjugation with radiation fog. If the air flow is too vigorous & beyond 8m/sec. in velocity then the fog will rise to form low altitude stratus cloud with a base of 100 m or so.

1)Steam Fog:- A type of advection fog, when cold air passes over a body of fresh water, temperature of which is considerably higher. Because the moisture condenses to form visible water droplets in the air the surface of the fresh water body appears to steam.

2)Arctic sea smoke / Arctic smoke:- A type of advection fog formed in high altitudes when cold air passes over a warmer water surface. The vapor pressure at the water surface exceeds the saturation vapor pressure at the particular air temperatures that evaporation from the water surface proceeds at a greater rate than can be accommodated by the air  at saturation point. The excess water vapor condenses & is carried continuously upwards evaporate in the drier air above so that the water surface appears to 'smoke'.

b)Frontal Fog:- A term given to short lived period of moist & drizzle associated with the passage of a warm front in a depression. It forms when the cooler layers of the air in contact with the ground surface become saturated by the evaporating warmer rain droplets which fall from the advancing front.

c)Smog:- Literally it means a contraction of 'smoke-fog' in it's original usage but in USA & elsewhere the term has become synonymous with 'Photochemical fog'.

The original use of the term from 1905 was intended to describe thick 'Radiation Fog' blanketing the urban &/or Industrial area under certain weather conditions. Thus, smoke, soot & other aerosols, together with gasses like sulphur-di-oxide & carbon mono-oxide etc. were unable to escape from below the lid of temperature inversion, thereby giving a sickly yellow color & acrid odor to smog. Smogs were common place in most industrial cities until the smoke-abatement laws of mid-20th century led to cleaner air but not before it has caused several disasters.Eg.-The London smog in 1952 led to some 4000 deaths due to bronchial disorders.

d)Radiation Fog:- it is a shallow layer of fog formed near the ground surface due to terresterial radiation. Loss of heat by the surface cools the air in contact with it, especially during weather with little turbulent mixing & clear skies & the dew point is soon reached whereupon condensation occurs.Since,the cold air flows downhill due to gravitational pull there is a tendency for the radiation fog to develop in hollows & in valleys.

                  In summer, the fog will disperse due to vaporization of water droplets near the ground after the sunrise. But in autumn & in winter it may be trapped beneath an inversion of temperature. In industrial regions a radiation fog may develop into smog under these conditions.

e)Photochemical Fog:- It's the state of poor visibility caused by chemical reaction of sunlight & hydrocarbons in the atmosphere. Water droplets are not present. It’s a haze produced when hydrocarbons & nitric oxides present in car exhausts. This type of fog is dangerous to human health if it reaches critical concentration levels.Eg.-Los Angeles,USA commonly suffers from this type of fog problem due to temperature inversion in its mountainous Girt basin couples with large scale emissions of nitrogen-di-oxide & ozone by vehicles during day hours.

f)Ice Fog:- It's a surface fog associated with ice sheets & ice shelves where the lowest layers of the atmosphere are cooled by the ice surface sufficiently for minute ice crystals to form in air .Here, they will remain till calm conditions prevail. When sunlight shines through this 'veil' of ice crystals it produces a dazzling light conducive to snow blindness. Owing to the refrigeration of sun rays it produces optical phenomenon like haloes.

g)Ground Fog:- It is also known as ground mist.It is caused by terrestrial radiation & is confined to low areas or valley floors. When the ground surface cools rapidly at night by excessive terrestrial radiation loss, it lowers the temperature of the lowest air layers, which then become denser & flow into topographic hollows owing to gravity.Here,condensation occurs as air is cooled below its dew-point.It may be only a few meters in thickness,so that tall buildings or trees may be obtrude from the upper surface of the fog. 

Fogs can be serious hindrance to the smooth movement of traffc, it hinders landing & take-off of aircraft, they might result in respiratory problems if it lingers long (like:-Photochemical Fog),they are harmful to crops, orchards etc. Several protective measures like dry ice seeding technique(in case of cold fogs) or by addition of heat/turbulence to accentuate the evaporation process of fogs(for warm fogs).Eg.-The scheme taken at Paris Airport called 'Turboclair' to keep runway free from fogs old jet engines sited along main runway.