Cold conditions persisted in the national capital as the minimum temperature remained below five degrees Celsius for the fourth consecutive day on Saturday, the India Meteorological Department said.

The Safdarjung Observatory, which provides representative data for the city, recorded a minimum of 4.4 degrees Celsius, three notches below normal.

"Shallow" fog reduced visibility to 1000 meters at Safdarjung and 800 meters in the Palam area, Kuldeep Srivastava, the head of the IMD's regional forecasting centre, said.

According to the weather department, "very dense" fog is when visibility is between 0 and 50 meters, 51 and 200 is "dense", 201 and 500 "moderate", and 501 and 1,000 "shallow".

The mercury is likely to increase slightly on Sunday and Monday under the influence of a fresh Western Disturbance affecting the upper reaches of the Himalayas. Moderate fog is likely during this period, Srivastava said.

Cold wave conditions will return December 29 onwards, he said.

For the plains, the IMD declares a cold wave when the minimum temperature is 10 degrees Celsius or below and is 4.5 notches less than normal.

Delhi's air quality improved slightly, but it was still "very poor".

The city's air quality index (AQI) was 324 at 9 am. The 24-hour average AQI was 357 on Friday, 423 on Thursday, 433 on Wednesday and 418 on Tuesday.

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".

A marginal improvement is predicted by Sunday as wind speed picks up under the influence of the WD, Srivastava said.

However, he said, the air quality will dip again, starting Monday, due to predicted high humidity, low wind speed and temperatures.