The Urban Heat Island, explained: Why are cities warmer than the countryside?

A child plays cools down in a fountain in Brive-la-Gaillarde, southwestern France, during last summer's heatwave. Image: Getty.

In cities, the air, surface and soil temperatures are almost always warmer than in rural areas. This effect is known as the Urban Heat Island – a term which first came into use in the mid-20th century.

Until the 1980s, this effect was considered to have relatively little practical significance. In fact, given that most studies were done in cities with cold winter climates, a warmer temperature was seen as a potential benefit, because it reduced the need for heating. But since then, we’ve found a number of reasons to be concerned.

For one thing, it became clear that the Urban Heat Island (UHI) effect of cities was influencing air temperature records, which are used to assess climate change. In other words, it became important to remove urban “contamination” from weather station records to ensure their accuracy.

What’s more, as populations in warm and hot cities have increased, so too has the demand for indoor cooling – typically met by air conditioning. This even applies in colder climates, where changing building uses has increased the demand for cooling; for example, in office buildings, to offset the heat generated by computers.

In these situations, the UHI adds to the heating burden: ironically, cooling buildings with air conditioners increases outdoor air temperatures.

Heatwaves have the power to kill; for example, during the 2003 heatwave in Europe, 70,000 additional deaths were recorded, making it one of the region’s deadliest natural disasters of the last 100 years. The UHI makes city dwellers more vulnerable to the dangerous effects of extreme weather events like this.

The potential medical impact is perhaps the most significant issue related to UHI, especially against the backdrop of continued climate change and global warming. For all these reasons, it’s crucial to understand how the UHI works, so that we can find ways to mitigate and adapt to its effects.

Understanding the UHI

The UHI is strongest during dry periods, when the weather is calm and skies are clear. These conditions accentuate the differences between urban and rural landscapes.

Cities are distinguished from natural landscapes by their form: that is, the extent of the urban land cover, the construction materials used, and the geometry of buildings and streets. All of these factors affect the exchanges of natural energy at ground level.

Much of the urban landscape is paved and devoid of vegetation. This means that there is usually little water available for evaporation, so most available natural energy is used to warm surfaces. Construction materials are dense, and many – particularly dark-coloured surfaces like asphalt – are good at absorbing and storing solar radiation.

The urban jungle. Image: mdalmul/Flickr/creative commons.

Meanwhile, the shape and positioning of buildings in the city slows the movement of air near the ground, creates complex patterns of shade and sunlight and limits natural energy exchanges. Urbanisation is also associated with the emission of waste heat from industry, transport and buildings, which contributes directly to the UHI.

There are, however, different types of UHIs, with different dominant causes.

Keeping our cool

“Surface UHI” refers, unsurprisingly, to warmer urban temperatures at the Earth’s surface. Typically, this type of UHI is measured using satellites with a plan view of the city, so that the temperature of roofs and roads (but not walls) can be measured. From this perspective, the surface UHI is highest during the daytime, when hard urban surfaces receive solar radiation and warm quickly.

Another type of UHI is based on observations of air temperature, which are made close to the ground; in the city, this means placing the instruments below roof height.

This UHI is usually strongest at night, as street surfaces and the adjacent air cool slowly. Above the roof level, the contributions of streets and building roofs together warm the overlying urban atmosphere. In some conditions, this warming can be detected up to 1km to 2km above the surface.

The geography of the Urban Heat Island. Image: Jamie Voogt/University of Western Ontario/Author provided.

The geography of the UHI is relatively simple – it’s magnitude generally increases from the urban outskirts towards the city centre. However, it also contains many micro-climates – for example, parks and green spaces appear as cool spaces.

The UHI is an inevitable outcome of the landscape changes that accompany urbanisation. But its magnitude and impacts can be managed by modifying some physical aspects of our cities. This can include increasing vegetative cover and reducing impermeable cover; using lighter coloured materials, designing urban layouts to allow for better ventilation through the streets and buildings, and managing urban energy use.

Of course, these solutions need to be tailored to the type of UHI. For example, a focus on building cool or green roofs will have an impact on the overlying air and the top floor of buildings, but may have little impact on the UHI at street level. Similarly, trees may be an effective means of providing street shade, but if the canopy encloses the street, then it can trap traffic emissions, resulting in poor air quality.

As a first step, many cities have completed UHI studies to identify the “hot-spots”, where design interventions could have greatest effect. But what most cities need is a coherent climate plan, which addresses interrelated environmental issues including flooding and air quality, as well as surface and air temperatures. The Conversation

Gerald Mills is senior lecturer in geography at University College Dublin.

