Severe fog: Met department in dark

the weather gods seem hell-bent on proving the India Meteorology Department (imd) wrong. Barely had it recovered from the embarrassment of forecasting the monsoon incorrectly when the winter wrong-footed it.

An unusually severe nightly fog, enveloping a vast stretch of the Indo-Gangetic belt from Punjab to Kolkata, has confounded the imd as much as it has disrupted normal life across northern India. Its intensity is such that visibility levels drop to near zero soon after dusk and sunlight is blocked out well into noon.

The fog and an accompanying bitter cold wave - which has so far claimed over 600 lives in north India - are almost unprecedented in magnitude, point out Indian meteorologists. Some of them concede that such foggy conditions have been on the rise in winter months during the past six years.

Such an occurrence over polluted areas can lead to a serious problem as it can trap aerosols - both natural such as mineral dust, and human-made like suspended particulate matter. Aerosols are minute particles in the atmosphere.
Bare facts Fog is a natural phenomenon and the elements that cause it to occur are quite common: clear, calm nights and high humidity. Under their combined effect, the heat radiated by the ground spreads to the cold sky leading to condensation around minute particles in the atmosphere. This is called radiation fog. In another commonly experienced form of the phenomenon, known as advective fog, a layer of haze is formed by the movement of moist air over a cold surface such as a waterbody or freshly fallen snow. While Delhi and its neighbouring areas are experiencing radiation fog, the advective form is enveloping the Uttar Pradesh and Bihar regions.

For many years, westerly winds (often accompanied by showers) originating from the Mediterranean Sea have been lashing north India in the winter months. A cold wave spawns these westerly disturbances, but it does not last more than a few days. "This time, however, the number of such disturbances was almost one in three days which is unusually high," reveals U S De, former assistant director general of imd. This is one of the reasons for the unrelenting cold wave. But scientists at the National Centre for Medium Range Weather Forecast (ncmrwf), New Delhi, are baffled by the persistent foggy conditions.

While there are many similarities between a cloud and a fog, there are a few differences as well. Firstly, unlike clouds which are formed at higher altitudes, fog condensation takes place a few hundred metres above the ground. Besides, fog droplets are at least 100 times smaller than those that are contained by clouds.

The ideal temperature range in which a fog is formed is 1