Urban heat island effect

13 min read


The effects that urban centers have on public health, the economy, and social interactions are documented. Urban centers have historically been crucial economic drivers and indicators of prosperity. For instance, China’s rise as an economic power has closely corresponded with accelerated urbanization. This pattern has been seen elsewhere. Countries like Bangladesh, Ethiopia, and India have seen rapid urbanization fueled by, and leading to, strong economic performance.

It is not only the economy that benefits, or drives, urbanization. Social interactions are stronger when people are closer together. In such close quarters, their circumstances are likely to be similar, fostering greater participation in social matters. It is no accident then, that cities have historically served as hotbeds for political activity.

Despite these advantages, several issues bedevil cities, sometimes in ways that endanger lives and dilute the social and economic opportunities they bring. Urban centers can easily become a breeding ground for disease, social segregation, and deprivation of access to economic opportunities.

The built-up areas of a city also reemit the heat they receive from the sun during the day. This heat combines with anthropogenic heat (heat generated from human activity, mainly through fossil fuel consumption) to build up a phenomenon known as an urban heat island. The island resembles a dome that envelopes an urban center. This makes it significantly hotter than the surrounding rural areas, and is the focus of this article.

Treeless cities increase the intensity of urban heat islands.

What is Urban Heat Island (UHI)?

An urban heat island effect refers to when an urban center becomes significantly hotter than the surrounding countryside. [1] Structures associated with urban centers such as houses, roads, and pavements reemit more heat into the atmosphere than natural vegetation and water bodies.

The result is that the temperature in urban centers, especially during warmer months, is as high as 3°C more than in the corresponding countryside. In many cases, structures absorb heat during the day, which they reemit during the night. In the process, the urban heat island effect is more pronounced at night.  

More and more land is coming under urban development, often at the expense of natural vegetation and cropland. The amount of heat that is re-emitted into the lower atmosphere keeps rising. This is especially so in instances where development is concentrated, that is, urban centers. As climate change makes the globe warmer, urban centers will feel the temperature rise more acutely. [2]

What causes an urban heat island?

In many modern cities, surfaces are almost always impermeable to water. Instead of stormwater being easily absorbed into the ground, it is directed to stormwater and sewerage systems. [3] Cities fail to benefit from the effects of evapotranspiration in the event of a storm. Water bodies are usually situated a safe distance out of town, such that even if the water does eventually get there, it cannot absorb excess heat in urban centers.

The size of an urban center and the nature of its structures affect the intensity and incidence of an urban heat island. Additionally, the nature of vegetation within the urban center also helps determine the intensity. For instance, a “concrete jungle” with no trees or grass within the built-up area is likely to have more severe temperature variation than a city that has more trees and other vegetation.

Color and materials used

Today’s superhighways, motorways, and all other types of roads are predominantly black. They are also likely to be made of asphalt. This combination makes the surfaces absorb more heat, making the places with a greater concentration of such surfaces warmer. This is especially so during the night, with variations between urban centers and surrounding countryside varying by as much as 9°C.[4]

San Francisco’s closeness to the sea significantly helps keep the temperatures down by absorbing heat.

Urban canopies mean that for meters on end, there is no break for say, soil, water, or any sort of vegetation. The collective heat stored is enormous, even considering that in many instances, trees line up the street. This is especially so with high-rise buildings, which either absorb the heat and trap it or reflect it on shorter buildings, with the same effect.

Effects of the urban heat island effect

The urban heat effect has significant effects on the environment and public health. It also affects some aspects of economic interactions and social well-being. For this article, we will discuss the effects of the urban heat island effect on air pollution, health, and the economy.

Environmental impact of the urban heat Island effect

The environmental effects of an urban heat island are mainly confined to how it interacts with already existing air pollution. Due to the dependence of most cities today on vehicles for transport, the dependence on air conditioning, and other activities, cities are highly polluted. The heat is a consequence of, as well as a cause of, worsening pollution.

Air conditioning and the urban heat islands

Heat islands directly affect air quality in the places they occur. [5] This happens in two ways. First, high temperatures usually mean less wind flows. This concentrates air pollution that would otherwise have been dispersed. Sunlight accompanied by high temperatures leads to more chemical reactions between the air and pollutants. The result is more pollution, but also, more dangerous elements in the air that we breathe. In other words, an urban heat island exacerbates conditions and reactions that are already present with normal weather conditions.

