New Delhi, Jan. 21: The unusual spell of cold weather and persistent fog across north Indian cities, especially Delhi, over the past few days is the result of pollution and a sharp drop in day temperature, meteorological department officials have said.
Spells of intense cold are ushered in by a series of low pressure zones moving south- eastwards from the Mediterranean region into the Indian subcontinent.
This year, northern India has experienced a wave of such zones with little respite.
“Air pollution is causing the fog to persist over northern Indian cities long past sunrise,” said Govind Ballabh Pant, director, Indian Institute of Tropical Meteorology, Pune.
“Pure fog lifts within a couple of hours after sunrise. But fog mixed with dust particles and pollutants blocks solar radiation, prevents ground heating and persists longer,” he said.
R.D. Singh, a director in the Delhi meteorological department, said some invisible pollutants such as sulphur absorb moisture available in the atmosphere, making the fog dense.
According to Pant, pollution slows the ground-heating process, that in turn keeps the maximum day temperature close to the minimum temperature. As a result, such days are unusually cold.
The maximum day temperature in Delhi has dipped below the normal of 21 degrees Celsius by 5-7 degrees, Singh said.
Since New Year’s day, the maximum day temperature has been hovering between 12 and 17 degrees.
According to the Met office data, the lowest maximum day temperature was 12 degrees on January 7, followed by 12.3 degrees three days later and 13.8 on January 11.
The coldest day of the month was January 10 when the minimum temperature was 3.5 degrees Celsius.
When the sky clears, the day temperature goes up because the fog gets scattered — like today in Delhi.
The Met office, which does not have an explanation yet why the severe cold spell and dense fog have hit the country after nearly 40 years, hopes the weather will improve in the next four days.
For tomorrow, Delhi will have relatively thinner fog in the morning and a mainly clear sky through the day.
The minimum temperature will be around 5 degrees.
The persistent biting cold and blinding fog have brought the entire capital to a near standstill for some weeks now.
Even today, air traffic at the Delhi airport ground to a halt for over 12 hours because of dense fog.
The air traffic was halted at 11 last night after runway visibility was reduced to 75 metres, airport sources said today.
With some improvement in the weather today, the first international flight arrived at 10.57 am and took off at 12.05 pm.
The first domestic flight took off at 11.33 am and arrived at 11.45 am, airport sources said.
Seven domestic flights and five international flights were diverted to Mumbai, Jaipur and Ahmedabad.
The Northern Railway, too, cancelled as many as 11 trains today. Several other trains ran behind schedule.