What is fog?
The official definition of fog is visibility of less than 1,000 meters. This limit is appropriate for aviation purposes but for the general public and motorists an upper limit of 200 meters is more realistic. Severe disruption to transport occurs when the visibility falls below 50 meters.
What causes fog?
Fog is caused by tiny water droplets suspended in the air. The thickest fogs tend to occur in industrial areas where there are many pollutants on which water droplets can grow.
Types of fog
Fogs which are composed entirely or mainly of water droplets are generally classified as per the physical process which produces saturation or near-saturation of the air. The main types of fogs are:
Radiation fog
Radiation fog usually occurs in the winter, aided by clear skies and calm conditions. The overnight cooling of land by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. Radiation fog usually dissipates soon after sunrise as the ground warms.
Valley fog
Valley fog forms where cold dense air settles into the lower parts of a valley, condensing and forming fog. It is often the result of a temperature inversion with warmer air passing above the valley. Valley fog is confined by local topography and can last for several days in calm conditions during the winter.
Advection fog
Advection fog occurs when moist air passes over a cool surface and is cooled considerably. A common example of this is when a warm front passes over an area with snow cover. It is also common at sea when moist tropical air moves over cooler waters. If the wind blows in the right direction then sea fog can become transported over coastal land areas.
Upslope fog
Upslope fog or hill fog forms when winds blow air up a slope (called orographic uplift). The air cools as it rises, allowing moisture in it to condense.
Evaporation fog
Evaporation fog is caused by cold air passing over warmer water or moist land. It often causes freezing fog. When some of the relatively warm water evaporates into low air layers, it warms the air causing it to rise and mix with the cooler air that has passed over the surface. The warm and moist air cools as it mixes with the colder air, allowing condensation and fog to occur.
Freezing fog
Freezing fog is composed of super cooled water droplets - which remain liquid even though the temperature is below freezing-point. One of the characteristics of freezing fog is that rime - composed of feathery ice crystals - is deposited on the windward side of vertical surfaces such as lamp posts, fence posts, overhead wires, pylons and transmitting masts.
So how does fog form in INDIA?
The type of fog which is observed in India in general and Northern India is Radiation fog. It forms when ground cools due to outgoing radiation in the night, and there is sufficient moisture available to cause condensation. So, fog will have very high probability of formation, on days when Northern India gets winter rains due to Western Disturbances (WDs) and it is followed by clear nights. This will meet both the requirements of availability of moisture & lowering of night temperatures due to radiation cooling. It is most likely to happen from middle of November to middle of February, with December and January being the peak months.