Authorities ordered closure of all schools in Lucknow up to Class 8 till Jan 13.
The regional Met Office said the minimum temperature in Lucknow was 2.1 degrees Celsius Saturday, which was five degrees below the season's average.
The minimum temperature in Kanpur settled at 3.7 degrees Celsius, while it was 4.7 degrees in Varanasi and 5.2 degrees Celsius in Gorakhpur.
In New Delhi, fog delayed trains by over six hours, though flights were not affected. Icy winds kept temperatures below average for the past few days.
The Met office has forecast a cold and foggy weekend in Delhi.
Saturday's maximum temperature was 12.6 degrees Celsius, eight notches below average for this time of the season. The minimum was 2.9 degrees, four notches below average, an official of the India Meteorological Department (IMD) said.
"There is a heavy backlog leading to many delays and rescheduling of trains. A total of 23 trains were delayed and 10 cancelled due to fog," a Northern Railway spokesperson told IANS.
An official at Indira Gandhi International Airport said flight operations were not affected.
"Due to a thick fog in the morning 23 flights operated in Low Visibility Procedure (LVP) and one flight was delayed," an airport official said.
The Met department has forecast similar weather conditions Sunday.
"The maximum and minimum temperatures are likely to be around 13 and 3 degrees Celsius respectively on Sunday," a Met official said.
The national capital is experiencing intense cold conditions this week with Friday being the coldest with the minimum temperature dropping to this season's lowest of 2.7 degrees.
In the northern reaches, Srinagar in Jammu and Kashmir recorded minus 5.4 degrees Celsius Friday night, the season's coldest.
The minimum temperature dropped to a bone-chilling minus 16.8 degrees in Leh town of the Ladakh region Saturday. It was minus 9.4 in Gulmarg ski resort and minus 5.4 in Pahalgam.
Parts of Dal Lake in Srinagar are frozen.
The cold wave continued to grip Himachal Pradesh Saturday with the higher reaches experiencing temperatures below freezing point.
Saturday's minimum temperature in Shimla was 0.6 degrees Celsius, two degrees below the average for this time of the year.
The minimum temperature in Manali settled at minus 1.8 degrees Celsius, while it was minus 4.4 degrees in Kalpa in Kinnaur district and minus 7.1 degrees in Keylong in Lahaul and Spiti district.
Dharamsala, the seat of Tibetan spiritual leader the Dalai Lama, saw a low of 5.2 degrees Celsius.
The minimum temperature in Agra settled at 2.1 degrees Celsius Saturday with a thick fog enveloping the city in the early hours. The regional Met department office in the state capital Lucknow said the temperature could drop to one degree Celsius in the next two days.