A "severe" cold day is when the maximum temperature is at least 6.5 notches below normal.
Delhi shivered under "cold day" conditions for a second consecutive day on Saturday, recording the season's lowest maximum temperature at 14.8 degrees Celsius, with thick fog blotting out the sun.
The minimum temperature at the Safdarjung Observatory, which is considered the official marker for the city, settled at 6.1 degrees Celsius, a notch below normal, it said.
"Today also there was a layer of moderate fog/low cloud over Delhi-NCR, Punjab, Haryana, Uttar Pradesh and north Rajasthan. It prevented the sunlight from reaching the surface. Light north westerlies are also blowing over the region. These conditions led to a cold day," an IMD official said.
Similar conditions are predicted for Sunday too.
Severe cold day conditions persisted at Narela and Jafarpur, where the maximum temperature dipped to 10.7 degrees Celsius and 10.9 degrees Celsius, seven and nine notches below normal, respectively.
Narela was also the coldest place in Delhi on Saturday.
On Friday, the automatic weather stations at Narela and Jafarpur had logged a maximum temperature of 11 degrees Celsius and 12.1 degrees Celsius.
According to the IMD, a "cold day" is when the minimum temperature is less than 10 degrees Celsius and the maximum is at least 4.5 degrees Celsius below normal.
A "severe" cold day is when the maximum temperature is at least 6.5 notches below normal.
Dense to moderate fog is predicted in Delhi for the next four days, the IMD said.
When visibility due to fog is reduced to the range of 0 to 50 metres, it is categorised as "very dense" fog. In the case of "dense" fog, visibility is between 51 and 200 metres, "moderate" is 201 and 500 metres, and "shallow" is 501 and 1,000 metres.
The capital's air quality improved a bit and the 24-hour air quality index (AQI) read 258 at 4 pm, as against 348 at the same time on Friday.
An AQI between zero and 50 is considered 'good', 51 and 100 'satisfactory', 101 and 200 'moderate', 201 and 300 'poor', 301 and 400 'very poor', and 401 and 500 'severe'.
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