In state capital Lucknow, the minimum temperature was 1.4 degree Celsius -- seven notches below normal.
Met officials have predicted a harsher spell of cold, with icy winds compounding the problems.
Gutsy north westerly winds were blowing across the state Tuesday, pulling down day temperatures further.
Temperatures were near sub-zero in Muzaffarnagar at 0.7 degrees.
Owing to the unrelenting cold weather, the government has ordered closure of all schools till Jan 12, after which a call would be taken again, Basudev Yadav, director (Primary Education), told IANS.
Regional Met Department director J.P. Gupta said the cold wave would persist for another week though foggy conditions would not stay for long.
Due to fog in some parts, vehicular and rail traffic were crippled.
Flights at the Chowdhary Charan Singh international airport in Lucknow and Babatpur airport in Varanasi were delayed.
More than a dozen trains were running late from and to Lucknow, railway officials said.
Photos: Shraddha Kapoor, Kim Sharma at 'Haseena Parkar' screening
Mumbai to Goa train: First look at the glass-top Vistadome coach
Shuttler Ashwini Ponnappa keeps it short and sexy on social media
Photos: Narendra Modi, others at Marshal Arjan Singh's funeral
Photos: Sussanne Khan and Nimrat Kaur spotted at a spa in Juhu