This article was originally published on The Conversation. Read the original article.



Businesses need less office and retail space than ever. So what does this mean for cities?

Boarded up shops in Quebec City. Image: Getty.

As policymakers develop scenarios for Brexit, researchers speculate about its impact on knowledge-intensive business services. There is some suggestion that higher performing cities and regions will face significant structural changes.

Financial services in particular are expected to face up to £38bn in losses, putting over 65,000 jobs at risk. London is likely to see the back of large finance firms – or at least, sizable components of them – as they seek alternatives for their office functions. Indeed, Goldman Sachs has informed its employees of impending relocation, JP Morgan has purchased office space in Dublin’s docklands, and banks are considering geographical dispersion rather concentration at a specific location.

Depending on the type of business, some high-order service firms will behave differently. After all, depreciation of sterling against the euro can be an opportunity for firms seeking to take advantage of London’s relative affordability and its highly qualified labour. Still, it is difficult to predict how knowledge-intensive sectors will behave in aggregate.

Strategies other than relocation are feasible. Faced with economic uncertainty, knowledge-intensive businesses in the UK may accelerate the current trend of reducing office space, of encouraging employees to work from a variety of locations, and of employing them on short-term contracts or project-based work. Although this type of work arrangement has been steadily rising, it is only now beginning to affect the core workforce.

In Canada – also facing uncertainty as NAFTA is up-ended – companies are digitising work processes and virtualising workspace. The benefits are threefold: shifting to flexible workspaces can reduce real-estate costs; be attractive to millennial workers who balk at sitting in an office all day; and reduces tension between contractual and permanent staff, since the distinction cannot be read off their location in an office. While in Canada these shifts are usually portrayed as positive, a mark of keeping up with the times, the same changes can also reflect a grimmer reality.  

These changes have been made possible by the rise in mobile communication technologies. Whereas physical presence in an office has historically been key to communication, coordination and team monitoring, these ends can now be achieved without real-estate. Of course, offices – now places to meet rather than places to perform the substance of consulting, writing and analysing – remain necessary. But they can be down-sized, with workers performing many tasks at home, in cafés, in co-working spaces or on the move. This shifts the cost of workspace from employer to employee, without affecting the capacity to oversee, access information, communicate and coordinate.

What does this mean for UK cities? The extent to which such structural shifts could be beneficial or detrimental is dependent upon the ability of local governments to manage the situation.

This entails understanding the changes companies are making and thinking through their consequences: it is still assumed, by planners and in many urban bylaws and regulations, that buildings have specific uses, that economic activity occurs in specific neighbourhoods and clusters, and that this can be understood and regulated. But as increasing numbers of workers perform their economic activities across the city and along its transport networks, new concepts are needed to understand how the economy permeates cities, how ubiquitous economic activity can be coordinated with other city functions, such as housing, public space, transport, entertainment, and culture; and, crucially, how it can translate into revenue for local governments, who by-and-large rely on property taxes.

It’s worth noting that changes in the role of real-estate are also endemic in the retail sector, as shopping shifts on-line, and as many physical stores downsize or close. While top flight office and retail space may remain attractive as a symbolic façade, the ensuing surplus of Class B (older, less well located) facilities may kill off town-centres.

On the other hand, it could provide new settings within which artists and creators, evicted from their decaying nineteenth century industrial spaces (now transformed into expensive lofts), can engage in their imaginative and innovative pursuits. Other types of creative and knowledge work can also be encouraged to use this space collectively to counter isolation and precarity as they move from project to project.

Planners and policymakers should take stock of these changes – not merely reacting to them as they arise, but rethinking the assumptions that govern how they believe economic activity interacts with, and shapes, cities. Brexit and other fomenters of economic uncertainty exacerbate these trends, which reduce fixed costs for employers, but which also shift costs and uncertainty on to employees and cities.

But those who manage and study cities need to think through what these changes will mean for urban spaces. As the display, coordination and supervision functions enabled by real-estate – and, by extension, by city neighbourhoods – Increasingly transfer on-line, it’s worth asking: what roles do fixed locations now play in the knowledge economy?

Filipa Pajević is a PhD student at the School of Urban Planning, McGill University, researching the spatial underpinnings of mobile knowledge. She tweets as @filipouris. Richard Shearmur is currently director of the School, and has published extensively on the geography of innovation and on location in the urban economy.