Ever noticed how the summer heat hits you differently on a stroll through the city than in a rural setting?

That’s not a figment of your imagination. Inner-city areas tend to trap heat more effectively than natural landscapes, creating a phenomenon known as the urban heat island.

The culprit for scorching city temperatures lies in the assembly of our urban landscapes. Stripped of trees and vegetation, buildings, roads, and pavements absorb and retain more heat.

Since the urban heat island effect can cause city air temperatures to reach several degrees higher than surrounding areas, it’s a growing consideration for urban planners.

In this blog post, we’ll get you up to speed on what urban heat islands are, the causes of urban heat islands, examples, and innovative solutions cities are implementing to keep cities cooler and more comfortable!

What is an urban heat island?

Although it might sound like a fictional island retreat, the urban heat island (UHI) has a much more sinister context.

The urban heat island effect occurs when a city experiences higher temperatures than nearby rural areas (as much as 8°C). Major cities worldwide are facing this problem, and climate change is only making it worse. Urban heat island effects contribute to extreme temperatures in urban areas, particularly during heat waves.

As metropolitan areas grow, new buildings, roads, and other infrastructure are needed to support growing populations. This often comes at the cost of natural landscapes. The addition of more heat-absorbing materials means cities retain extra heat, causing air temperatures to rise and a significant temperature difference between urban and rural areas.

Because skyscrapers, pavements, car parks, and inner-city traffic wouldn’t exist without humans, UHI is one of the most prominent ways we can see how human impact can damage our planet.

Trees can block 70–90% of direct sunlight and reduce the temperature inside parked cars by 25°C.

Trees can block 70–90% of direct sunlight and reduce the temperature inside parked cars by 25°C.

Causes of urban heat island

If you’re feeling the extreme heat in your city and wondering what the cause is, here are some of the main culprits that make cities hotter than surrounding areas:

Urban materials replacing nature

Reduced greenery in city centres is one of the main reasons why they’re hotter than rural areas. It’s simple: the more human-made objects that replace nature, the worse the UHI effect is. For example, if a city has more pavements than trees and fewer parks than car parks, the effects will be more drastic.

This is because the materials used to build cities have a high thermal mass, meaning they absorb and store heat, reaching high surface temperatures.

At night, asphalt, concrete, and dark-coloured buildings release the accumulated heat energy back into the environment, creating a constant warming effect. This is in contrast to trees and other plants that provide shade and release water vapour, which cools the surrounding air.

Tall buildings block airflow

When a city is densely packed with high-rise buildings, the mazy, narrow streets restrict air circulation. Hot air becomes trapped and stagnates at street level.

It’s easy to imagine how walking through a jam-packed city centre with towering buildings feels hotter than an open field with a cool breeze. This is a perfect example of how urban design plays a significant role in the effects of UHI.

Waste heat from personal vehicles

Unbeknownst to many city dwellers, our daily habits can feed the urban heat island effect. The physical urban landscape isn’t solely to blame for UHI. We all have a part to play in keeping our cities cool.

Cars are the primary contributors on a personal level. Personal vehicles release significant waste heat into our cities, and traffic congestion worsens the problem. Idle engines pump out heat, creating a heat bubble around areas with gridlock traffic.

Other human activities that generate waste heat include the overuse of air conditioners and excessive energy consumption in our homes, whether it be appliances or taking long, hot showers. High urban temperatures create a vicious cycle: increased demand for air conditioning leads to higher energy consumption and waste heat production, further raising air temperatures.

Building materials like brick and concrete absorb heat, while high-rise buildings restrict air circulation.

Building materials like brick and concrete absorb heat, while high-rise buildings restrict air circulation.

How urban heat islands affect you

Living in a bustling city offers many advantages — cultural attractions, career opportunities, and a vibrant buzz. However, few consider the hidden side effects of urban heat islands.

Health risks

UHIs exacerbate the effects of hot weather, making heat stroke, dehydration, and respiratory issues more likely, especially amongst vulnerable demographics.

Increased nighttime temperatures in urban areas prevent the body from cooling down and recovering before the next day’s heat. The extra heat trapped within the city makes the body work overtime to keep you cool, making your city a health hazard through summer.

