What is an Urban Heat Island?

An urban heat island, or UHI, is a much warmer metropolitan area than the rural areas that surround it. It is a cause of concern because of the additional energy consumption and air pollution that it causes. This proves to be an unsustainable factor leading to excessive use of resources for cooling and puts the urban population at increased risk of morbidity and mortality.

In big cities like Delhi, Chennai and Kolkata, heat is created by energy from all the people, cars, buses, and trains. Areas that have lots of activity and lots of people create the urban heat islands. A recent study by Raj, Paul, Chakraborty, & Kuttippurath 2020, which covered 44 Indian cities with a population of over a million observed that, despite having greenery in surrounding rural or suburban areas, urban built-up areas are taking longer to shed heat.

The World Meteorological Organisation (WMO, 2019) has highlighted that the year 2019 concluded a decade of exceptional global heat and high-impact weather, including the prolonged heatwave in India. The country sizzled as it fought one of the longest heatwaves in three decades.

How Do Heat Islands Form?

Size and Shape of Cities

Cities have a very different shape than rural areas, aerodynamically. Urban heat island effect is created, when houses, shops, and industrial buildings are built closely. Tall buildings and narrow streets heat the trapped air between them and reduce airflow.

Urban Deserts

Cities can be regarded as virtual deserts with scarce vegetation and materials that are almost impermeable to rain. Displacing trees and vegetation minimise the natural cooling effects of shading and evaporation of water from soil and leaves (evapotranspiration) which in turn increases sensible heat. Metropolitan areas tend to become heat islands when infrastructural development (asphalt and concrete for roads, buildings, and other structures) necessary to accommodate increasing populations is pursued at the cost of the green cover.

Cities like Kolkata and Chennai, have a higher vegetation cover in its outlying rural areas, that heightens heat island effect in the daytime but the opposite effect is observed in cities that have less green cover surrounding it e.g. Delhi, where the effect of heat island is eminent at nights. This is because buildings, sidewalks, and parking lots block heat originating from the ground from rising into the cold night sky. The temperature gets warmer, as the heat is trapped on lower levels.

Humidity Effects

While the amount of water that urban areas and its fringes maintain in their atmospheres (absolute humidity) does not vary significantly, higher urban temperatures substantially reduce relative humidity (since warm air can retain more water than cold air).

Urban Canyons

Urban areas are densely populated, meaning more people in a small space. They are also densely constructed, meaning buildings are constructed very close to each other. When there is no more room for an urban expansion, engineers start to build upwards, designing skyscrapers. All this construction creates a “canyon effect” trapping radiant energy in their walls which means waste heat—and heat that escapes insulation has no place to go. It lingers in and between buildings in the urban heat island. This suggests that areas with denser and taller buildings will more rapidly develop heat islands.

Urban Haze

The haze of air pollution which hangs over many cities can act as a miniature greenhouse layer, preventing urban areas from escaping outgoing thermal radiation (heat).

Anthropogenic Heat

The release of heat from the open burning of stubbles can also raise urban temperatures. The burning of crop residues in Punjab, Haryana and Western Uttar Pradesh contributes directly to environmental pollution, and is also responsible for the haze in Delhi and melting of Himalayan glaciers (Yadav, 2019). States practicing open burning of stubbles release large amounts of energy more than the amount of energy that comes into the urban area from the Sun.

Waste Heat

People and their resources, such as cars and factories, are always burning off energy, whether they’re jogging, driving, or just living their daily lives. The energy that people burn off usually escapes in the form of heat. And if one place has a lot of people, that’s a lot of heat.

Rolling Blackouts

When it’s really hot, many of us run straight to the fan or the air conditioning. This is especially true in metropolitan areas which are suffering from urban heat island effects. UHIs contribute to energy demands in the summer, thus straining energy resources. UHIs often experience “rolling blackouts,” or power outages. Utility companies start rolling blackouts when they don’t have enough energy to satisfy the demands of their customers. The energy used in electric fans and air conditioning ends up contributing to an even hotter UHI.

Construction Materials

Construction materials are usually very good at insulating, or holding in heat. This insulation makes the building surroundings warmer. Building materials and surfaces such as asphalt absorb the suns heat. This leads to a rise in surface temperatures and overall temperatures.

