Urbanisation has not led to hotter summer days for many Indian cities

Urbanisation has not led to hotter summer days for many Indian cities

  • 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 & concrete structures are supposed to have higher temperature than non-urban regions due to urban heat island effect.
  • The results highlight the importance of increasing the vegetation cover in cities to effectively mitigate the urban heat island effect.
  • 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 temperature in the cities during March to May.
  • 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.

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