Environmental impact

With UHI trapping extreme heat within cities, emissions from vehicles and factories get stuck in the layer of hot air rather than dispersing. The subsequent air pollution strains human health and harms the environment.

The temperature difference can also lead to changes in water quality. Roads, pavements, and buildings don’t absorb water and are often 30°C hotter than the surrounding environment. They can warm up rainfall, causing a loss of aquatic life in nearby bodies of water by rapidly raising water temperatures more than 10°C.

Reduced liveability

What’s the use of living in a bustling major city if you’re faced with an uncomfortable, polluted environment every time you head outdoors? Since urban heat islands turn cities into sweltering concrete jungles, the consequences are far-reaching and impact the liveability of major cities.

Examples of urban heat islands

Every city experiences the urban heat island effect to a different degree. As we’ve seen, natural and human-made factors both have a part to play. However, some cities are far better than others at making planning choices that mitigate UHI.

Here are some examples of cities, each with its own unique set of challenges and solutions to help keep their city cool.

Phoenix, Arizona

Pheonix is indeed located in a naturally hot and dry desert environment. So it’s only natural that the city would experience high temperatures. What might come as more of a shock is the temperature difference between the city and the surrounding desert. It can be up to 11.7 degrees Celsius in favour of the urban area!

With a lack of vegetation and water bodies throughout the city, Phoenix isn’t blessed with natural cooling systems. Add to the mix a sprawling network of asphalt roads that absorb and retain the heat; Phoenix is a prime example of a city suffering from the urban heat island effect.

Despite the many challenges faced by the city, Phoenix has been working to mitigate the effects of UHI in recent years. New initiatives include developing parks and green corridors throughout the city to create a cooler, more liveable and sustainable environment. Phoenix also promotes reflective roofing materials and encourages residents to plant trees on their properties.

Tokyo, Japan

With skyscrapers covering almost every inch of Tokyo, this major metropolis is no stranger to scorching inner-city temperatures. Tokyo’s dense urban layout — dominated by concrete, asphalt and dark-coloured buildings — exacerbates the effect of the urban heat island. Since these materials trap heat and high-rise buildings obstruct air ventilation, hot air becomes stagnant at street level.

Tokyo’s tightly packed skyscrapers are the answer to the city’s land scarcity. However, as a major contributor to UHI, what can city planners do to lessen the effect of UHI on the towering cityscape?

Tokyo is incentivising the use of green roofs on skyscrapers. Similarly, vertical gardens on building facades are another strategy to add greenery while maximising space. Such measures provide insulation and reduce heat absorption from buildings, improving air quality.

Additionally, Tokyo is shifting to lighter-coloured materials for pavements and applying reflective coatings to existing buildings. These coatings reflect sunlight and reduce heat absorption by streets and buildings, contributing to a cooler urban environment.

Zurich, Switzerland

Zurich is one of the world’s frontrunners for mitigating the effects of UHI. The city’s ongoing commitment to sustainable practices and green infrastructure has created a comfortable living environment for residents and serves as a model for other major cities.

Zurich is fortunate because it benefits from a higher altitude and the presence of water bodies, which help regulate temperatures naturally. However, Zurich doesn’t solely rely on its natural advantages to prevent heat spikes. The Swiss capital incorporates many sustainable practices throughout its urban planning.

As you stroll through the city, you notice the abundance of green spaces promoting natural cooling. More recently, lighter-coloured materials are being used for rooftops and pavements. Combined with urban planning that preserves open spaces for improved airflow, these efforts create a cooler and more comfortable urban climate for Zurich’s residents.

Greenery, open spaces, light surfaces, and using public transport instead of a personal car help reduce the air temperature in our cities.

Greenery, open spaces, light surfaces, and using public transport instead of a personal car help reduce the air temperature in our cities.

How to reduce the heat island effect in urban areas

Urban heat islands can make our urban areas brutally hot. But it’s not all doom and gloom, city dwellers; there are ways to turn things around. By implementing certain solutions, we can make our cities cooler and more liveable!

Plant power

Cities often lack greenery. Not only is this a blot on the urban landscape, but it also means cities miss out on nature’s cooling tricks. Whether it be trees, vines, shrubs, flowers, or grasses, vegetation fights against the extreme heat and protects us from stifling temperature spikes.

