What exactly is "Urban Heat Island (UHI) Effect"
Lorraine Njeri
Climate-Resilience | Environment | Energy | Sustainable Urban-Development across Secondary Sub-Saharan African Cities
Understanding Urban Heat Islands: How Cities Can Beat the Heat
Have you ever noticed how much hotter it feels in the city compared to the countryside, especially on a sunny day? And how the city parks are cooler than the CBD/ built up sections of the city?
Observations indicate that urban centers can be up to 12°C warmer than neighboring rural regions (Voogt &Oke 2003).
That's the Urban Heat Island effect at work, and it's not just a minor inconvenience—it's a significant environmental challenge; with implications for urban planning, energy consumption and public health; that cities around the world are grappling with.
In this article, we will go over:
What is UHI?
Urban Heat Island (UHI) is defined as the relative warmth of a city compared to the cooler surrounding rural areas (Heisler & Brazel, 2010).
Cities demand extensive natural resources, including energy, food, water, and raw materials, contributing to up to 80% of greenhouse gas (GHG) emissions and exacerbating global warming (Cetin et.al, 2019))
Where the key contributors to the UHI effect include heat released from vehicles, air conditioners, power plants, industries, the replacement of natural vegetation with concrete surfaces, and the configuration of tall, high-density buildings.
UHI effects are particularly noticeable at night when stored heat is released, increasing the temperature differential between urban and rural areas.
How is UHI Measured?
Traditionally, urban heat islands were measured by taking the difference in temperature between the city center and surrounding rural areas as measured by ground-based temperature monitors
However, Thermal remote sensing via satellite, aircraft or ground-based sensors provides a more comprehensive view of urban heat patterns (Shi et al., 2021)
In Nigeria, micro-climatic studies primarily use ambient temperature measurements with vehicle-mounted sensors due to limited resources (Chibuke, 2018).
UHI occurs on the surface and in the atmosphere. Therefore, when measuring the UHI, they can be measured at the surface or through air temperature. The Land Surface Temperature (LST) maps can be used as they measure the radiance at the top of the atmosphere in the thermal infrared, which is the temperature measured at the Earth’s surface and is regarded as its skin temperature.
Below is an example of a thermal image of a city using a thermal imagery camera, where the buildings and pavements have the highest temperatures in comparisons to the greenery.
What are the effects of UHI?
Extreme heat has adverse impacts on health, economy, leisure activities, and overall well-being. Vulnerable populations, such as the elderly and poor, are particularly affected by thermal stress (Hajat et al., 2007). Urban warming can also worsen air quality, increasing surface ozone concentrations and health risks.
UHI raises the demand for air conditioning, leading to higher energy consumption and associated emissions. With each 1°C increase in temperature, there’s an energy demand rise by 0.5 percent to 5 percent, depending on the local level of air conditioning penetration (Santampuris et al., 2015)
For instance, a study by (Shen, Chow, & Darkwa ,2013) found that cooling demand in office buildings in Hangzhou, China, increased by 10.8% with a 0.5°C rise in temperature.
2. Decreased Air Quality
Increasing energy consumption, results in further greenhouse gas emissions and more air pollutants during energy production, which lowers air quality in cities and contributes towards smog. Smog is a ground level ozone, that then increases urban temperatures by acting as a thermal layer(Geddes, Jeffrey & Jennifer , 2012
3. Health Implications
Heat-related illnesses such as heat stroke and respiratory problems are more prevalent in urban areas during heatwaves. The elderly, children, and those with pre-existing conditions are at higher risk (Lungman et al., 2023).
Chicago is one example of how extreme heat events can seriously affect human health, where during the summer of 1995, there were 485 heat-related deaths and 739 excess deaths
4. Decreased Water Quality
When the water comes in contacted with these super-heated surfaces as surface run off the water temperature increases. A slight increase in water temperature can have detrimental effects on the aquatic ecosystems. Increased water temperature can reduce the dissolve oxygen concentration causing harmful effects on the metabolisms and respiratory systems of aquatic life (Chapra, Luis & Graham, 2021)
5. Reduced longevity of grey infrastructures
Both of these have suitable temperature ranges to function effectively. One example of potential grey infrastructure damage is the pavement degradation of roads, which tends to accelerate when the temperature rises beyond the suitable range of the selected pavement materials
What are the mitigation measures? [With an emphasis on urban parks]
To minimize UHI impacts ,sustainable urban development is crucial , as outlined in the UN’s New Urban Agenda.
Effective measures include:
These interventions can be categorized as:
How Urban Parks help alleviate the UHI effect?
Urban heat islands are created when the natural landscape is removed and replaced with impervious surfaces. By reintroducing vegetation into the cityscape can help lower the surface and air temperature by providing shade and through evapotranspiration (Knowles,2020). Green vegetation improves air quality, serves as biodiversity hotspots (Cornelis & Hermy, 2004).
Unlike impervious surfaces, vegetation has low thermal storage and admittance, emitting less thermal radiation to the environment. Vegetation with highly reflective sur-faces (high albedo) may reduce surface temperature by reducing the amount and intensity of thermal radiation which may also lower local and downwind ambient air temperatures because of smaller convective heat fluxes from cooler surfaces.
Conclusively Parks;
** Case Study: Abuja Recreational Park (Chibuke, 2018)
A study of Abuja Recreational Park with an aim to assess green parks cooling effect on Abuja urban microclimate using geospatial techniques revealed that it was 2.04°C cooler than its surrounding area at a 350m buffer zone.
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The mean Land Surface Temperatures (LSTs) inside various parks (Millennium Park, Abuja Recreational Park, and Zone 6 Neighborhood Park) were found to be 27.87°C, 29.25°C, and 30.66°C, respectively.