Impacts of Urban Heat Islands

Poor Air & Water Quality

Urban heat islands can have worse air and water quality than their rural neighbours. This is caused due to the toxins (waste products from vehicles, industry, and people) being pumped into the air. The urban landscape which includes houses, highways, sidewalks and parking lots, blocks these pollutants from scattering and becoming less toxic. Water quality also suffers. Warm water stresses the native species that have adapted to life in a cooler aquatic environment, when it ends up flowing into local streams from the UHI.

Health Effects

Heat islands contribute to higher daytime temperatures, reduced night time cooling, and higher air-pollution levels. These, in turn, contribute to heat-related illnesses such as general discomfort, dehydration, heat syncope, respiratory difficulties, heat cramps, heat exhaustion, non-fatal heat stroke and even sometimes death.

Climate Change

Since the heat islands in urban spaces mainly depend on the surface temperature in those spaces, the effect of heat islands in regions with higher temperatures would be the strongest. As the average global temperature goes up (global warming) the heat island effect is also expected to increase. This is a critical concern in the context of climate change. Scientists are exploring how urban heat islands could lead to global warming. The most recent trend of climate change which involves the gradual warming of the Earth’s temperature (Rutledge, et al., 2011).

Because of these negative effects, scientists say city dwellers, architects, and planners all have to work to reduce people’s impact on urban areas.

Mitigating Strategies

  • Selection of building materials is important in this context. Changes in urban surfaces (impervious surface or changes in land use) modify the temperature of a city.  Infrastructure can considerably alter the temperature of a region/city. Much research has been done on technologies for mitigating urban heat island effects by making urban surfaces cooler by increasing their albedos.
  • Using green roofs or eco-roofs, which are roofs of buildings covered in plants, helps cool things down. Plants absorb carbon dioxide, a leading pollutant. They also reduce the heat of the surrounding areas. Using lighter-coloured materials on buildings helps, too as it reflects more sunlight and trap less heat.
  • Increasing tree and vegetation cover lowers surface and air temperatures by providing shade and cooling through evapotranspiration. Trees and vegetation can also minimise storm water runoff and protect against erosion.
  • Integration of new mobility concepts such as car sharing, car-pooling, bike stations and cycling lanes, and an increased share of electric mobility will help in mitigating urban heat island effect and carbon dioxide emissions.

It is important to keep the cities cool. If city planning does not account for the management of its green cover, pollution, water sources and wetlands, local weather and environment (temperature and precipitation), socio-economic factors (e.g. population), and urban infrastructure (e.g. green buildings and eco-friendly building materials), the surface heat island effect will be intensified.

Author:

Snigdha Dev Roy
Member of NOSPlan
School of Planning and Architecture, Delhi


References

  • Ghosh, S. (2020, January 28). Cities taking longer to shed heat despite surrounding greenery: Study. Retrieved from Mongabat: https://india.mongabay.com/2020/01/cities-taking-longer-to-shed-heat-despite-surrounding-greenery-study/
  • Pomerantz, M. (2018). Are cooler surfaces a cost-effect mitigation of urban heat islands? ELSEVIER, Volume 24, pp. 393-397.
  • Raj, S., Paul, S. K., Chakraborty, A., & Kuttippurath, J. (2020). Anthropogenic forcing exacerbating the urban heat islands in India. Journal of Environmental Management.
  • Rutledge, K., Ramroop, T., Boudreau, D., McDaniel, M., Teng, S., Sprout, E., . . . Hunt, J. (2011, January 21). Urban heat island. Retrieved from National Geographic: https://www.nationalgeographic.org/encyclopedia/urban-heat-island/#:~:text=Powered%20by-,An%20urban%20heat%20island%2C%20or%20UHI%2C%20is%20a%20metropolitan%20area,York%2C%20Paris%2C%20and%20London.
  • WMO. (2019, December 3). 2019 concludes a decade of exceptional global heat and high-impact weather. Retrieved from World Meteorological Organization: https://public.wmo.int/en/media/press-release/2019-concludes-decade-of-exceptional-global-heat-and-high-impact-weather
  • Yadav, R. S. (2019, June 4). Stubble burning: A problem for the environment, agriculture and humans. Retrieved from Down To Earth: https://www.downtoearth.org.in/blog/agriculture/stubble-burning-a-problem-for-the-environment-agriculture-and-humans-64912