Cities like Mumbai 1-2 degrees Celsius hotter than surrounding areas: CSE report

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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.

The extra heat is trapped and is unable to dissipate after sunset due to pollution and continuing waste heat generation. As a result, the city fails to cool down leading to warmer conditions compared to rural areas. The CSE study looked into the temperature and humidity data collected by the real-time air quality monitoring network and found massive variations in temperature within the cities.

In terms of absolute air temperature, Mumbai's was 5.5 degrees Celsius variation while the same for Delhi was 6.2 degrees Celsius. From a heat index perspective, Mumbai leads the group with a 17.3 degrees Celsius variation. Hyderabad with a 7.1 degrees Celsius variation had the most pronounced heat islands, while Kolkata with just 1.3 degrees Celsius had the least pronounced ones. "From the land surface temperature perspective, it is Delhi in the lead with a 24.6 degrees Celsius variation," the analysis said.

The difference among the cities can be explained by the environmental difference in the nature of the heat in their regions, as well as the differences in their topography and land use patterns. But the fact that all four cities show significant variations in their intra-city temperatures (in all three forms) is strong evidence of urban heat island problems within the cities, it said.

Heat index that accounts for both temperature and humidity shows Mumbai, Kolkata and Hyderabad more stressed than Delhi this summer. Even though the seasonal average daily maximum air temperature in Delhi has been 1.5-2.8 degrees Celsius higher than the other three metros, in terms of heat index Kolkata was significantly hotter.

Monsoon is hotter than the pre-monsoon period on average, while winter and post-monsoon seasons are warming up faster, the analysis said. "At an all-India level, the monsoon season - June, July, August and September, as per IMD classification - has been 0.3-0.4 degrees Celsius hotter than pre-monsoon (or summer)," CSE said adding, "What's more? It is getting hotter with time!"

The decadal average temperatures for pre-monsoon or summer period are now 0.49 per cent hotter than the long term normal (1951-80 baseline). "This is a significant increase, but it pales in front of the increase noted among the decadal average temperatures for the other three seasons. The post-monsoon period - October, November and December as per IMD classification - is hotter by 0.73 per cent. Similarly, the winter - January and February as per IMD classification - has been warmer by 0.68 degrees Celsius, and during the monsoon by 0.58 degrees Celsius," the CSE analysis said.

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