Standing in the shade of a tree on a hot summer day is often relaxing and refreshing. One would appreciate this all the more had they experienced the sweltering heat of Delhi’s summer. Being one of the most densely populated cities on the planet, the temperatures in the core areas of the city are much higher as compared to the peripheral regions. This unusually high temperature that exists in the city can be explained by a phenomenon known as the ‘urban heat island effect’ and the areas experiencing this effect are known as urban heat islands (UHI). The phenomenon was first described by scientists in the 1800’s.
UHI’s are areas that are often densely populated and composed of large portions of built-up areas that are characterized by heat-absorbing surfaces. These built-up areas coupled with relentless developmental activities that clear large portions of green spaces are responsible for the generation and accumulation of heat in the city. Further, dark paved surfaces that are ubiquitous in the city are known to have a low albedo (a measure of how much light/heat that hits a surface is reflected without being absorbed). In other words, dark paved surfaces absorb most of the incoming heat from the sun while reflecting very little back into the atmosphere. The heat from built-up areas and paved surfaces coupled with anthropogenic heat production are conducive for the creation of UHI’s.
A study conducted in Japan by Takebayashi and Moriyama (2012) showed that asphalt surfaces can be 20°C hotter than grassed surfaces in normal daytime temperatures. In a similar study of the effect of Land Cover Fractions on Changes in Surface Heat Islands by Chen et. al. (2019), an increase in land surface temperature was found to be caused by impervious artificial objects created in the urban expansion process. Impervious surfaces not only absorb more heat but also lead to faster runoff from land.
The issue of UHI is compounded when cooling strategies are adopted to cope with the increasing urban heat. Although cooling mechanisms such as air conditioning provide human comfort, their high energy consumption primarily leads to more heat production (Grimmond, 2007). Thus, cities are inherently spaces where activities and practices that are meant to provide comfort to people, result in the entrapment of heat and the consequent urban climate.
However, why is urban heat a matter of concern at all? With more than 68 % of the world’s population expected to be living in urban areas by 2050 (World Urbanisation Prospects), the question arises, how liveable will cities be especially when faced with the effects of UHI? UHI is responsible for an increasing number of heatwaves and heat stresses, with critical impacts on health and wellbeing.
In order to counter some of these negative impacts, a few steps should be taken. To begin with, green roofs and vertical gardens can help in reducing the impacts of UHI. Further, it is important to understand the intricacies of the urban environment while modifying existing structures and designing new ones.