Contrary to common notion, a “majority” of 84 cities across India, particularly those in central India and Gangetic Basin, have lower daytime temperature 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 temperature than non-urban regions due to urban heat island effect.
The results published on January 9 in the journal Scientific Reports based on a 13-year land surface temperature record from satellite sensors is not in agreement with the general understanding of urban climate and surface urban heat island effect in tropical cities. The results 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 temperature 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.
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“This is prominent in cities that are located in the arid region. We didn’t see this in coastal cities as they are not in the arid region,” says Prof. Ghosh, the corresponding author of the paper.
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 temperature in the cities during March to May.
Since cities tend to have lower daytime temperature 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 as they are not in the arid region,” says Prof. Ghosh.
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During winter (December to February) crops that grow in the non-urban areas result in increased vegetation cover and more evaporative cooling leading lower temperature than in the cities. Also, there is increased biomass burning for cooking and heating in the cities during winter leading to increased emission of black carbon. “The black carbon emission increases the air temperature which may have a feedback to land surface temperature. But this has to be investigated further,” says Prof. Ghosh.