Earlier this year, the National Green Tribunal (NGT) had halted concretisation of all roadsides in Noida, Greater Noida to minimize urban heat island (UHI) and water logging, and to maximise ground water recharge and bio-diversity. The heat wave was phenomenal this year.
In Delhi, the average summer temperature is 36-39°C. In mid-May, the temperature shot up to 49°C in some parts of the city. Such sudden alarming increase in ambient temperatures cannot be attributed alone to seasonal or metrological factors but also due to heavy concretization of cities and urban spaces. These surfaces when exposed to the sun absorb heat during the day. At night they release heat, leading to high night temperature as well. This phenomenon is called Urban Heat Island (UHI).
What is Urban Heat Island?
Urban heat island is a kind of heat accumulation phenomenon within urban areas due to urban construction and human activities
Cities are much warmer than villages nearby. Have you noticed that you feel much cooler and refreshed when you drive into less dense areas with tree cover, lakes, and water bodies? Water fronts are cooler and breezier than urban alleys and roads. The situation in urban area is no better at night as the heat gets trapped and hard surfaces that absorb heat during the day, reradiate it back to earth.
Heat islands are formed when cities replace natural land cover and water bodies with pavements and buildings. These actions lead to higher urban temperature as removal of trees and green cover reduce the cooling and shading impact of plants and the cooling effect from evapotranspiration. Pavements, rooftops, and other non-reflective surfaces absorb heat and retain the heat during daytime. In the evenings, the heat gets released to atmosphere thus increasing the urban temperature.
The heat build-up is stronger if there is no air movement and the impact of heat island is felt strongly in places where the heat is trapped in urban dense spaces.
Urbanisation also adds to more vehicles on street, air-conditioners in buildings, and heat releasing industries such as from furnaces and boilers. All these activities and elements release heat to environment and aggravate the heat island impacts.
Why are heat islands bad for us?
Higher temperatures affect people’s health, air and water quality and increase energy consumption. It also has great impact on people’s quality of life and livelihoods. It is more likely that casual street vendors and businesses operating in open areas shall keep away from hot urban areas and confine themselves to indoors during hot summer days. This single action has two impacts, loss of livelihoods and use of indoor equipment such as air-conditioning or fans that consume energy.
Heat Islands and health
Heat islands can significantly raise temperature. A 1 - 12°C increased temperature is easily possible in dense urban cities during summertime. This causes breathing problem; heat strokes and the impacts are widely visible in vulnerable population such as young children and senior citizen.
Heat Islands and air quality
Heat build-up also impact air quality by causing chemical reaction that increases ozone levels in air. Higher ozone concentration triggers cough, chest pain, and flares asthma. It also impacts lung function negatively. Ozone damages plants by inhibiting photosynthesis. Thus, elevated level of ozone due to heat island can reduce crops and vegetation yields.
Heat Islands and water systems
Pavements and hard surfaces that cause heat islands, also prevent water percolation in ground and damage rooting system of trees. No wonder, big trees get easily uprooted, in slightest storms.
Heat Islands and Air conditioning
Heat islands increase air conditioning and cooling needs in a city significantly. A study done by TERI in the city of Bengaluru, estimated a cooling load reduction of 10-16% in commercial buildings if suitable heat island mitigation measures are taken.
Can we minimise heat islands?
The solution is in our hands. Actions to reduce heat island impact can be implemented at two levels- Marco perspective of town planning and micro perspective of building design and planning.
The planning on the Bay in the City of Singapore was done by keeping this perspective in mind. Buildings were spaced out to allow air movement and heat dissipation. Enough green areas were introduced to have micro cooling effect.
The following measures can be taken at urban and building level to reduce heat island impacts,
1. Arrange buildings in masterplan to allow air flow and avoid stagnation of air.
2. The tall buildings should be spaced out more to allow heat dissipation as the heat releasing surface area is more in taller structures
3. Heat reflecting glasses also re-radiate and reflect back heat to atmosphere. Thus, use of glazing should be optimised for buildings
4. Green spaces and tree cover should be adequate. Tree cutting should be avoided as much as possible and maximum effort should be made to retain the existing trees, particularly the shade giving mature and evergreen trees.
5. Pavements should allow water percolation and tree growth. Full concretisation should be avoided.
6. Roofs of buildings should be shaded /painted as cool roof/have urban farming.
What can we do?
Urban heat island, as the name suggests is an urban phenomenon that arises from cumulative impact of our actions at urban level. While we can rely on our urban local bodies to take suitable actions to minimize urban heat islands, there are some actions that we can take at individual level. For example, if we have control on the roof area of where we stay, we can paint it with a high albedo paint and make it a cool roof. Else, we can also put up a roof garden by taking measures to ensure that waterproofing is adequate. We can minimize tree cutting and plant shade giving trees. All external reflective glass surfaces should be shaded.
There are good examples of city wide programs to mitigate urban heat islands. The city of New York has a cool roof programme to mitigate heat island.
The LEED certified city of Louisville in Kentucky , USA has a successful UHI management program.
Effective urban planning policies that look at integrated ways of managing heat islands go a long way in reducing emissions, and improving air quality and human health.