Secondly, heat islands mean that offices and cars have to use air conditioning more extensively. The emissions that using air conditioning occasions means that significantly more emissions are pumped into the atmosphere than would otherwise be the case. This is not all, however. Air conditioners use hydrofluorocarbons as a refrigerant. Older systems may occasion small leakages of this gas, which has the capacity to, per molecule, trap much higher amounts of heat when compared to other greenhouse gases such as carbon dioxide.

Air conditioners work by trapping the heat within cars and buildings, and expelling it outside. The effects of thousands of running cars and buildings using air conditioning systems help to further pollute the air, or, as it is called in some quarters, thermal pollution.  

Ground-level ozone pollution

In a typical urban center, cars, boilers, air conditioners, buildings, households, and industries emit different gases into the air. [6] In the presence of sunlight, these gases react and give birth to new gases, many of which are harmful, for a variety of reasons. The chemical reactions that happen usually result in the formation of smog. The effect is also known as ground-level ozone. The implication here is that ozone is at a level where it should not be. It traps heat, minimizes the dispersion of pollutants through wind, and likely makes cities inhabitable.

Particulate matter pollution

Particulate matter pollution (PMP) refers to the presence in the air of solid and liquid droplets between 2.5 and 10 micrometers in diameter. [7] These particles are emitted directly through mechanical processes such as vehicles and industrial machinery, though they can also be the product of reactions between different gasses emitted into the atmosphere. Alone, particulate matter is a serious air pollutant.

It has been hypothesized that in the cases where the particulate matter is a result of reactions between different gases, whose reaction is facilitated by heat, higher temperatures lead to a higher concentration of particulate matter. Since the particulate matter can absorb or trap more heat than normal atmospheric gases and particles, the phenomenon feeds itself, whereby higher temperatures mean higher temperatures, which in turn mean higher particulate matter pollution, and therefore, higher temperatures.

Acid deposition

Acid rain causes corrosion to metal and concrete.

Sulfur dioxide and nitrogen dioxide are two gases commonly emitted by burning fossil fuels. When these gases come into contact with water, they may turn into their respective acids. More broadly, the gases cause acid deposition, which also includes dust or even snow with acidic particles which form after moisture dissolves the gases mentioned above.

While acid rain is a common occurrence due to the dispersal of sulfur dioxide, nitrogen oxide, and other gases across wide areas, it is particularly worsened by higher temperatures. An urban heat island where the wind is low will have more severe acid rain since it provides the perfect conditions – a high concentration of pollutant gases, higher temperatures, and low dispersal.

Warmer winters

While most of the consequences of an urban heat island are negative, the heat effect has an advantage as well. Recent research has found that urban heat islands could potentially be beneficial during winter. A study carried out in Birmingham, UK, found that on average, the urban heat island effect helps decrease cold-related deaths by 15% during winter. This is because, according to the researchers, winters are milder than they are in the countryside, and since more people live in urban areas, fewer people are exposed to extreme cold.

Impact of the urban heat island effect on public health

Traditionally, exposure to cold weather kills more people than heat. Cold weather is associated with respiratory illnesses that exacerbate underlying conditions, resulting in death. Increasingly, however, more and more people are succumbing to heat, not cold. Globally, 5 million people die every year as a result of exposure to extreme temperatures. The numbers are going up, with one odd characteristic: cold-related deaths are decreasing, while those related to heat are increasing.

Heat-related deaths are most common in Eastern Europe, but it is also a cause of concern in the US. Today, heat is the leading weather-related killer in the country. It manifests in the form of heat stroke, heart attack, and other cardiovascular conditions which are triggered by heat, and result in death.  

Heat claims more lives than any other weather-related deaths.

Besides direct deaths, heat also causes several serious illnesses. Hyperthermia is an abnormally high body temperature, caused by exposure to too much heat. In its mild form, it causes heat cramps. Cramps and spasms usually occur after exercise and sweating in hot conditions. Heat exhaustion is worse, and involves the body’s inability to regulate its temperature after exposure. If untreated it develops into a heat stroke, which can be fatal. The incidence of such conditions is becoming more commonplace as temperatures rise in urban centers, a phenomenon directly attributable to the urban heat island effect.

Air pollution and public health

Besides the direct effects of heat, air pollution is a major public health concern. [8] Various types of cardiovascular and respiratory illnesses and some cancers have been linked directly to air pollution, which is worsened by rising temperatures in urban centers. Particulate matter and ground-level pollution trigger and worsen respiratory illnesses. They increase the risk of developing lung cancer and heart disease while making asthma attacks more severe.

The use of central heating and air conditioning systems is an integral part of modern real estate development. However, sharing air without sufficient purification mechanisms means that airborne diseases can easily be transmitted across a wide range. During the coronavirus pandemic, people, safely in their apartments, contracted the virus due to contact with recycled air.

