India’s urban open spaces have become unbearably hot. A planning expert shows how to reclaim these areas

This is the first of a two-part series on effective planning to make cities cooler. Read the second part.

The world is witnessing ever-increasing temperatures. In the summer of 2023, extreme heat struck worldwide, making July 2023 the hottest month ever recorded. Average global temperatures broke records three times in the first two weeks of July. This year in Delhi, temperatures have been hovering dangerously close to the 50 degrees Celsius mark throughout May and parts of June.

While global warming is responsible for an overall rise in temperatures, it is the urban heat island (UHI) effect which is the main culprit for soaring temperatures in urban areas. UHI effect occurs when cities are warmer than surrounding rural areas due to human activities and structures like buildings and roads that absorb and retain heat. This results in higher temperatures, increased energy consumption and potential health issues for residents. A 2024 study by the Delhi-based think tank Centre for Science and Environment on the morphology and heat patterns of nine Indian cities over the past decade showed how urban centres are turning into heat islands, posing a serious threat to human health.

The extreme heat fundamentally threatens our ability to use outdoor spaces. Open markets, public areas, and streets are becoming less viable, hindering community interaction. Those most affected are individuals whose livelihoods depend on outdoor work, forcing them to endure the sweltering conditions. As per estimates, up to 75 per cent of India’s workforce depends on heat-exposed labour. A research conducted in India revealed that 71 per cent of 442 workers were exposed to heat from the sun, incineration or other sources. In the summer, 82 per cent faced heat stress, a physical hazard that can cause discomfort, headaches, psychological issues, heat stroke, and, in extreme cases, death. 

A study by the National Hawkers Federation surveyed 721 street vendors across 15 city markets and discovered high levels of irritability, anxiety and sleeplessness among vendors during heatwaves. Additionally, seven out of eight women vendors reported high blood pressure, and some expressed concerns about delayed menstrual cycles due to extreme heat. Unfortunately, considering the trend of rising temperatures year after year, it seems the heat is here to stay and this will be the new normal. Given the situation, implementing climate-appropriate heat-proofing measures to reclaim our open areas becomes essential.

Understanding our experience of heat outdoors

When we talk about feeling uncomfortably hot, what we experience can vary greatly depending on whether we are in direct sunlight or in shade. The temperature that is reported is generally the dry bulb temperature, also called the ambient air temperature. To put this into context, if the Internet tells you that it is 45 degree in the city, it is the temperature one experiences while being under a shaded area in open space. However, the moment one steps out into the sun as experienced often by labourers, hawkers and others working outdoors, one would experience something higher than the ambient air temperature. Under direct sunlight, we experience what is known as the sol-air temperature, which also considers the ambient air temperature as well as the solar radiation. However, there is one more critical component that makes a difference in our experience of heat: The ground itself absorbs and radiates back the heat to the surrounding air, thus raising the ambient air temperature further.

CSE conducted a study to find out how the surface temperatures varied in different situations. An infrared thermometer was used for the experiment. The experiment was conducted in June 2024 in and around Lodhi Road in New Delhi. Surface temperature is an effective metric for understanding the direct sunlight a surface receives and the quantity of heat it retains.

Shade makes a massive difference

The results revealed that providing a shade over a surface makes a massive difference. The surface temperatures of the black topped road was found to be 12 degree higher under the sun, reaching a staggering 61.4°C. 

The red paver tiles faced a similar situation — the temperature under shade was 38.7°C but jumped to around 60°C in areas with the same material that were exposed to direct sunlight, a whopping increase of 21.5°C.

Colour changes way heat is absorbed

Surface temperatures revealed that the red paver tile under shade had temperature of 38.7 C. In similar conditions, the black topped road had a surface temperature of 49.4°C. This is because the lighter coloured surfaces reflect more heat and retain less of it.

Further, the surface temperatures of different grasses were compared. Under sun, artificial grass was found to have surface temperature of 47.8°C, which is around 13 degrees lower than the temperature of road and tiles under similar conditions. This can be attributed to the fact that the light green colour of artificial tiles is lighter than the black of the road and dark red of the tiles and, hence, reflects more sunlight and absorbs less heat.

Evaporation brings down temperatures further

The temperatures further improved in the case of natural grass reaching 43.4°C. This could be attributed to evapotranspiration in grass, wherein water vapour is released from the plant leaves to cool the leaf surface. This effect further gets amplified when the grass and the surrounding soil is made wet, as the evaporating water cools down the surface. The temperature in this scenario was found to be less than 40°C even under the sun.