Blog Post

Overheating in the City – Understanding the Urban Heat Island

16.07.2019 | 12 min read

Summer heat wave in the city

“Urban heat island.” The phrase has a nice ring to it, doesn´t it? Is it a breezy islet tucked away in a shady courtyard, a place to hide from the scorching summer sun? Or does it evoke the sensation of lounging under a leafy tree in a downtown park, having a quiet moment, marvelling at skyscrapers glistening in the sun?

One might even imagine relaxing by the side of a rooftop pool, sipping a sunset-coloured cocktail—topped off with a tiny wooden parasol, naturally—while dipping one´s toes in the cool, refreshing water.

How wonderful it would be if that were what an urban heat island really was—a refuge for city dwellers, a break from their daily routines, an escape from the ever more frequent and intense heatwaves that are mercilessly occurring all over the globe. Sadly, it is quite the opposite.

What Is an Urban Heat Island?

When examined with even the merest pinch of scrutiny, these seemingly innocent three words describe an uncomfortable truth—cities with potent heat traps, becoming up to 10 degrees Celsius hotter than their surroundings during summertime heatwaves.

This bizarre phenomenon occurs when a city or town has a higher mean air temperature than the rural area in its immediate vicinity. Not great for all those millions upon millions of us who live in cities all over the world.

Beyond being wildly uncomfortable, urban heat islands can take a massive toll on human health, energy consumption, and the economy.

Shanghai skyline and cityscape at sunset

When Is the Urban Heat Island Effect the Strongest?

Urban heat islands are most powerful during dry periods, benefitting from calm weather and clear skies. As global temperatures slowly and mercilessly rise, the problem posed by the urban heat island phenomenon is becoming even more severe.

Our love for building visually striking glass towers, enormous housing blocks made of concrete, and covering natural surfaces with hard, dark asphalt has laid all the groundwork for the environment’s harsh response to changes in air temperature.

Asphalt and concrete, the materials of choice for modern roads, are darkly coloured, black, or grey. Objects that are dark absorb almost all the light that lands on them.

And when light is absorbed, it is converted into thermal energy and emitted back out in the form of heat. Because black or dark grey surfaces absorb more light than light-coloured ones, they also emit more heat into the surrounding air. This is a problem.

The ETH Zurich, one of the world´s leading science institutions, has published a study that analyses and anticipates the climate shifts that over 200 cities will experience over the next 30 years.

The study states that the climatic conditions of over 77% of the world´s major cities will change to such a degree that the 2050 climate of London, will be like that of Barcelona today. 22% of the world´s cities are likely to be in a climatic regime that does not yet exist on the planet.

Apartment building in Hong Kong with air conditioning units on the facade

An Urban Canyon – Doubling Down on Heat

Another charmingly named warm-weather occurrence is the “urban canyon effect”. Most downtown areas consist of several tall buildings standing in close proximity to one another. This prevents the natural flow of cool night air, causing the formation of hot air pockets, thus preventing urban areas from cooling down at night.

Convection cooling is a process of heat transfer from hot objects through the fluids surrounding them. The fluid (which in the case of an urban canyon is air) expands during the cooling process while its density reduces.

The alteration of density causes the fluid to flow, carrying thermal energy (heat), which it transfers from the hot object into the surrounding air, cooling down the object it has just left. With no free-flowing wind, convection cooling cannot occur, allowing air pollution and heat to linger.

In modern cities, various surfaces are just waiting to soak up sunlight. Buildings can absorb much of the sun’s thermal energy during the day and then emit that heat into the atmosphere at night. The heat is transferred within the structural elements and radiated into the surrounding air.

The closer a city’s buildings are, the more thermal energy will be stored within the densely urbanised island. Since they cannot disperse or cool down overnight because of the buildings’ proximity to one another, they radiate and absorb each other’s heat deposits.

Moreso, many buildings feature dark surfaces which heat up rapidly, making the entire building store far more thermal energy than it would if its surfaces were brightly coloured. Not to mention those spectacular glass facades that look so impressive and stylish when featured in architectural and tourist magazines.

