Contrary to the common notion, a “majority” of 84 cities across India, particularly those in central India and the Gangetic Basin, have lower daytime temperatures from March to May compared with the surrounding non-urban areas (taken as 1 km radius of the city). Cities with heavily built-up areas and concrete structures are supposed to have higher temperatures than non-urban regions due to urban heat island effect.
The results — published on January 9 in the journal Scientific Reports and based on a 13-year land surface temperature record from satellite sensors — are not in agreement with the general understanding of urban climate and surface urban heat island effect in tropical cities. They once again highlight the importance of increasing the vegetation cover in cities to effectively mitigate the urban heat island effect.
A study by a team of researchers led by Prof. Subimal Ghosh from the Department of Civil Engineering, Indian Institute of Technology (IIT) Bombay has shown that while cities have lower daytime temperatures than surrounding non-urban areas from March to May, it is the reverse during nights. During night time, the cities, particularly those in the Gangetic Basin, were hotter than non-urban areas.
The relatively high vegetation cover leading to higher evapo-transpiration compared with nearby non-urban areas is the main reason why cities are relatively cooler than the adjacent non-urban areas during the day in summer. While the cities have more trees, the non-urban areas are mostly crop lands and are barren during the summer months. The absence of evapo-transpiration during night and the heat contained in the concrete structures increases the night time temperatures in the cities during March to May.
Since cities tend to have lower daytime temperatures during March to May, the intensity of heat waves will be lower in the cities compared with non-urban areas. “This is prominent in cities that are located in the arid region. We didn’t see this in coastal cities,” says Prof. Ghosh, the corresponding author of the paper.