Blog Post

Hot town, summer in the city – 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 oneself 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 was 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. When examined with even the merest pinch of scrutiny, these seemingly innocent three words describe an uncomfortable truth – cities are potent heat traps, becoming up to 10 degrees Celsius hotter than their surroundings during summertime heatwaves. This bizarre phenomenon takes place 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.

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 made ever more dire. 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 built environment’s harsh response to changes in air temperature.

Our roads, streets and parking lots perform brilliantly when it comes to retaining thermal energy. Asphalt and concrete, the materials of choice for modern pathways, 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 lightly coloured ones, they also emit more heat into the air that surrounds them. This is a problem.

The ETH Zurich, one of the world´s leading science institutions, hast 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, England will be like that of Barcelona, Spain today. 22% of the world´s cities are likely to be in a climatic regime that does not yet exist on the planet.

Shanghai skyline and cityscape at sunset


Another charmingly named warm-weather occurrence is the “urban canyon effect”. An agglomeration of tall buildings, standing in close proximity to one another – as most downtown areas are built – 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 the mechanism that transfers heat from a hot surface by having the fluid which surrounds it flow. The fluid (in the case of an urban canyon – air) expands during the cooling process, its density reducing. The alteration of density causes the fluid (air) to flow, carrying thermal energy along with it, thereby transferring energy (heat) from the hot object into the surrounding air – ergo cooling down the object it has just left. With no free-flowing wind, convection cooling cannot occur, which in turn allows air pollution and heat to linger.

In modern cities, a myriad of surfaces is just waiting to soak up sunlight. Buildings can absorb a lot of the sun’s thermal energy during the day and then emit that heat into the atmosphere at night. The heat is transferred both within the structural elements as well as radiated into the surrounding air. The closer together a city’s buildings are built, the more thermal energy will be stored within the densely urbanized island, unable to disperse or cool down over night because of the buildings´ proximity to one another, radiating and absorbing each other’s heat deposits in a really warped take on the sharing economy.

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 they are featured in architectural and tourist magazines, but unfortunately act like giant greenhouses, trapping even more heat within the buildings in the summer, and letting heat escape in winter – 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, after the city starts to heat up, the residents are forced to somehow help themselves get through the ever more frequently occurring and longer-lasting heatwaves. They turn to air-conditioning, a now more-or-less ubiquitous process of cooling down man-made spaces which, as a by-product of doing its job (reducing the air temperature in the interiors of buildings), emits even more hot air into the surrounding landscape, contributing to the heat build-up. Our machines are excellent at exacerbating heat islands. Not just air conditioners that pump waste heat into the air, but also our beloved cars – vehicles stuck in traffic emit heat.

All that thermal energy transfer comes at a grave cost – air conditioning accounts for 1/5 of building-related electricity use. Globally. 2.5 times the total use of electricity in the entire continent of Africa. The use of cooling energy in buildings has doubled since 2000, due to an increase in population size and economic growth. The International Energy Agency estimates that the use of space cooling energy could more than double until 2040 because of the steadily increasing use of air conditioning.

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


But that’s not all that factors into the formation of an urban heat island. In unaltered circumstances, the natural environment has its own ways of dealing with the sun and its thermal discharge. Cities disrupt that balance. Paving green areas into water-resistant surfaces, such as roads, eliminates the cooling effect of evaporating water – the pavement happily radiating the heat that was stored in the asphalt back into the air. Sidewalks, roads and parking lots 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 could 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, that change driven by the very heat in the air. During this process, evaporation, 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 is makes the pavement so hot, is emits that heat back into the air.

The intensity of the urban heat island effect depends on several factors. The surrounding area – if for example a city is situated in a valley, surrounded by mountains that obstruct the flow of air and hinder cooling winds, the heat has no choice but to remain accumulated. Another important factor is the 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 own midst, but also influence local weather conditions. They interfere with precipitation levels, 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.

The weather takes a toll on us too. 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 urbanized areas, many can become compromised by climatic conditions. Those who cannot afford air conditioning, the young, the elderly and the ill are all at a higher risk of heat-related health issues brought on by the heat.

Windows in a green wall covered with ivy


Luckily, there are so many simple and efficient ways to mitigate the causes of urban heat islands, aesthetically pleasing and proven solutions that can greatly reduce the heat retention of our built environment – while at the same time 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 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. Green facades provide better air quality, dampen noise and beautify the city with their calming colours. They improve the residents´ psychological wellbeing and cost next to nothing to maintain. The biodiversity offers a habitat for wildlife, which in turn pollinates other plants and makes 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.

And best of all, green facades increase the thermal insulation of buildings, acting as natural and emissions-neutral air conditioners. The leaves 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

Guingi Tower in Lucca, Tuscany

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 – using sprinklers on roofs, wetting 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. On street level, the more trees are planted, the more shade they will provide, the less the pavement will heat up, the more evaporation there will be. The Hanging Gardens of Babylon don´t need to remain a myth.

A great way to keep the asphalt from getting so hot that an egg can be boiled on it is to paint the streets white, enabling them to reflect solar energy instead of absorbing it. According to some Australian researchers, conventional paving can get up to a 67 degrees Celsius surface temperature and roofs up to 50-90 degrees. “Cool pavement” is a road surface that uses brightly coloured paint to reflect solar radiation, instead of it 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. After painting roads white, initial measurements indicate a drop in surface temperature of around 10 degrees Celsius.

Using water is 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 – making sure that the evaporation of water can cool the hot, dry Spanish air. Today urban planners could incorporate pools, fountains, misting systems and sprinklers into their designs.

Institut du Monde Arabe, Paris, France

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, also borrowed from history, would be to design shading systems that protect buildings from the sun´s heat during the summer.

This isn´t a new concept. In the 1980’s, 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.

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

If we used all these measures to minimize the duration and intensity of urban heat islands, our lives would be so much easier, as so many of us live in cities – and so many of us will continue to migrate to urban areas in the coming decades. 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 such a bad idea after all.

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