Mega cities like Mumbai, Delhi, Kolkata and Hyderabad are hotter than the larger region surrounding them due to a phenomenon known as 'heat island effect' caused by surface absorption of heat and local-waste heat generated by traffic, industrial pollution, air-conditioning and other urban activities, found an analysis by the Centre for Science & Environment (CSE) on Thursday.

"The seasonal average of Mumbai, Delhi, Kolkata and Hyderabad was 1-2 degrees Celsius higher than the all-India average and 2.5-3.8 degrees Celsius higher than northwest India. This is due to the urban heat island phenomenon," CSE said. Due to excessive hard and dark surfaces, cities tend to absorb the heat coming from the sun during the day and also generate a lot of waste heat due to human activities, which add to the natural heat, such as the use of air conditioners.