A cold wave swept Delhi on Thursday as people in the city gear up to ring in a chilly New Year with the weather department predicting similar conditions till January 3.
The minimum temperature at the Safdarjung Observatory, considered the official marker for the national capital, dropped sharply to 3.4 degrees Celsius, four notches below normal. On Wednesday it was 8.4 degrees Celsius.
The automatic weather stations at Delhi's Ayanagar and Narela on Thursday recorded a minimum temperature of 3.8 degrees Celsius and 3.2 degrees Celsius, respectively.
In the plains, the India Meteorological Department (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 less than normal.
Delhi had experienced cold wave conditions on December 20 and 21 when the minimum temperature settled at 3.2 degrees Celsius, the lowest this season so far, and 4 degrees Celsius.
Thereafter, two back-to-back western disturbances and the resultant slowing down of cold northwesterly winds and cloudy conditions gradually pushed the minimum temperature up to 9.8 degrees Celsius.
Clouds trap some of the outgoing infrared radiation and radiate it back downward, warming the ground.
The IMD has predicted cold wave to severe cold wave conditions in northwest India till January 3.
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.
The weather department predicted the minimum temperature will start increasing from January 4 under the influence of an active western disturbance which is very likely to cause fairly widespread rainfall and snowfall over Jammu and Kashmir and Himachal Pradesh between January 4 and 7.
It will also lead to "light to moderate scattered to fairly widespread" rainfall over Punjab, Haryana, Chandigarh Delhi, north Rajasthan and west Uttar Pradesh between January 5 to January 7.