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.


 

 
 
 
 

Leeds is still haunted by its pledge to be the “Motorway City of the Seventies”

Oh, Leeds. Image: mtaylor848/Wikimedia Commons.

As the local tourist board will no doubt tell you, Leeds has much to be proud of: grandiose industrial architecture in the form of faux-Egyptian temples and Italian bell-towers; an enduring cultural legacy as the birthplace of Goth, and… motorways. But stand above the A58(M) – the first “urban motorway”  in the country – and you might struggle to pinpoint its tourist appeal.

Back in the 1970s, though, the city council was sufficiently gripped by the majesty of the motorways to make them a part of its branding. Letters sent from Leeds were stamped with a postmark proudly proclaiming the city's modernity: “Leeds, Motorway City of the Seventies”.

Image: public domain.

During the 1960s, post-war optimism and an appetite for grand civic projects saw the rapid construction of motorways across England. The construction of the M1 began in 1959; it reached Leeds, its final destination, in 1968. By the early 1970s the M62 was sweeping across Pennines, and the M621 loop was constructed to link it to Leeds city centre.

Not content with being the meeting point of two major motorways, Leeds was also the first UK city to construct a motorway through the city centre: the inner ring road, which incorporates the short motorway stretches of the A58(M) and the A64(M). As the council put it in 1971, “Leeds is surging forward into the Seventies”.

The driving force behind Leeds' love of motorways was a mix of civic pride and utopian city planning. Like many industrial cities in the North and Midlands, Leeds experienced a decline in traditional manufacturing during the 1960s. Its position at the centre of two major motorways seemed to offer a brighter future as a dynamic city open for trade, with the infrastructure to match. In response to the expansion of the roads, 1970s council planners also constructed an elevated pedestrian “skywalk” in an attempt to free up space for cars at ground level. Photos of Leeds from that time show a thin, white walkway running through blocky office buildings – perhaps not quite as extensive as the futuristic urban landscape originally envisaged by planners, but certainly a visual break with the past.

Fast forward to 2019 and Leeds’ efforts to become a “Motorway City” seems like a kitsch curiosity from a decade that was not always known for sustainable planning decisions. Leeds’s historic deference to the car has serious consequences in the present: in February 2019, Neville Street – a busy tunnel that cuts under Leeds station – was found to contain the highest levels of NO2 outside London.

City centre planners did at least have the foresight to sink stretches of the inner motorways below street level, leaving pedestrian routes largely undisturbed. Just outside the centre, though, the roads can be more disruptive. Sheepscar Interchange is a bewildering tangle of arterial roads, Armley Gyratory strikes fear into the hearts of learner drivers, and the M621 carves unsympathetically through inner-city areas of South Leeds with pedestrian access restricted to narrow bridges that heighten the sense of a fragmented landscape.

 

Leeds inner ring road in its cutting. Image: author provided.

 

The greatest problem for Yorkshire's “Motorway City” in 2019, however, is not the occasional intimidating junction, but the complete lack of an alternative to car travel. The dire state of public transport in Leeds has already been raised on these pages. In the early 20th century Leeds had one of the most extensive tram networks in the country. The last lines closed in 1959, the same year construction began on the A58m.


The short-sightedness of this decision was already recognised in the 1970s, as traffic began to build. Yet plans for a Leeds Supertram were rejected by successive Conservative and Labour governments unwilling to front the cost, even though smaller cities such as Newcastle and Sheffield were granted funding for light transport systems. Today, Leeds is the largest city in the EU without a mass transit system. As well as creating congestion, the lack of viable public transport options prevents connectivity: the city's bus network is reasonable, but weaker from East to West than North to South. As a non-driver, I've turned down jobs a short drive away that would be a logistical impossibility without a car.

Leeds' early enthusiasm for the motorway was perhaps premature, but there are things we can learn from the 1970s. Whatever else can be said about it, Leeds' city transport strategy was certainly bold – a quality in short supply today, after proposals for the supertram were watered down to a trolleybus system before being scrapped altogether in 2016. Leeds' rapid transformation in the 1960s and 70s, its grandiose visions of skywalks and dual carriageways, were driven by strong local political will. Today, the long-term transport strategy documents on Leeds City Council's website say more about HS2 than the need for a mass transit system within Leeds itself, and the council has been accused of giving up the fight for light rail and trams.

Whilst central government's refusal to grant funds is the greatest obstacle to Leeds' development, the local authority needs to be far more vocal in demanding the transport system the city deserves. Leeds' desire to be the Motorway City of the Seventies might look ludicrous today, but the political drive and utopian optimism that underpinned it does not.