Heat creates the ideal conditions for microbes to thrive and multiply, especially when there is sufficient moisture. Urban heat islands create these conditions, potentially aiding pathogens to proliferate, and disperse much faster than would otherwise be the case. The result – communicable diseases becoming more prevalent. This is despite current prevention mechanisms. The phenomenon is already causing concern among public health experts, with rising temperatures being seen as a major culprit.

PMP is responsible for various skin ailments seen in urban centers today. It is also a source of oxidative stress. Oxidative stress and the mutations and imbalances it causes in the body have been cited as possible causes of disease. Diabetes, cancer, and depression are steadily on the rise in urban centers.

The economic impact of the urban heat effect

An obvious impact of an urban heat island effect is the increased energy bills facing households and businesses. Phoenix, which is a desert, is also one of the fastest-growing large cities in the world. The architecture of its sprawl creates a heat dome that engulfs the entire city at night, with a difference as high as 12°F in nighttime temperatures. This means that in many instances, air conditioning systems run throughout, occasioning steep energy costs for businesses and households. [9]

The amount of time that people spend on sick off is another economic cost. [10] They also incur costs seeking treatment, a cost that is not shouldered individually but by the community at large. In addition, excess deaths also mean that some careers and economic productivity are cut short prematurely. Though these costs may be difficult to quantify, they are nonetheless significant enough to warrant a rethink of the way our cities are built, and how they are growing.

Mitigating the urban heat island effect

Adding greenery

Green roof, Chicago’s city hall.

Trees are an important agent in curbing the urban heat island. They absorb heat, while cooling off surrounding areas through evapotranspiration. Trees also provide shade, which makes pavements that would otherwise help build up the heat island effect cooler. According to the EPA, researchers suggest planting trees to the west to maximize their effect.

Planting trees does not limit the heat that buildings reemit. To manage this, green roofs have been championed, due to their effect on buildings’ interiors. Working in the same way as trees, green roofs reduce a building’s temperature and the urban heat effect. An experiment of this nature was carried out in Chicago’s City Hall, with encouraging results for researchers. Since then, the building has become much more pleasant to work in and has seen significant energy savings.

Green walls can also be used where applicable to complement the effects of green roofs and trees on the street. Green roofs insulate buildings while absorbing heat and synthesizing it. Singapore is a trendsetter in this respect, with high-rise buildings having a collection of vines scaling their length.

Greenwall, Singapore.

Water bodies

Water usually gains and loses heat at a slower rate when compared to adjacent land and solid surfaces. By evaporating, water can absorb more heat from the surrounding areas, thereby significantly reducing the temperature. Normally, cities situated at the coast or near major rivers, such as Rio de Janeiro, Cairo, Khartoum, and Mumbai benefit from the presence of large water bodies; they could otherwise be uninhabitable.

The trick then is to recreate the water bodies’ effect in places without the natural water bodies. In South Korea, the city of Seoul demolished an elevated expressway and replaced it with an artificial “river”, known as Cheonggyecheon. This had the effect of significantly reducing the city’s temperatures, both through the heat that the water absorbed, and the vegetation that it allowed to sprout along its banks.

Swimming pools and small dams can have a similar effect, albeit on a much-limited scale. [11] This is suggested to be the case in Phoenix, where scientists found that areas with a swimming pool nearby had lower temperatures than those without. To make it really count, however, it would need a large number of such water bodies, close enough to each other, to decisively deal with an urban heat island.

Reducing anthropogenic heat emissions

Human activity is a major contributor to the urban heat island effect. It is therefore obvious that measures to reduce activities that produce heat would have a direct effect on the intensity of the phenomenon. One such way should be to reduce the dependence on cars, and the reduction of low-density settlements that help spread out an urban concrete jungle, in the process taking up the area that was formerly cropland or covered by natural vegetation.

Besides car dependence, more efficiency is needed to reduce the incidence of wasted heat escaping into the atmosphere, such as from industrial activities, and domestic appliances. Additionally, more responsible air conditioner use by offices, households, and vehicles can significantly cut back on the amount of heat emitted into the atmosphere. 

Rethinking urban planning and Design

Modern cities are dominated by asphalt and concrete. These surfaces are another big culprit of the urban heat island, due to their ability to absorb and retain heat. Some cities have tried painting such surfaces in brighter colors to reflect the light into space; while this may work, it also has other issues associated with it, including visibility and cost implications.

