According to the IMD, 'very dense' fog is when visibility is between 0 and 50 metres, 51 and 200 metres is 'dense', 201 and 500 'moderate', and 501 and 1,000 'shallow'.
Dense to very dense fog swept over parts of the country's north and northwest on Monday with the Met department predicting the conditions to persist over the next few days. A severe cold wave gripped the region with temperatures plummeting in several towns and cities.
In Delhi, dense fog lowered visibility to 50 metres in some areas, affecting road and rail traffic.Ten trains were reported running late by 1.45 to 3.30 hours, a Railway spokesperson said. The mercury dropped to 3 degrees Celsius in the Ridge area, 4.9 degrees below normal, making it the coldest place in the national capital. The Ridge and the Ayanagar weather stations recorded a minimum temperature of 4 degrees Celsius and 4.1 degrees Celsius, respectively.
The Safdarjung observatory, Delhi's primary weather station, logged a minimum temperature of 5 degrees Celsius -- three notches below normal. Biting cold conditions and dense fog were reported in several parts of Punjab and Haryana, with Narnaul being the coldest in the region at 2.4 degrees Celsius.
In Haryana, Hisar recorded a piercing cold as the minimum temperature plunged to 2.5 degrees Celsius. Ambala recorded a low of 7.7 degrees Celsius, Karnal 6.8 degrees Celsius, Rohtak 6.6 degrees Celsius, Bhiwani 5.5 degrees Celsius and Sirsa 5.2 degrees Celsius.
Chandigarh, the joint capital of Punjab and Haryana, recorded a minimum temperature of 7.4 degrees Celsius. In Punjab, Amritsar recorded a low of 6.5 degrees Celsius, Ludhiana 6 degrees Celsius, Pathankot 8.8 degrees Celsius, Bathinda 3.6 degrees Celsius, Faridkot 6 degrees Celsius and Gurdaspur 4.5 degrees Celsius. Intense cold conditions and fog affected normal life in parts of Rajasthan, with sub-zero temperatures being recorded in Sikar's Fatehpur.
During the last 24 hours, cold wave to severe cold wave conditions were observed at isolated places in northern parts of the state, the weather office said. Very dense fog also occurred at isolated areas. Bathinda in Punjab and Bikaner in Rajasthan reported zero visibility, while it dropped to 50 metres and below at Ambala, Hisar, Amritsar, Patiala, Ganganagar, Churu and Bareilly.
Foggy conditions will persist in these areas over the next few days, the India Meteorological Department (IMD) said. According to the IMD, 'very dense' fog is when visibility is between 0 and 50 metres, 51 and 200 metres is 'dense', 201 and 500 'moderate', and 501 and 1,000 'shallow'.
In the plains, the IMD declares a cold wave if the minimum temperature dips to 4 degrees Celsius. A cold wave is also declared when the minimum temperature is 10 degrees Celsius or below and is 4.5 notches below normal. A 'severe' cold wave is when the minimum temperature dips to two degrees Celsius or the departure from normal is more than 6.4 degrees Celsius.