After a brief respite, the weather turned extremely cold in most parts of Uttar Pradesh, including Lucknow and neighbouring areas on Tuesday, officials said.
Fog descended on the state capital since Monday night and continued till early Tuesday morning.
The mercury was recorded at 5.5 degrees Celsius.
Poor visibility hampered movement on roads and bonfires were spotted at most public places and markets.
More than 12 people have died due to the inclement weather and fog-related accidents in the last 24 hours.
Three deaths were reported from Machlishahr in Jaunpur, one death each in Unnao, Rae Bareli, Lakhimpur, Azamgarh, Muzaffarnagar and Shamli. Three persons, including a couple were killed in a road accident in Barabanki.
Najibabad with 1.6 degrees Celsius was the coldest in the state. The mercury dropped to 1.8 degrees Celsius in Sultanpur and Lakhimpur Kheri.
The weather department predicted that the cold wave would continue till Friday and it is expected to clear up after Sunday.
Six trains including the Shaheed Express have been cancelled due to long delays caused by fog and poor visibility. Other trains that have been cancelled include Sealdah Express, Kashi-Vishwanath Express and Kaifiyat Express.
Many state-owned buses also remained off the important Lucknow-New Delhi and Lucknow-Gorakhpur sectors.