To deal decisively with the heat island issue, it is important to rethink how more surfaces can be reclaimed and turned into greenspaces. In other instances, the materials used could be changed, or, as is the case with Barcelona, covered in canopies where possible to prevent too much exposure.

Barcelona’s umbrellas make the streets colorful but also help reduce the heat that reaches the pavement.


It is impossible to discuss the urban heat island effect without mentioning one of its biggest causes – urban sprawl. The transformation of huge tracts of land into a concrete jungle has brought economic and social development. It has also brought health issues, pollution, and now, seemingly, a potential climate-altering phenomenon. It is necessary to point out that while the effect on the global climate is still negligible, due to the relatively small area of land that urban centers occupy, this is changing. Urban sprawl is taking up land at unprecedented rates. More and more people are moving to towns, where they will be exposed to the problems associated with urban sprawl, including the urban heat island.

While the mitigation issues discussed above may significantly help, decisive action is needed. Rolling back the use of one of the biggest heat emitters – vehicles, and redesigning urban centers to more adequately respond to the environmental issues that we currently face will help.

References and further reading

Feng, Zong-Wei, et al. “Effects of ground-level ozone (O3) pollution on the yields of rice and winter wheat in the Yangtze River Delta.” Journal of Environmental Sciences 15.3 (2003): 360-362.

Kim, Kyung Eun, Daeho Cho, and Hyun Jeong Park. “Air pollution and skin diseases: Adverse effects of airborne particulate matter on various skin diseases.” Life sciences 152 (2016): 126-134.

Kleerekoper, Laura, Marjolein Van Esch, and Tadeo Baldiri Salcedo. “How to make a city climate-proof, addressing the urban heat island effect.” Resources, Conservation and Recycling 64 (2012): 30-38.

Macintyre, H., Heaviside, C., Cai, X., and Phalkey, R. “The winter urban heat island: Impacts on cold-related mortality in a highly urbanized European region for present and future climate.” Environment International 154 (2021).

Molina, L., Morales, S., and Carrion, L. “Urban Heat Island Effects in Tropical Climate.” Vortex Dynamics Theories and Applications, edited by Zambri Harun, IntechOpen, 2020. 10.5772/intechopen.91253.

Stewart, Iain D. “A systematic review and scientific critique of methodology in modern urban heat island literature.” International Journal of Climatology 31.2 (2011): 200-217.

Wang, Y., Guo, Z., and Han, J. “The relationship between urban heat island and air pollutants and them with influencing factors in the Yangtze River Delta, China.” Ecological Indicators 129 (2021): 1-10.

  1. The Environmental Protection Authority (EPA) defines the urban heat island as an urbanized area that experiences higher temperatures than the surrounding rural areas. The heat island effect is not observable in many cases, being just a perception. In other cases, it is more acute and scientifically verifiable, especially in large cities (more than 2 million people in the population.
  2. In places like the Netherlands, warmer summers have been more acutely felt in cities. This has forced a rethink of urban design to reduce the ability of artificial structures to reemit heat, and studies on how to climate-proof Dutch urban centers.
  3. Urban centers experience greater rain runoff, diminishing the ability of rain to significantly reduce the urban heat effect.
  4. Newark, New York, Chicago, and Houston are some of the cities with the severest heat islands. It is blamed on impermeable surfaces, building height, and albedo.
  5. Higher temperatures lead to an elevated demand for electricity, sometimes as much as 9% for every °F. Since fossil fuels are burnt to generate electricity, urban heat islands directly lead to poorer air quality.
  6. Away from urban centers, ground-level ozone pollution also affects agricultural production, as seen in China’s Yangtze basin, where rice and wheat production has significantly reduced. The reduction is blamed on this pollution by research
  7. PMP is not a single pollutant, but an assortment of emissions from several sources, which may or may not react while in the air.
  8. There is a direct and positive correlation between 03 concentration and the intensity of an urban heat island. There is also a strong correlation between land surface temperature and air pollution.
  9. Additionally, Arizona State University says that, while the urban heat island effect is most apparent during the warmer months, it is experienced in Phoenix throughout the year. The primary reason for this, according to the university, is urban sprawl – people are building farther and farther into the desert.
  10. OECD estimates that if we were to follow the current trajectory, there will be between 6 and 9 million pollution-related premature deaths by 2060. By then, it will cost 1% of GDP (2.6 trillion).
  11. Phoenix’s swimming pools have long been blamed for making the air humid. This is incorrect. Instead, though the overall effect is limited, man-made water bodies can indeed reduce temperatures by absorbing excess heat.
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