doi:10.1038/nindia.2017.8 Published online 23 January 2017
Cities around the world are warmer than their suburbs due to a phenomenon described as the ‘urban heat island effect’. But Indian cities seem to be bucking the trend, courtesy an anomaly in ‘evapotranspiration’ patterns between urban and non-urban areas, new research suggests1.
Researchers from Indian Institute of Technology Bombay (IITB) and Charotar University of Science and Technology in Anand, Gujarat have found that between the summer months of March and May, most Indian cities are actually cooler than the rural/suburban regions around them.
The researchers carried out the first ever comprehensive study of the urban heat island effect by analysing temperatures from 84 Indian cities and their surrounding non-urban regions, and found that between March and May, more than half of the cities had lower day time land surface temperature than their surrounding non-urban localities. This contradicts the prevalent theory of urban heat island effect.
The reason for this anomaly, they say, lies in the difference in urban and non-urban patterns of evapotranspiration – a process by which water is transferred from land to the atmosphere by evaporation and transpiration by plants. During the hot pre-monsoon months, rural areas have lower evapotranspiration as they turn into dry arid regions with low vegetation cover. However, the evapotranspiration level in cities is relatively higher because of human consumption of water and the presence of trees and grass. This difference leads to complex heat exchange and makes cities cooler than their surrounding regions. However, they are cooler only for a few hours – as night approaches, the surrounding regions become cooler than the cities.
“The urban regions sometimes maintain some vegetation artificially through gardening and management of lakes,” observes one of the researchers Subimal Ghosh, an associate professor at IITB, Mumbai. “This results in evaporative cooling during summer days and we see urban oasis in the arid barren regions of interior India,” he explains. However, at night the exact opposite happens as there is no evaporation and the urban heat islands become very prominent, Ghosh says.
Higher temperatures in non-urban regions make for more intense heatwaves than the cities close by. This also is in sharp contrast with the general understanding of the climate of tropical cities.
In winter, the black carbon-laden cities of the Indo-Gangetic plains are warmer than their surrounding regions. This is interesting because the presence of black carbon tends to lower the land surface temperature. “This needs to be investigated further with model driven studies,” Ghosh says.
Atiqur Rahman, a former professor at New Delhi-based Jamia Millia Islamia says the study could be made more impactful by exploring how rainfall affects the urban heat island phenomena of cities.
1. Shastri, H. et al. Flip flop of day-night and summer-winter surface urban heat island intensity in India. Sci. Rep. 7, 40178 (2017) doi: 10.1038/srep40178