Only one Western Disturbance has affected Northwest India till now and due to this, dry and cool north westerly winds are prevailing over the plains keeping the humidity level below normal. These winds blowing from the west and northwest direction have been preventing the formation of fog. In the normal course, at least 2 or 3 WDs would have occurred by now, increasing the moisture level favourable for fog formation. In the absence of any significant system at least for a week, formation of fog is not likely over Delhi/NCR.
Here's an understanding of what is fog, why and how it forms and in what way it affects weather.
Fog is an atmospheric obscurity which reduces the visibility to less than 1000 metres during highly humid conditions. In simple words, we can say that fog is a collection of very tiny water droplets or ice crystals suspended in the air at or near the Earth's surface.
How is fog formed?
Fog forms as a result of the cooling of the air in contact with the ground. The cooling process is due to loss of heat to the ground by warmer air that moves over the colder surface or due to loss of heat by the outgoing radiations. The former is ‘advection’ fog and the latter ‘radiation’ fog. Advection fog over land is a result of advection followed by cooling. Radiation fog forms when the temperature falls at night due to an excess of outgoing radiation and development of deep inversion. (Inversion means the ground is cooler than the air lying above it, at least in the lower levels)
Requisites for fog formation
Light wind conditions and cool temperatures on a highly humid day are favourable for formation of fog. Wind speed is a critical parameter for fog formation as strong and accelerating winds are generally a deterrent for fog formation. Overnight cloud cover is also a negative parameter for its formation.
The different types of fog includes shallow, thick, dense and very dense. Many-a-times fog occurs in patches but it is only dense fog, which usually affect rail, road and air traffic. Fog over northern plains including Delhi/NCR is seen after a spell of widespread rainfall due to Western Disturbance in winters.
Picture courtesy- sandesh.com
