Explainer| What are urban heat islands?

Why are urban heat islands formed?

Urban heat islands are formed due to many characteristics of cities. First, the loss of tree cover in cities allows much less cooling of the area through evapotranspiration – a combination of evaporation (movement of water to air from surfaces such as soil, water bodies, etc.) and transpiration (movement of water through plant roots into the air via tiny pores in leaves called stomata).

The second reason is due to geometric effects – buildings, especially tall ones, provide multiple surfaces that reflect and absorb heat from sunlight. In addition, multiple tall buildings in close proximity to each other also act as barriers to air flow and wind, which block cooling by convection. These two factors together lead to the urban canyon or street canyon effect.

Third, the dark surfaces and thermal bulk properties of the most abundant materials in cities – asphalt (roads) and concrete (most buildings) – makes them absorb much more heat than surrounding rural areas. Differences in night-time temperatures between UHIs and surrounding rural areas are usually more than differences in daytime temperatures.

This is because concrete, which is used widely in cities has a very high heat capacity and acts as a reservoir of heat. In addition, atmospheric conditions above cities often lead to urban air being trapped near the ground surface, where it is heated by the warm urban surfaces. Furthermore, the urban canyon effect blocks the loss of heat from ground surfaces through radiation.

The fourth reason is due to anthropogenic heat generation and air pollution. Anthropogenic heat is produced by vehicles and buildings (through electric devices like fans, computers, refrigerators, and air conditioners), although this is considered to be almost negligible in commercial and residential areas as compared to the other three factors.

The high levels of air pollutants in cities, however, especially carbon dioxide and ozone, both of which are greenhouse gases, are thought to contribute significantly to the UHI effect.

How do urban heat islands affect life in cities?

A 2012 analysis of 419 cities worldwide shows that UHI causes, on average, increases of 1.5 °C and 1.1 °C in day-time and night-time temperatures, respectively. The UHI effect has been shown to be dependent on population size, vegetation cover, and climate.

Studies in Europe and North America have shown that UHIs affect rainfall patterns (by increasing rainfall downwind of cities) and the formation of clouds and fog. In addition, UHIs may also affect plant growing seasons in temperate regions.

An analysis published in 2020, of UHI in 32 Indian cities, shows that the increase in city temperatures can range from 2 °C to as high as 9 °C in the daytime.

In another study of 44 Indian cities, the extent of heating was found to be dependent on vegetation cover (higher the vegetation cover, lower the heating effect) and season (heat differences between cities and surrounding rural areas are higher in monsoon and post-monsoon seasons).

However, a negative UHI effect (also called an ‘urban cool island’) where surrounding rural areas had higher temperatures than the city has been observed in several Indian cities.

“Now, this does not mean that cities are effectively becoming cooler. Instead, something is happening either on the surface or in the atmosphere in the rural areas to temporarily warm rural areas and cool urban areas,” says V. Vinoj, a professor at the School of Earth, Ocean and Climate Science in IIT-Bhubaneswar.

A recent article from his research group shows a weakening of daytime UHI effects in Bhubaneswar in Odisha in the past two decades. “We think this could be a consequence of multiple reasons such as increasing particulate air pollution levels over the city region that reduces surface-reaching solar radiation thereby cooling urban areas,” he adds.

Another possibility is the loss of vegetation in surrounding rural areas, especially during the hot, dry summer, which makes surrounding rural areas warmer, making cities appear to be cooling on an extended time scale. But these are mere speculations and need to be verified through detailed research.

Additionally, the changing characteristics of the UHIs themselves may modify the local weather. When the impacts of climate change and global warming are added to the equation, the UHI effects become highly dynamic, and understanding them will be essential for sustainable urban planning and expansion.

One of the main effects of UHIs in cities is on the health and welfare of city residents. Every summer, most major Indian cities experience heat waves that leave residents, especially those in poor neighborhoods suffering from heat strokes, heat exhaustion, heat syncope (fainting) and even death.

In addition, increased temperatures can exacerbate health issues in people who are overweight, and suffer from conditions such as diabetes and insomnia, or have cardiovascular problems. The intensity of UHI effects can also create a positive feedback cycle on accumulation of air pollution – floating soot, nitrogen oxides, carbon monoxide, etc. contribute to the formation of ozone, which in turn, intensifies heating over urban areas.

Another major consequence of UHIs is the need for increased refrigeration and air conditioning in cities with hot climates, while in cities with cold climates, UHIs help in decreasing the demand for heating.

“The UHI effect, when combined with global warming and increased incidences of heatwaves, will have a significant impact on life in cities like Bhubaneswar, which lie in the regions generally affected by heatwaves in India.

Thus, there’s a possibility of a cascading series of events. As temperatures rise, the use of refrigeration devices and air conditioners increases, which increases electricity demand, putting an increased load on distribution grids that could fail. This would lead to power cuts, further exasperating urban dwellers”, adds Vinoj.

In addition, UHIs affect water quality within cities and surrounding areas. Due to the UHI effect, water from storm drains in cities may be several degrees warmer than surrounding water bodies. The warmer runoff from city drains and sewers could increase temperatures in nearby lakes, ponds or streams and affect aquatic biodiversity.

Finally, UHIs also affect animals living in urban areas. Studies on ants, lizards, spiders, have shown that UHI can affect heat tolerance and reproduction in several species.

How can urban heat islands be mitigated?

There are several approaches to mitigate the formation of UHIs. These involve a range of strategies that use vegetation and building materials with low absorbance/reflectance, coupled with reducing anthropogenic heat generation to reduce the amount of heat present in the cityscape.

One set of strategies, which focus on roofs, promote the use of coatings with high reflectance and emittance (ability to emit absorbed heat quickly) to reduce heat absorption in buildings.

Another roof-based system to mitigate UHI is ‘green roofs’, which uses a coated, waterproof membrane-covered roof with a layer of soil and growing plants. This system uses evapotranspiration to reduce heating in city buildings. However, these strategies are yet to be widely used in India – currently, there are mathematical studies and ongoing experiments to test their effectiveness.

Non-roof-based strategies focus on increasing vegetation cover in cities by planting trees in open areas (especially places such as parking lots which are usually asphalted) and promoting ‘green walls’ (vertical plant growth systems). Other ideas involve increasing numbers of water bodies, and the use of ‘climate-responsive’ architecture.

Climate responsive architecture focuses on using specific materials and ventilation techniques that promote passive cooling such as the Tube house in Ahmedabad.

This article was first published on Mongabay here