What is the Urban Heat Island (UHI) effect?

What is UHI?

Urban Heat Island is a phenomenon that causes air temperatures in man-made urban areas to be significantly higher than the temperatures of the surrounding rural areas. This effect was first noticed in the mid-20th century and, until the 1980s, a warmer temperature in cities was considered a potential benefit as it reduced the need for heating.

However, growing populations and changing building usage have resulted in an increase in heat being emitted from electrical appliances, while demands for indoor cooling have risen too. Ironically, cooling buildings with air conditioners further increases outdoor air temperatures and adds to the problem.

What causes UHI?

Our cities differ from natural landscapes in terms of their form, size and density, the construction materials used and the overall geometry of the buildings and streets.

  • Reduced air flows – The shape and the positioning of buildings in cities slows down the movement of air near the ground. It creates complex patterns of shade and sunlight which results in limited natural energy exchanges.
  • Reduced green and blue spaces – Since much of the urban landscape is paved and lacks vegetation, there is usually little water available for evaporation as it is diverted into drainage due to widespread water-resistant surfaces.
  • Reduced albedo (solar reflectivity) effect – Large asphalt surfaces that are dense and often dark-coloured are very good at storing and absorbing solar radiation with only five per cent of it being reflected. This increases surrounding air temperatures.
  • Increased energy consumption – Urbanisation is associated with the emission of waste heat from buildings, transport and industry, directly contributing to the UHI effect.

Different types of UHI

There are different types of UHI depending on what level (surface level/below roof height/above roof height) the temperature is measured at. It generally increases from the urban outskirts towards the city centre and usually contains micro-climates around parks and allotments, and rivers and lakes, which create cooler climates.

Different measures will be suitable when mitigating at various levels.

  • Surface UHI – This refers to warmer temperatures at the Earth’s surface. It is typically measured using satellites so that temperatures of roads and roofs can be measured. The highest levels are during the daytime when hard urban surfaces receive solar radiation and warm quickly.
  • Air temperature UHI – This type refers to temperatures close to the ground, in cities this means below roof height. The highest levels are usually at night as street surfaces and the adjacent air cool slowly.
  • Above the roof level, the temperature of roads and roofs combined also affects the overlying urban atmosphere, and in some cases, warming can be detected up to one and two kilometres above the surface.

The effects of UHI on our health and well-being

The increased temperatures and higher air pollution can impair our health. According to the United States’ Environmental Protection Agency, the problems incurred by UHI can lead to respiratory difficulties, heat exhaustion, heat stroke and heat related mortality in sensitive populations.

Heatwaves have the power to kill. For example, during the 2003 heatwave in Europe, 70,000 additional deaths were reported, making it one of the region’s deadliest natural disasters of the past 100 years. Currently, about half of the world population lives in urban areas but it is estimated that by 2050 this will be closer to two-thirds.

The potential medical impact is perhaps the most significant issue related to UHI, especially against the backdrop of continued climate change and global warning. For all these reasons, it is crucial to understand how UHI works so that effective ways can be found to mitigate and adapt to its effects.

What can we do to mitigate the problems?

The UHI is an inevitable outcome of the landscape changes that accompany urbanisation. Its magnitude and impact can be managed by modifying some physical aspects of the cities. Urban design should aim to:

  • Increase air flows – Design urban layout that allows for better ventilation through the streets and buildings.
  • Increase green and blue spaces – Increase vegetative cover and reduce impermeable cover.
  • Increase the albedo (solar reflectivity) effect – Use lighter colour materials that reflect solar radiation.
  • Decrease energy consumption – Helping to mitigate urban heat gains by using effective solar shading will reduce the need for artificial cooling.

These solutions need to be tailored to the type of UHI. For example, a focus on building greener, cooler and reflective roofs will have an impact on the overlaying air and the top floor of buildings but may have little impact on the UHI at street level.

Similarly, trees may be an effective means of providing street shade, but if a canopy encloses the street, it can trap traffic emissions, resulting in poor air quality.

Cities can complete UHI studies which will identify hot spots where design interventions could have the greatest effect.

Holistic approach which addresses inter-related environmental issues such as flooding and air quality, as well as surface and air-temperature is needed.

Interesting facts and figures

  • Summer temperatures in urban areas are on average 8°C warmer than their surroundings (4).
  • At night, urban temperatures can be as much as 12°C higher (3).
  • These higher temperatures can double the air conditioning load of buildings and triple peak electricity demand (2).
  • In America, 5-10% of community-wide electricity demand is used to compensate for the higher temperatures caused by the urban heat island effect (3).
  • Urban heat islands raise demand for electrical energy in the summer, which intensifies greenhouse gas emissions and air pollution (3) and it contributes to global warning.
  • Higher temperatures caused by heat islands facilitate chemical reactions that turn atmospheric compounds into smog (1) further adding to urban air pollution.
  • The increased temperatures and higher air pollution associated with urban heat islands can lead to respiratory difficulties, heat exhaustion, heat stroke, and heat-related mortality in sensitive populations (3).

 Sources:

  1. 1. Wang, Yupeng, Umberto Beradic, and Hashem Akbarib. “Comparing the effects of urban heat island mitigation strategies for Toronto, Canada.” Energy and Buildings (2015). Web. 21 Aug. 2015.
  2. 2. Santamouris, M. “Cooling the cities – A review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments.” Solar Energy 103 (2012): 682-703. Web. 21 Aug. 2015.
  3. 3. “Heat Island Effect.” United States Environmental Protection Agency. N.p., n.d. Web. 21 Aug. 2015.
  4. 4. Mills, G. “Why are cities so much hotter than surrounding areas?” Web. 25 Jan. 2016
  5. 5. Stephens, A. “Roofing Surface & The Urban Heat Island Effect.” Web. 2015.
  6. 6. “Urban Heat Islands”. Met Office. Web. 2012

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