The Urban Heat Island Effect

As most of us who live in the concrete jungle of Colombo would agree, sometimes it’s “just too hot to even walk outside”. Taking a walk along the busy Galle Road pavement at midday is almost unbearable; it feels like there is heat radiating from the ground and everywhere around you…you just can’t wait to get inside an air-conditioned building, to drink something cold.  Am I right?

This is due to the ‘Urban Heat Island Effect’; a phenomenon felt in urban areas the world over.  Urban Heat Islands (UHIs) are ‘built up’ areas within urban or metropolitan boundaries that are warmer than suburban and rural areas due to human activity.  Hotter days naturally drive up the temperature further and on hot days cities are found to be up to 10 degrees hotter than surrounding rural areas

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What is the Urban Heat Island Effect?

Generally, dark objects/areas, especially man-made ones such as roads made of concrete, asphalt, buildings made of darker materials, roofing sheets etc., tend to absorb more of the sun’s radiation and, therefore, heat up more quickly than lighter coloured and vegetation filled rural countryside environments.

Cities and towns are designed to drain surface-water into drains and sewers, where it cannot evaporate as easily and remains trapped under bitumen roads and sidewalks.  In contrast in rural areas, the groundwater and surface run-off from rain drains easily through the top-soil and is transpired by plants.  Evaporation of water in natural environments also adds a cooling effect to the local climate making villages and the rural countryside much cooler.

Cities have drastically lower vegetation and tree cover and, therefore, do not have the advantage of natural (and free) cooling from tree-shade and evapotranspiration, which occurs around the clock in natural environments. 

The Urban Heat Island Effect is made worse by things like air conditioning and the heat generated by vehicles; both large and small.  In cities like Colombo, when we try to escape the heat by turning on the air-conditioning and choosing to drive, or catch a taxi, rather than walking, we are, in fact, contributing further to the Urban Heat Island Effect.

How does the Urban Heat Island Effect impact us?

  1. It further exacerbates climate change – Urban heat islands increase the demand for energy consumption during the summer when temperatures rise.  As a result of increased energy consumption, there is an increase in air pollution and greenhouse gas emissions.
  2. It affects our general health and wellbeing – Heat islands contribute to higher daytime temperatures, reduced night-time cooling, and higher air pollution levels. These, in turn, contribute to heat-related deaths and heat-related illnesses such as general discomfort, respiratory difficulties, heat cramps, heat exhaustion, and non-fatal heat stroke. Aggravated heat stress can have more serious repercussions for vulnerable people such as the elderly, the very young, and those with pre-existing medical conditions.
  3. It can kill off urban vegetation – Extreme heat can have damaging impacts on certain tree species.  Higher ambient temperatures, combined with low soil moisture during droughts, can scorch the foliage and bark of trees, especially those that are not yet well established.  This can eventually kill the tree.  This is seen with newly planted trees along busy urban roads in Colombo.
How can we mitigate the Urban Heat Island Effect?

Creating green spaces within urban areas is the most effective method for mitigating heat retention and the Urban Heat Island Effect.  Establishing trees along busy roads and sidewalks, as well as inside car parks, creating ‘urban forests’ or parks within busy metropolitan cities, and increasing the overall vegetation within city boundaries, helps in reducing the Urban Heat Island Effects.  This, it does by lowering surface and air temperatures by providing shade, deflecting the radiation from the sun, and providing natural cooling through evapotranspiration.

In Sri Lanka with its burgeoning human population, cities like Colombo, Galle, Kandy are growing exponentially and smaller towns are transforming into larger towns and cities overnight. Well thought out ‘town-planning’ which incorporates plenty of green-spaces like parks, tree-lined roads etc are essential in mitigating the Urban Heat Island Effect.

Making informed decisions in choosing energy-efficient appliances and equipment for urban homes and office spaces is also important. 

Whilst tree planting in cities and towns is important, it is also essential to maintain newly planted trees and saplings, as it can take up to 20 years for a tree to grow to a size that can help in mitigating the Effect. When selecting trees for planting, it is important to collaborate with someone with knowledge on local tree species as we should target species that are able to tolerate hotter conditions due to increasing impacts of climate change like extended droughts and above average daily temperatures. 

If you would like contribute towards reducing the Urban Heat Island Effect in Colombo, you can join ‘Greening Colombo’ – an initiative led by the Federation of Environmental Organizations (FEO).  For more information visit 


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Author: Mandi Daluwatte
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