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.


 

 
 
 
 

Does it matter that TfL are renaming White Hart Lane station Tottenham Hotspur?

New White Hart Lane. Image: Getty.

Pretend for a moment that you’re travelling in the London of 1932. You’re taking the Piccadilly Line northbound and alight at Gillespie Road station. The name should be obvious: it’s inscribed in bespoke brown tiling on the platform.

But that 31 October, following an intense campaign by the eponymous football club, the London County Council changed the station’s name to Arsenal (Highbury Hill). The area’s growing association with the name “Arsenal” ended in a lengthy negotiation that changed maps, signs and train tickets alike. Football had acquired so much power that it changed the name of not just a Tube station but an entire suburb, even before the era of Wenger or the Emirates.

Now the spectre of name changes is on the horizon once again. As Tottenham Hotspur FC inches closer to completing its new stadium, the club is clamouring for a renamed Overground station. Despite the fact the new stadium is located on almost exactly the same site as the old just off White Hart Lane, and fans have long been calling the scaffolding-laden mess “New White Hart Lane”, the club’s executive director is adamant that the station’s existing name cannot stand. White Hart Lane station, on the Overground line leaving Liverpool Street, is set to be renamed “Tottenham Hotspur”, at a cost to the club of £14.7m.

Little has been made of the fact that this peculiar PR kerfuffle is tied to Spurs’ failure to convince Nike to sponsor the venue. Some sources have even claimed that the sponsorship is yet to be finalised because it is somehow contingent on the renaming of the Overground station; beyond the ridiculous Johnson-era vanity project that was the Emirates Air Line, it seems improbable that TfL will allow any more corporate-flavoured information pollution. There will be no “Nike Stadium” station on the way to Enfield, much as there is no “Emirates” on the way to Cockfosters, especially if public consultation gets a look in.

The scene of the crime. Image: TfL.

But there’s a problem with the new name, all the same. “White Hart Lane” already means “football stadium”, in the same way Loftus Road or Stamford Bridge do. Changing it to “Tottenham Hotspur” risks opening the floodgates to an “O2 North Greenwich” or a “Virgin Euston” at some point in future, names as banal as there are dystopian. The Greater London Authority has promised to spend the £14.7m fee on community programmes in the local area – but that’s not much money to set the precedent that a private company can mess about with the Tube map.


What’s more, as CityMetric has often observed, there are plenty of station names across London that could do with a tidy up. Picking one that’s perfect already and asking for £14.7m to change it is adding insult to injury. How much would it cost a community group if they asked to change the name of Goodge Street to Fitzrovia? Why does a vast corporate entity backed by international sponsors and thousands of season ticket holders get to set the standard?

Back in Arsenal’s day, changing names on the Tube must have been easy; changes could be accommodated gradually without bothering the every day traveller. But in our world of online information, maps and apps, name changes are rather more complicated.

The question is – if TfL can bring itself to balefully accept this particular proposition, why can’t it accept ours? Why sort out a single non-issue on the Tube Map when you can catch lots of real ones in one go? A day’s pandemonium might just be a price worth paying to fix the Bethnal Greens problem once and for all.