The city is bracing itself for a few months of warm summer nights. But the winter nights were no better.
An analysis by the India Meteorological Department (IMD), Pune, showed the frequency of warm nights in Mumbai during winter has almost doubled in the past two decades because of the growth of high-rise structures, more vehicles on the roads and fewer open spaces.
The study was done by IMD and University of Western Ontario, Canada, on urban heat islands from November to February. An urban heat island is an area significantly warmer than its surroundings because of human activity.
The analysis found that between 1969 and 2009, during the sunset hour, the temperature difference between the Colaba and Santacruz weather stations was only 0.5 degree Celsius . This distinction, however, grew as the night progressed. The hourly temperature recorded at Colaba was 3 to 4 degrees Celsius more than at Santacruz.
“The urban heat island phenomenon in metros is because of rapid urbanisation and modern building structures that retain more heat,” said AK Srivastava, the lead investigator, IMD. “Heat islands are more prominent during winter nights, a trend seen in cities across the world.”
The study analysed how often such heat island events took place in Mumbai over four decades.
The decade between 1991 and 2000 recorded 98 nights - when the difference between hourly temperature at Colaba and Santacruz weather stations was more than 3 degrees Celsius for most hours after sunset. This is followed by 90 events from 2001-2009.
The first two decades of the study period witnessed fewer warm nights - 65 warmer nights from 1971-1980 in south Mumbai as compared to the suburbs, and 25 occurred between 1981 and 1990.
“An increase in concrete surfaces and the lack of tree cover is responsible for urban heat islands. Green cover and soil do not absorb much sunlight, helping to moderate the sun’s heat. But when sunlight gets absorbed in concrete surfaces, it radiates heat at night, therefore the nights get warm,” said Rakesh Kumar, chief scientist and head, National Environmental Engineering Research Institute, Mumbai.
The team found in the course of 40 years, the occurrence of heat island events was the highest in the month of December (98) followed by January (62), November (55) and February (37).
“While sea surface temperature plays a role in temperature rise in Colaba, there are other factors as the sea has always been there, but the frequency of heat island events has almost doubled in the past two decades,” Srivastava said .
“There seems to be an increase in the number of vehicles and urbanisation near the Colaba weather station as there are relatively less restrictions compared to the airport site (where the Santacruz weather station is) where high-rise buildings are not allowed.”