A cold wave swept parts of Delhi on Sunday with the minimum temperature settling at 5.3 degrees Celsius, three notches below normal, the weather office at Safdarjung said.

The mercury dropped to 3 degrees Celsius in the Ridge area, 4.9 degrees Celsius below normal, making it the coldest place in the capital.

Dense to very dense fog prevailed over certain parts of the National Capital Region and states of Haryana, Punjab, west Uttar Pradesh and north Rajasthan.

Foggy conditions will persist in these areas over the next few days, the India Meteorological Department (IMD) forecast.

According to the IMD, ''very dense'' fog is when visibility is between 0 and 50 metres, 51 and 200 is ''dense'', 201 and 500 ''moderate'', and 501 and 1,000 ''shallow''.

The Safdarjung observatory, Delhi’s primary weather station logged a minimum temperature of 5.3 degrees Celsius, three notches below normal.

The maximum temperature settled at 16.2 degrees Celsius, five notches below the normal and the lowest this season so far.

The Aya Nagar weather station recorded a minimum temperature of 3.5 degrees Celsius, 4.5 notches below the normal.

The Air Quality Index (AQI) at 6 pm in the national capital was recorded at 319, the Central Pollution Control Board (CPCB) said.

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".

In the plains, the IMD declares a cold wave if the minimum temperature dips to 4 degrees Celsius. A cold wave is also expected 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.

When the minimum temperature is less than or equal to 10 degrees Celsius and the maximum temperature is at least 4.5 degrees Celsius below the normal, it is said to be a ''cold day''.