Hazy about untimely fog that caused blackouts

a large part of northern India, including capital Delhi, went powerless due to tripping of transmission lines of the Northern Grid. It first happened on March 8 and then on March 10, resulting in large-scale blackouts and affecting the Northern Railways network. The reason dense fog, unusual for March.

Tripping of transmission lines is not a new phenomenon. It happens almost every year during peak winters--December and January. As a precautionary measure, transmission lines are cleaned from October to January. And the unusual incident of tripping of about 50 transmission lines in Delhi, Haryana, and western Uttar Pradesh in March caught power department officials completely off guard.

Fog requires certain amount of moisture in the air but the "winter rains" did not happen this year. Western disturbances usually bring sudden winter rain and snow to the north-western parts of the Indian subcontinent.It is a non-monsoonal precipitation pattern driven by the westerlies.

This year, however, the westerlies by-passed northern India and affected only central and north-west India. What caused the fog then? There is much confusion and too many loose ends in the explanations.

Fog develops when there is a standard mix of moisture, aerosols (which act as nuclei for moisture to condense), low temperatures, slow wind and a clear sky. Typically, when humidity levels are above 75 per cent and the horizontal visibility on the ground is less than one km, the condition is declared foggy. Humidity levels on March 8 and March 10 were 94 per cent and 77 per cent.

Experts say conditions for fog can be triggered in a very short time. "Tropospheric winds (slightly above the ground) could have caused the fog. Experiments conducted earlier have shown that free tropospheric winds trigger sudden fog," says M K Tiwari, head of the radio and atmospheric science division at the National Physical Laboratory, New Delhi.

These winds cause sudden cooling of atmosphere at the surface level, leading to temperature inversion. In case of temperature inversion, the temperature increases with height. Normally, temperature decreases with altitude. Inversion results in dust and pollutants being trapped in the lower atmosphere.

The National Physical Laboratory, along with the Central Pollution Control Board in 2001 conducted experiments to study fog in Indo-Gangetic plains. Tiwari says there was an additional temperature inversion layer found at the ground level then, besides the regular inversion layer that forms in winters. "Inversion was seen mainly at the height of 10 m above the ground and could have important implication in fog formation," he says. But the reason for presence of additional inversion layer is still not known. The 2001 study, which was also mandated to look into the role of pollution in fog formation, was never done.
How much pollution?Some say increased fog is due to rise in air pollution,