Contrary to global evidence, a study by IIT Gandhinagar researchers has found that during daytime the urban areas are relatively cooler than the surrounding non-urban areas during heat waves. However, during night time, all urban areas are hotter than the surrounding non-urban areas. The reason being that unlike non-urban areas, the urban areas have perennial vegetation cover in the form of trees and more water bodies too.
A study of 89 urban areas in India has found that though there is an absolute increase in temperature during heat waves in both urban and non-urban areas, the urban areas are relatively cooler than the surrounding non-urban areas. At 1.94 degree C, the absolute increase in temperature during daytime in non-urban areas during heat wave was significantly higher than the urban areas (0.14 degree C).
The general notion and global evidence suggest that compared with non-urban areas, the urban areas become hotter during daytime during heat waves. However, in the case of India, the urban areas were found to be relatively cooler than the surrounding non-urban areas — at 44.5 degree C, the non-urban areas were warmer than urban areas (43.7 degree C) during heat waves. However, during night time, all urban areas were hotter than the surrounding non-urban areas.
“This result was quite unexpected. Though our earlier study showed that compared with non-urban areas the urban areas experience daytime cool island effect, we hypothesised that this might not be the case during heat waves. But the results of our study showed otherwise,” said Prof. Vimal Mishra from the Department of Civil Engineering at IIT Gandhinagar, who led the study. “Our study has implications for urban planning in India.” The results of the study were published in the journal Environmental Research Communications.
Why urban areas are cooler
According to Prof. Mishra, the urban areas witness lesser temperature increase during heat waves compared with non-urban areas due to significantly higher tree cover and more number of water bodies. In contrast, a majority of non-urban areas are located in agriculture-dominated regions. In non-urban areas, the vegetation cover in the form of crops and soil moisture from cropland irrigation decline sharply after crops are harvested and well before the onset of heat waves during summer. The urban areas, on the other hand, have perennial vegetation in the form of tree cover and lawns, and more number of water bodies, which help in keeping the urban areas relatively cooler than non-urban areas.
The land surface temperature was estimated by analysing satellite data collected between 2003 and 2016. Between 1951 and 2016, a majority of urban areas experienced about five hot days and nights per year. About 44% of urban areas showed an increase in frequency of hot days while 34% showed a significant decline in frequency of hot days.
Between 1951 and 1980, the frequency of hot days in urban areas located in the Indo-Gangetic plain region was more than in urban areas lying outside this region. But post-1980, the urban areas in the Indo-Gangetic plain region witnessed a decline in the frequency of hot days and hot nights. The decline in the frequency is due to intensive irrigation in the Indo-Gangetic plain.