Vegetation provides a natural shade umbrella from the sun and releases water vapour into the air, keeping the surrounding area cooler. Better still, vegetation helps air circulate through the city, channelling the hot air that can get stuck between buildings.

So, consider planting trees on the south side of your house to block the hot sun. And campaign for your city to plant more urban trees within the city limits. If space is tight, vertical vines, rooftop gardens, and hanging baskets are a great way to add greenery, even in small areas.

Every bit of vegetation helps insulate buildings and cool the air around them.

A splash of colour

When we’re young, we learn that dark objects absorb the sun’s heat while light colours reflect it. The darker colours prominent in the buildings that line our cities from the asphalt, steel, and brick absorb and soak up heat, making everything warmer. Just as wearing a black T-shirt on a hot day would make you warmer than a white T-shirt, the results are the same for dark-coloured buildings.

Picture Greece. With soaring summer temperatures, is it any wonder why all the houses are painted white and blue? Yes, it’s charming, but it’s also practical. The light-coloured paint helps reflect sunlight and reduce heat absorption.

It’s not just buildings that can be lightened. Black asphalt streets, car parks, and dark roofs can be covered with a more reflective grey coating. Doing this may drop urban air temperatures dramatically, especially during summer heat waves.

In Greece, houses are painted white and blue to reflect sunlight and keep buildings cooler.

In Greece, houses are painted white and blue to reflect sunlight and keep buildings cooler.

Smart urban planning

Urban planning significantly impacts the degree of UHI in a city. Poor urban planning decisions that focus on dense building developments, neglect greenery, and rely on heat-trapping materials can have dire consequences on the urban environment.

Smart urban planning decisions made today can create cooler, healthier urban environments for everyone.

Promote green infrastructure: Strategically integrating parks, trees, and rooftop gardens into urban design plans increases shade and reduces heat absorption. Vines, for example, can reduce wall surface temperatures up to 20ºC.
Developments with airflow in mind: New developments that keep streets wide and promote open spaces allow for better air circulation, preventing heat from getting trapped.
Reflective, light-coloured materials: Encouraging reflective surfaces and light-coloured materials for development contribute to a cooler urban area. For example, a white coating can reduce pavement surface temperature by 18–25 °C.
Water features: Fountains, ponds, and sprinklers scattered throughout the city can cool the immediate area through evaporation by an average of 0.7 to 3°C.
Encourage sustainable transport: Sustainable urban planning that encourages walking, light vehicles (bikes, e-bikes, scooters), shared mobility, and public transit reduces car dependency, preventing traffic heat generation within the city. To reduce the reliance on private cars, cities must ensure a solid infrastructure for other modes of transport.

Opting for sustainable modes of transport can help reduce the urban heat island effect.

Opting for sustainable modes of transport can help reduce the urban heat island effect.

Urban heat island FAQs

What can I do to help?

By working together, we can reduce the extreme heat and make our cities more comfortable. Small, personal changes can sometimes feel useless in the grand scheme of things. But if we were to all make little alterations to our home environment, it would go a long way.

Planting trees, using clothing dryers and air conditioning less, and opting for sustainable transport methods over personal vehicles can all help.

Why are urban heat islands a problem?

Experiencing the UHI effect in your city is nothing to be proud of. UHI results from poor urban planning and can harm residents’ health, create dangerous living conditions in heatwaves, and stress wildlife. Urban planners worldwide are now recognising the problems that UHI creates and are working on solutions to cool down our cities.

What are some signs of UHI?

Higher air and surface temperatures and the temperature difference between urban and rural areas are the number one sign that your city is experiencing UHI effects. UHI causes the area to become noticeably warmer than surrounding rural areas, especially at nighttime. Other signs include restricted inner-city air circulation, low air quality, and more frequent heat waves.

Bolt’s commitment to liveable cities

Bolt is committed to making urban living better for everyone.

Whether it’s an e-scooter, e-bike, flexible ride-hailing, or shared rental car, Bolt offers all the transport options necessary to easily move around without a private car.

Download the Bolt app and start your ride today.

Understanding the urban heat island: why cities get so hot

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