The study indicated that urban park size and perimeter had a positive relationship with Park Cooing Intensity (PCI) up to a 300m buffer, suggesting that larger parks enhance PCI intensity and mitigate UHI effects.
These findings highlight the importance of park size and shape in mitigating UHI effects, providing valuable insights for urban planners and designers.
The following thermal image of an urban park in Sydney, shows how much cooler the water fountain and greenery are in comparison to the buildings.
Conclusion
Understanding the cooling effects of urban green spaces is crucial in mitigating UHI effects. Urban planners can optimize park size, shape, and vegetation to maximize cooling benefits.
Integrating green infrastructure like street trees, private gardens, and green roofs alongside sustainable design principles such as narrower streets and strategic tree planting can enhance airflow and provide shade. These improvements contribute to better urban thermal environments and enhance the overall quality of life for city residents, creating healthier and more livable urban spaces.
Cities are dynamic laboratories for sustainable innovation. Just as Singapore has led in green initiatives, African cities can also pioneer solutions suited to their unique contexts. By sharing knowledge and adopting effective strategies, cities worldwide can combat UHI and build resilient, sustainable urban environments.
Next time you step out into the city on a scorching day, think about how simple changes—like planting more trees or using cooler materials—can make a world of difference.
With an understanding of the complexities of Nature based solutions and green infrastructures, we must acknowledge the challenges and controversies over these mitigation measures:
Regardless, the bottom line is for us to reduce the concrete surfaces by at least planting and taking care of trees. It’s not that simple, but it is that is simple. 😊
-THE END-
Parting Shot
I have attached resource links, that I found rather insightful and would highly recommend.
This was quite fun putting together and you better enjoy it as much I did, putting it together 😉
And if you have opportunities for collaboration, feel free to reach out, I’d love to expand my experience UHI modelling, computational analysis and cities’ case-study research for think tanks.
Resource Links:
1. My UHI introductory LinkedIn Post:
2. 3 min You Tube Video: Sky News- Heatwave: What is the urban heat island effect?
3. Article: Urban Heat Islands 101 [very easy , infographic detailed read]
4. Article: 5 Barriers That Hinder Green Financing
5. YouTube Video: How Singapore uses science to stay cool
6. PDF Document: How Researchers Measure Urban Heat Islands
7. PDF Document: How Cities Use Parks for Climate Change Management
8. Article: Urban Heat Island Effect- Principles of Sustainability
9. PDF Doc: Projecting global urban land expansion and heat island intensification through 2050
References:
Cetin, M., Adiguzel, F., Gungor, S., Kaya, E., & Sancar, M. C. (2019). Evaluation of thermal climatic region areas in terms of building density in urban management and planning for Burdur, Turkey. Air Quality, Atmosphere & Health, 12, 1103-1112.
Chapra, S. C., Camacho, L. A., & McBride, G. B. (2021). Impact of global warming on dissolved oxygen and BOD assimilative capacity of the world’s rivers: modeling analysis. Water, 13(17), 2408.
Chibuike, E. M., Ibukun, A. O., Abbas, A., & Kunda, J. J. (2018). Assessment of green parks cooling effect on Abuja urban microclimate using geospatial techniques. Remote Sensing Applications: Society and Environment, 11, 11-21.
Cornelis, J., & Hermy, M. (2004). Biodiversity relationships in urban and suburbanparks in Flanders. Landscape and Urban Planning, 69, 385–401
Geddes, J. A., & Murphy, J. G. (2012). The science of smog: a chemical understanding of ground level ozone and fine particulate matter. In Metropolitan sustainability (pp. 205-230). Woodhead Publishing.
Hajat, S., Kovats, R. S., & Lachowycz, K. (2007). Heat-related and cold-related deathsin England and Wales: Who is at risk? Occupational and Environmental Medicine,64, 93–100
Heisler, G. M., & Brazel, A. J. (2010). The urban physical environment: Temperature and urban heat islands. Urban ecosystem ecology, 55, 29-56.
Santamouris, M., Cartalis, C., Synnefa, A., & Kolokotsa, D. (2015). On the impact of urban heat island and global warming on the power demand and electricity consumption of buildings—A review. Energy and Buildings, 98, 119–124. doi:10.1016/j.enbuild.2014.09.05
Knowles, C. L. (2020). Planning Strategies for Improving Resilience of Cities in Developing Countries to the Urban Heat Island.
Iungman, T., Cirach, M., Marando, F., Barboza, E. P., Khomenko, S., Masselot, P., ... & Nieuwenhuijsen, M. (2023). Cooling cities through urban green infrastructure: a health impact assessment of European cities. The Lancet, 401(10376), 577-589.
Shi, H., Xian, G., Auch, R., Gallo, K., & Zhou, Q. (2021). Urban heat island and its regional impacts using remotely sensed thermal data—a review of recent developments and methodology. Land, 10(8), 867. (Shen, Chow, & Darkwa ,2013
Voogt, J. A., & Oke, T. R. (2003). Thermal remote sensing of urban climates. Remote sensing of environment, 86(3), 370-384
Visionary technologist and lateral thinker driving market value in regulated, complex ecosystems. Open to leadership roles.
1moLorraine Njeri -- Great work on the assembly of research and organization of your article! I appreciate the details. Had you encountered any sources for statistics on the extent of efforts to alleviate these issues? I am seeking to understand the aggregate scope of investments by major cities in North America to reduce UHI effects through the remediation mechanisms you'd noted.
Clean heat | Green electricity | System integration
2movery helpful.
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3moThank You for this Lorraine, very informative. Been trying to look for local heat maps (Nairobi, Kenya), i haven't come across any, any resources you have come across?