How Cooling Our Homes Adds Heat

The urban canyon, unfortunately, acts like giant greenhouses, trapping even more heat within the buildings in the summer and letting heat escape in winter. This is a very inefficient way to regulate the temperature within a building, making building cooling and heating a huge drain on the world´s energy resources.

In a brutal plot twist, the residents are forced to help themselves get through the ever more frequently occurring and longer-lasting heatwaves after the city start to heat up. They turn to air-condition units, a now more-or-less ubiquitous process of cooling down man-made spaces.

Air conditioning emits even more hot air into the surrounding landscape, contributing to the heat build-up. Sadly not just air conditioners that pump waste heat into the air but also our beloved cars, especially when we’re stuck in traffic.

All that thermal energy transfer comes at a grave cost. Air conditioning, for instance, accounts for much of building-related electricity use. Statistics show that there are an estimated 1.9 billion air conditioning units worldwide.

The International Energy Agency (IEA) predicts that these figures will increase to 5.5 billion by 2050. It goes without saying that this heightened energy demand will pose potential problems for the world, a blind spot in current energy debates.

How Nature Prevents the Urban Heat Island Effect

Besides the urban canyon, more factors lead to the formation of urban heat islands. In unaltered circumstances, the natural environment has its ways of dealing with the sun and its thermal discharge. Cities disrupt that balance.

Paving green areas into water-resistant surfaces, such as roads, sidewalks, and parking lots, eliminates the cooling effect of evaporating water. They also create a barrier atop the soil, inhibiting the evaporative cooling process of the ground, plants, treetops, and bodies of water.

Plants soak up water through their roots so they can survive. They process that water, allowing what remains unused by their lifecycle to be absorbed into the surrounding dry air. The liquid water is turned into vapour, which is driven by the air’s very heat. During this evaporation process, the air is robbed of its heat and can cool down.

The same thing happens to us when we sweat. The air around us absorbs the moisture from our dewy skin and becomes cooler. As we expand our cities, we replace more and more vegetation with asphalt and concrete and lose the evaporative cooling advantage and shade that plants so kindly provide us.

What’s more, asphalt, our material of choice, absorbs so much solar heat that makes the pavement so hot it emits that heat back into the air.

Windows in a green wall covered with ivy

Common Causes of Urban Heat Islands

The intensity of the urban heat island effect depends on several factors, some of which we consider below:

1. Location

The surrounding area plays a major role in determining whether heat is retained or not. For example, suppose a city is situated in a valley surrounded by mountains that obstruct airflow and hinder cooling winds. The heat will have no choice but to remain accumulated.

2. Population Size

The more people live in a city, the more heat they generate by using cars, air conditioning, and heating. It is a vicious circle. Urban heat islands do not just increase the temperatures in their midst but also influence local weather conditions.

They interfere with precipitation, cloud coverage, fog, humidity, and wind patterns. Thunderstorms even form more frequently close to cities. Local wildlife is greatly affected as its habitat becomes altered by the thermal pollution exuded by the city.

3. Weather

The prevailing weather condition in a place plays a significant role in determining whether there will be some form of urban island effect or not. When the weather is unfavourable, people always turn to artificial cooling systems contributing to global warming.

The mechanisms of the human body can be easily overpowered by the heat, making it the main environmental cause of death.

Heat exhaustion is a common reaction to severe temperatures. Dizziness, headaches, fainting, dehydration, and confusion are minor effects. The health ramifications of sunstroke and heatstroke can be very serious.

As cities grow and more people move to urbanised areas, many can become compromised by climatic conditions. Those who cannot afford air conditioning will be at a higher risk of heat-related health issues.

How to reduce the Urban Heat Island Effect

The big question is, “How can urban heat islands be reduced?” Luckily, there are many simple and efficient ways to mitigate the causes of urban heat islands.

These aesthetically pleasing and proven solutions can greatly reduce the heat retention of our built environment while simultaneously creating more attractive, beautiful, and comfortable places to live, work and play.

For starters, we can reintroduce nature into our cities. Vegetation can be a powerful ally in the fight against the heat. Instead of cutting down plants, we could let them grow up the facades of our buildings. Vertical vegetation is a simple and incredibly successful mechanism for reducing the heat strain of buildings.

