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Why there is fog in Chennai and suburbs? (TN Weatherman)


Posted on : 12/Jan/2019 19:38:49

Many questions are seen in the page and in inbox like PJ was this because of Climate Change or Global Warming or Pollution? Why is Chennai suddenly seeing fog? Lets see what is the reason with some facts / data. For the past few days, the difference between Temperature and Dew Point Temperature is less then 1.6 C which is ideal for formation of Fog. This lesser the difference means air is near to saturation point and it cant hold any more water with Relative Humidity close to 100% like the past three days. We can even say Fog as one type of very very low level cloud. Remember when fog happens as a result of pollution, its called smog.

In our monsoon season, this dew point temperature drop to the actual temperature would have resulted into a Rainy day but in our winter season High pressure dominates Peninsular India incl Tamil Nadu.

What type of Fog is seen over Chennai and other parts of Tamil Nadu - It is Radiation Fog
This is called as Radiation fog. Lets see how it forms. As u all know. High Pressure is domination and all the heat is reflected back and escapes our atmosphere without getting trapped as there are no clouds to trap them (Refer the OLR image post put up 5 days ago in this page). The Radiation fog forms over land on calm, clear nights when loss of heat by radiation cools the ground and chills the air in the lowest few metres to below the dew-point temperature. Once dense fog has formed, the top of the fog replaces the ground as the effective surface cooled by radiation, and the fog increases progressively in depth as long as there is sufficiently moist air above it. The development of a strong temperature inversion tends to stabilize the fog and suppress air motions, but slow, turbulent stirring motions usually are present and probably are important in maintaining the fog. They do so by replacing the air in the lowest layerswhich is losing moisture by deposition on the groundwith moister air from above. Typical inland radiation fogs reach to heights of 100 to 200 metres..

Even in Navalur (Chennai fog taken two days ago) we can see fog is only few hundred meters from the ground level till the height where there is moisture (see the pic enclosed). Above which the air is very dry.

Courtey: TN Weatherman