The India Meteorological Department on Thursday said temperatures in parts of northern India showed slight improvement, ANI reported. However, low visibility conditions delayed 21 trains in the Northern Railway region, officials said.

The weather department said the temperatures have increased by three to four notches in parts of Uttar Pradesh, Rajasthan and at isolated places in Haryana, Delhi, Punjab and Madhya Pradesh during the past 24 hours.

Delhi airport authorities asked passengers to contact their respective airlines for flight updates.

However, cold wave conditions persisted in Bihar and authorities have extended the school holidays till January 5. “It has been made to appear to me that due to continuing prevalent cold weather and low temperature particularly in the early morning in the district, health and life of children are at risk,” Patna District Magistrate Kumar Ravi said. “Therefore under section 144 of Code of Criminal Procedure, 1973 the academic activities up to Class X which requires the presence of children in all the private and government schools of Patna District are prohibited.”

Around a dozen people were reportedly injured as several vehicles collided due to the low visibility in Rajasthan’s Alwar district. The injured have been shifted to a hospital.

The IMD said the wind direction changed from north-westerly (which brought cold winds from the Himalayas to the northern plains) to easterly (which brings moist air from the Bay of Bengal), leading to marginal rise in minimum temperatures.

Last week, Delhi had registered its 16th successive “severe cold day”, with minimum temperature recorded at 3.4 degrees Celsius.

The national Capital is witnessing the longest cold spell in December in 22 years. Since 1993, Delhi has had a cold spell only in four years – 1997, 1998, 2003, and 2014. According to the weather department, a “severe cold day” is registered when the minimum temperature drops to less than 10 degrees Celsius and the maximum temperature is at least 6.4 degrees Celsius below normal. A “cold day” classification is used when the minimum temperature is less than 10 degrees Celsius and the maximum is 4.4 degrees Celsius below normal.