How Do Green Roofs Reduce the Urban Heat Island Effect?

Here are a two main ways that green roofs can reduce the urban heat island effect:

1. Improved Air Quality

Green facades provide better air quality, dampen noise and beautify the city with their calming colours. They improve the residents´ psychological well-being and cost next to nothing to maintain.

The biodiversity offers a habitat for wildlife, polluting other plants and making the city lush. Green facades do not require any additional structural support. They can attach themselves to almost any surface and are mostly free-standing.

2. Thermal Insulation

Green facades increase the thermal insulation of buildings, acting as natural and emission-neutral air conditioners. The leaves also bind pollutant microparticles in the air and create oxygen, improving air quality.

Evergreen plants help insulate the buildings in winter, lowering power costs. In summer, the temperature measured in buildings with green facades is up to 13 degrees Celsius lower than that of their neighbouring edifices.

Fast-growing plants such as wild grape and wisteria only need one to two years to fully cover an area of 8 square meters. A low-maintenance, low-energy, healthy alternative to air conditioning.

The same can be done on roofs. Instead of dark, heat-absorbing surfaces, putting vegetation on the tops of buildings will not only prevent the roof from heating up but will make use of the evaporative cooling effect and diminish the intensity of the urban heat island.

An alternative approach is roof sprinkling, which involves using sprinklers on roofs to wet the surface so that the air above it can be cooled via evaporation. We could even put trees up there, just as the medieval people of Lucca, Italy, did atop the Guinigi Tower in the 12th century, where they remain to this day.

Guingi Tower in Lucca, Tuscany

How else can we reduce the Urban Heat Island effect?

1. Cool Pavement

Cool pavement” is a road surface that uses brightly coloured paint to reflect solar radiation instead of being absorbed by the dark asphalt. In this way, the surface temperature of the asphalt can be decreased by more than 5 degrees Celsius.

Cities, such as Los Angeles in California, are testing out this method in an effort to combat the effects of climate change. 10% of that city´s surface is covered in black asphalt that absorbs 95% of solar energy, greatly contributing to the urban heat island effect. After painting roads white, initial measurements indicate a drop in surface temperature of around 10 degrees Celsius.

On the street level, the more trees we plant, the more shade they will provide, the less the pavement will heat up, and the more evaporation there will be, cooling the surrounding air.

2. Add water

Using water to cool built environments is a tried and tested method. In the Middle Ages, the builders of the Alhambra, a pearl of Andalusian medieval architecture, filled its courtyards with pools and fountains.

The essence was to ensure that water’s evaporation could cool the hot, dry Spanish air. Following this trend, modern urban planners incorporate pools, fountains, misting systems, and sprinklers into their designs. Of course, this is only feasible in countries and regions which don’t suffer from water shortages.

Institut du Monde Arabe, Paris, France

3. Changing Building Styles

The way we build is also a problem. Those shimmering glass facades, our giant modern glasshouses, trap heat in summer and let it dissipate in winter. An interesting alternative approach borrowed from history would be to design shading systems that protect buildings from summer sun heat.

This isn´t a new concept. In the 1980s, French starchitect Jean Nouvel designed an intricate façade that draws inspiration from the lattice work famously found in Islamic architecture, which shields the building’s occupants from the sun.

Nouvel took a traditional element into a new age by creating a system of several hundred light-sensitive, mobile diaphragms, which, by opening or closing, regulate how much light will enter the building. This dynamic façade was ground-breaking at the time.

4. Reduce Traffic

Adding the obvious to the mix, scientists are also recommending reducing traffic to minimise pollution and the heat emissions of vehicles. More efficient public transport, cycling, and other alternative means of transportation are becoming increasingly popular in large, densely built, and populated cities all across the globe.

Final Words

If we used all these measures to minimise the duration and intensity of urban heat islands, our lives would be so much easier, and the seasonal discomfort of living in urban areas would reduce.

The weather is one of the few things on this planet that we cannot conquer, so if we want to keep thriving as a species, the smart thing to do would be to prepare for the upcoming effects of climate change. Perhaps, living in an urban jungle is not a bad idea after all – providing we design it properly.

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