More than 60% of the total 89 urban areas are 1-5 degree C cooler than their surrounding non-urban areas during April and 70% of cities are cooler in May.

Between February and May, most of the 89 Indian cities that are to be developed as Smart Cities have been found to be 1-5 degree C cooler during the day relative to the surrounding non-urban areas. More than 60% of the total 89 urban areas are 1-5 degree C cooler during April and 70% of cities are cooler in May. This observation is in variance with the globally witnessed phenomenon of urban areas getting significantly warmer during the day compared with the surrounding areas as a result of urban heat island effect.

In contrast, during the post-monsoon period (October to January), about 80% of the urban areas show typical urban heat island effect during the day and are 1-6 degree C warmer than the surrounding non-urban areas.

But warmer nights

But at night, all the cities studied are 1-5 degree C warmer than the surrounding non-urban areas due to urban heat island effect regardless of the season and location. Compared with other cities, urban areas in semi-arid and arid regions of western India show higher warming in the night. The night time warming is driven mainly by heat stored in buildings and impervious concrete areas.

A team of researchers led by Prof. Vimal Mishra from the Civil Engineering department at IIT Gandhinagar found that cities are cooler during the day than the surrounding non-urban areas only when the non-urban areas lack vegetation and moisture either due to lack of irrigation or water bodies. These cities (Kurnool, Vijayawada, Badami, Bijapur,  Aurangabad and cities in Gujarat and Rajasthan) are typically located in western and central parts of India.

However, cities (Varanasi, Lucknow, Allahabad, Kanpur, and Patna) in the Gangetic Plain, north-western India (Punjab and Haryana) and southern tip of the west coast show typical urban heat island effect during the day; these cities are 3-5 degree C warmer than the surrounding non-urban areas during the pre-monsoon (February-May) and post-monsoon (October-January) periods. The non-urban areas in these areas have vegetation in the form of trees or agriculture and have moisture due to irrigation.

What causes the cooling

“There are two reasons why urban areas in western and central parts of India become cooler than non-urban areas during summer. The non-urban areas have no crop and moisture, the soil is dry and day-time air temperature is above 40 degree C. On the other hand, the urban areas have vegetation cover and water bodies. This is why cities are cooler than the surrounding non-urban areas during the day,” explains Prof. Mishra.

More than 70 of the 89 cities studied are surrounded by non-urban areas which have more than 50% of total land cover under agriculture between November and March. This results in non-urban areas being cooler than the cities during the post-monsoon season. The results were published in the journal Scientific Reports.

Aerosols too have an effect in reducing the temperature but their contribution to cooling during day time is very less compared with vegetation and irrigation.

Preventive strategies

“Cities being significantly warmer than surrounding non-urban areas during night has policy-related implications,” says Prof. Mishra. “During heat-waves, the prominent night urban heat island effect which is prevalent across cities could worsen the levels of discomfort.”

“Since the government is planning to develop these as smart cities, we should think of using more sustainable building materials that absorb less hear during the day. We also need to include passive cooling measures such as increased tree cover, increased ventilation in buildings and orientation of buildings in modern building designs to reduce the night-time urban heat island effect,” he says. There should be an optimal combination of impervious cover, vegetation cover, and water bodies within the cities.

The researchers used satellite data (2000-2014) and community land model to identify the impact of irrigation and show the cooling seen in cities is due to lack of vegetation and moisture in non-urban areas relative to cities.

Published in The Hindu on October 25, 2017