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On the brink

Sci Tech Jun 18, 2023

Over the years, urbanization has increased at an unimaginable pace in India and all over the world. People have moved to the nearest cities in search of jobs and livelihood. Along the way, nature has borne a severe toll because of our actions. Numerous places in Delhi, the NCR, Mumbai, Chennai and Indore have reported significantly higher night temperatures than other places inside and outside the city. This is due to a phenomenon called the "Urban Heat Islands".

Concrete, asphalt, steel and glass are among the majority of materials that make up an urban environment. All these materials are heat absorbing and have a very low albedo, meaning they can absorb more heat and sunlight than they can reflect. That can cause them to heat up very fast under direct sunlight, which means that they absorb the radiated heat during the day and re-radiate at night. That leads to an increase in local temperatures, which we term “Heat Islands”. Although a large part of a city is made from these materials, only those few that are densely populated and heavily inhabited suffer from this problem. Now you might think, “So what? Why is this even a concern?”. It all comes down to cooling requirements. Since school days, we talked about systems and energy, and we are familiar with the concept that every system wants to be at a lower energy state, well, usually. But in the case of cooling, removing the heat energy from the room is an extremely unlikely process to happen on its own. Refrigeration and air conditioning are costly because we move energy up the gradient. When your apartment heats up significantly, there is a limit up to which you feel comfortable. When that cap is breached, you look for ways to cool your apartment. A very popular method is the usage of an air conditioner, and that’s a lot of energy to be spent. Now remember, we said that this often happens in densely populated areas. Now, when all the apartments in your neighborhood turn on their air conditioners, imagine the power consumed and the immense load on the grid. In many places across India, there isn't enough grid infrastructure to bear such loads. The heat removed from the apartment is let out, along with the heat generated from the running of the machinery. When the local density of A/Cs becomes large, the air around that zone heats up more than in other places. The extra heat is again absorbed, radiated and re-radiated by the roads and buildings.

Now that we have understood what a heat island is and how it works, let’s look at why and how they form in the first place. Ample studies have attributed this to the decrease in vegetation with increasing urbanization. Plants transpire throughout the day and add to the moisture in the air. This moisture takes up the heat and keeps the surroundings cool. You can experience this effect if you go under a tree’s shade on a hot summer afternoon. Furthermore, trees planted along roadsides also provide shade, which lowers the heating up of roads. Increased impervious surfaces are another factor adding to the trouble. Soil, unlike concrete and asphalt, absorbs the sun’s heat. Roads and buildings cover up all exposed soil and entrap the heat within. The geometry of the city is yet another factor influencing this effect. Places with tall buildings and narrow roads, on the one hand, entrap more heat than those with wider streets, while on the other hand, the tall buildings do not expose the streets to direct sunlight and hence prevent them from heating up. Findings strongly affirm that wider streets are more prone to the heat island effect. The architecture and location of buildings also play a vital role in the wind flow patterns around the city, hugely impacting heat extraction. Climate change has its hand in adding fuel to this burning fire. The ENSO oscillation (El-Nino southern oscillation) has dramatically changed the wind patterns and monsoon seasons in countries like India. That has led to problems like the late onset of monsoon season, frequent cyclones, etc; to name a few. Tropical cyclones usually form over oceans and pull in all the moisture from the land towards the low pressure. That further reduces the cooling effect of moisture, as discussed earlier.

Now let us look at the effects that the heat islands have on people, plants, animals and infrastructure. Recent observations in metropolitan cities have shown that the difference in temperature between two places within the same city could be as much as 15 C. That means the temperature in the hotter areas is 15 C above usual.[1][2] That leads to the failure of roads and bridges due to overheating and buckling. That poses a safety threat to the public using them and a loss of infrastructure. The elevated day temperatures also put the animals like dogs, cows and pigeons inhabiting the place on the red line. There has been an increase in reports of dogs and birds dying due to extreme heat in major cities across India. It also affects the birds’ egg-laying and hatching periods, due to which there’s a marked decrease in the birth and survival rate of urban birds. There are other socio-economic effects and considerations of heat islands, which are beyond the scope of this article. Since the last decade or so, there has been a lot of research going into finding a solution. One of the most effective ways prescribed is to implement rooftop gardening. Rooftop gardening bundles a double advantage: it offers shade to the rooftop and doesn’t let the terrace heat up quickly. Secondly, the plants transpire throughout the day, thereby taking up the heat from the surroundings. Vertical gardens and hanging gardens are other ways of growing more plants on and around buildings because, ultimately, plants are the most effective means of controlling heat and pollution. Creating more parks and gardens with misting systems is highly desirable in an urban environment. Scientists also prescribe reducing the width of streets and planning nearby buildings to be of different heights. But these suggestions were not welcomed by many urban authorities as they lead to public inconvenience. Painting the buildings and sidewalks in lighter colours is known to reduce heat absorption by reflecting most of the incoming sunlight.

Despite all efforts, heat islands still prevail in many cities across India and the world. It is victimizing people, animals and infrastructure. Cities like Singapore and New York have come up with innovative strategies to mitigate this phenomenon. It is imperative that city authorities work closely with environmental experts, biologists and scientists to bye-pass this existential risk.


Vishaal Harikrishna Kumar

I write because I have something to say. You are encouraged to think outside the blog and not to read in between the lines.