Here, we review the ‘’Urban Heat Island’’ effect, the processes that drive it, way in which it may be characterized, review studies in Sri Lanka and suggest mitigation/ adaptation options.

Have you noticed?

If you are a person living in the city or the suburbs, have you ever felt that the air feels so much cooler when you move to the outskirts of your town or city? You might think this is due to global warming. In fact, global warming does increase the ambient temperature but the consequences of this are not localized just to the urbanized regions. There is another reason as to why urban areas are becoming warmer. Here’s to why that is.

Urban Heat Islands (UHI)

An urban heat island is a micro-climatic condition created as a result of elevated temperatures due to the abundancy of surfaces that absorb and retain temperature. Heat trapped in between tall buildings, unable to escape causes distress to humans. Residents of urban and suburban areas are exposed to this phenomenon rather than people living in rural areas. According to research the annual average temperature of an urban residential tends to be 10C to 30C more than their neighbouring rural areas. Likewise, on a clear calm night this difference can be as much as 120C. Hence, by studying warming trends we can determine where and when a UHI is created.

UHI Processes

Urban heat islands occur as a result of surfaces absorbing and retaining heat. This effect in the long run can cause heat related mortalities in people and increase levels of air pollution in cities. Following is a list of few factors that increase the temperature in cities:

• Building materials – concrete, tar
• Lack of vegetation & shade
• Lack of proper ventilation & cooling devices
• Heat released by traffic & heat waste
• Urban geometry
• Pavements
• Urban growth
• Global Warming

Trees on the other hand are capable of reducing this effect due to a process called ‘Transpiration’ whereby a plant releases water vapour making the air surrounding it feel cooler. This is the reason that even in the city you feel a lot less warm when you sit under a tree or walk beneath a tree-lined street.

With increasing global warming, and more unfavorably built environment, people living in urbanized areas will be more at risk of experiencing elevated temperatures and its consequences such as the heat island effect. Therefore, people living in these areas should get to know of the risks so they are motivated and able to mitigate them.

Other Causes of Warming – Environmental Change and Global Warming 

Role of Environmental Change and the Built Environment: The loss in green areas around cities, the construction of ever taller buildings, the loss of wetlands, the construction of more paved roads, land use change such as housing on past coconut and rice lands are already causing the environment to be uncomfortably warm and humid. In addition, buildings constructed of late do not adequately consider cooling and ventilation as they use materials that do not help and are on shrinking parcels of land. 

Role of Global Warming:  According to a review done by FECT on global warming related climate change, the temperature in Sri Lanka shows a 2.60C per 100 years warming trend for maximum temperature and a 1.70C per 100 years warming trend for minimum temperature from 1960 – 1990. In fact, in Colombo district the mean annual average temperature has gone from around 270C to 28.50C in the last century. Moreover, districts like Anuradhapura, Galle and Badulla are showing higher warming levels. 

Thus the warming trend, adds to the impact of the UHI that we describe. The built environment in the cities are not equipped to deal with these impinges. We presume that urbanization could account for some of these temperature changes in addition to global warming.

Impacts of the Warming 

Human bodies operate effectively at 370C ± 10C. Increased levels of heat can cause the body’s heat regulating mechanisms to break down resulting in dire circumstances. Thus, leading to discomfort and heat related illnesses like heat rash, heat cramps and heat strokes. Moreover, marginalized communities living in the city will be more impacted economically due to having to invest on cooling devices which have typically high maintenance and energy consumption values. Also, high heat conditions facilitate epidemic and invasive species outbreaks. Moreover, the use of cooling devices causes anthropogenic heat to be released into the atmosphere, adding to the UHI effect. 

UHIs in Sri Lanka 

Temperature in Sri Lanka does not undergo a wide range of changes annually. Therefore, an elevation in the temperature beyond the ambient conditions may cause distress to Sri Lankans. Research carried out in Pettah-Colombo shows that the heat radiation energy measurements in the day time can go up to 560C and at night up to 350C in some places. 

Short Term Adaptation and Mitigation

Residents of urban and suburban areas can follow some of the methods given below in order to reduce the impacts of urban heat islands.

• Planting trees
• Adapting green roofs – keeps indoors cool by transpiration and by providing shade
• Adapting white roofs – keeps indoors cool by reflecting most of sun’s radiation away 

These can help provide shade, deflect radiation from the sun and release moisture in to the atmosphere thereby reducing heat levels.

References

Emmanuel, R., & Fernando, H. J. (2007). Urban heat islands in humid and arid climates: role of urban form and thermal properties in Colombo, Sri Lanka and Phoenix, USA. Climate Research, 34(3), 241-251.

Johansson, E. and Emmanuel, R., 2006. The influence of urban design on outdoor thermal comfort in the hot, humid city of Colombo, Sri Lanka. International journal of biometeorology, 51(2), pp.119-133.

Lareef Zubair, Tuan Hadgie, Ashara Nijamdeen, Nipuni Alahakoon, Kokila Wikramasinghe, Charuni Kulasinghe, Azra Munas, Ushan Adithya, Sarasi Pamunuwa, SanduniGammanpila, Zain Iwais, Zeeba Javard, 2021. Change: The Global, Regional and Sri Lankan Evidence. BHC Project Report, Federation for Environment, Climate and Technology, Sri Lanka. 

Lareef Zubair et al., Climate Change Impacts and Adaptation in Sri Lanka: Impacts on the Plantations, IRI/FECT Technical Report, 2007