Why are cities so much hotter than the surrounding areas?

Scorchio! London. Image: pic fix/Flickr/creative commons.

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.

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 Ontari/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.

Laying a cool roof. Image: NNSANews/Flickr/creative commons.

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.

The ConversationAs 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.

Gerald Mills, Senior Lecturer in Geography, University College Dublin.

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

 
 
 
 

To make electric vehicles happen, the government must devolve energy policy to councils

The future. Image: Getty.

Last week, the Guardian revealed that at least a quarter of councils have halted the roll-out of electric vehicle (EV) charging infrastructure with no plans to resume its installation. This is a fully charged battery-worth of miles short of ideal, given the ambitious decarbonisation targets to which the UK is rightly working.

It’s even more startling given the current focus on inclusive growth, for the switch to EVs is an economic advancement, on an individual and societal level. Decarbonisation will free up resources and push growth, but the way in which we go about it will have impacts for generations after the task is complete.

If there is one lesson that has been not so much taught to us as screamed at us by recent history, it is that the market does not deliver inclusivity by itself. Left to its own devices, the market tends to leave people behind. And people left behind make all kinds of rational decisions, in polling stations and elsewhere that can seem wholly irrational to those charged with keeping pace – as illuminted in Jeremy Harding’s despatch from the ‘periphery’ which has incubated France’s ‘gilet jaunes’ in the London Review of Books.

But what in the name of Nikola Tesla has any of this to do with charging stations? The Localis argument is simple: local government must work strategically with energy network providers to ensure that EV charging stations are rolled out equally across areas, to ensure deprived areas do not face further disadvantage in the switch to EVs. To do so, Ofgem must first devolve certain regulations around energy supply and management to our combined authorities and city regions.


Although it might make sense now to invest in wealthier areas where EVs are already present, if there isn’t infrastructure in place ahead of demand elsewhere, then we risk a ‘tale of two cities’, where decarbonisation is two-speed and its benefits are two-tier.

The Department for Transport (DfT) announced on Monday that urban mobility will be an issue for overarching and intelligent strategy moving forward. The issue of fairness must be central to any such strategy, lest it just become a case of more nice things in nice places and a further widening of the social gap in our cities.

This is where the local state comes in. To achieve clean transport across a city, more is needed than just the installation of charging points.  Collaboration must be coordinated between many of a place’s moving parts.

The DfT announcement makes much of open data, which is undoubtedly crucial to realising the goal of a smart city. This awareness of digital infrastructure must also be matched by upgrades to physical infrastructure, if we are going to realise the full network effects of an integrated city, and as we argue in detail in our recent report, it is here that inclusivity can be stitched firmly into the fabric.

Councils know the ins and outs of deprivation within their boundaries and are uniquely placed to bring together stakeholders from across sectors to devise and implement inclusive transport strategy. In the switch to EVs and in the wider Future of Mobility, they must stay a major player in the game.

As transport minister and biographer of Edmund Burke, Jesse Norman has been keen to stress the founding Conservative philosopher’s belief in the duty of those living in the present to respect the traditions of the past and keep this legacy alive for their own successors.

If this is to be a Burkean moment in making the leap to the transformative transport systems of the future, Mr Norman should give due attention to local government’s role as “little platoons” in this process: as committed agents of change whose civic responsibility and knowledge of place can make this mobility revolution happen.

Joe Fyans is head of research at the think tank Localis.