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.


 

 
 
 
 

Community-powered policies should be at the top of Westminster’s to do list

A generic election picture. Image: Getty.

Over the past five decades, political and economic power has become increasingly concentrated in the UK’s capital. Communities feel ignored or alienated by a politics that feels distant and unrepresentative of their daily experiences.

Since the EU referendum result it has become something of a cliché to talk about how to respond to the sense of powerlessness felt by too many people. The foundations of our economy have been shifted by Brexit, technology and deindustrialisation – and these have shone a light on a growing divergence in views and values across geographies and generations. They are both a symptom and cause of the breakdown of the ties that traditionally brought people together.

As the country goes through seismic changes in its outlook, politics and economy, it is clear that a new way of doing politics is needed. Empowering people to take control over the things that affect their daily lives cannot be done from the top down.

Last week, the Co-operative Party launched our policy platform for the General Election – the ideas and priorities we hope to see at the top of the next Parliament’s to do list. We have been the voice for co-operative values and principles in the places where decisions are made and laws are made. As co-operators, we believe that the principles that lie behind successful co‑operatives – democratic control by customers and workers, and a fair share of the wealth we create together – ought to extend to the wider economy and our society. As Labour’s sister party, we campaign for a government that puts these shared values into practice.

Our policy platform has community power at its heart, because the co-operative movement, founded on shop floors and factory production lines, knows that power should flow from the bottom up. Today, this principle holds strong – decisions are best made by the people impacted the most by them, and services work best when the service users have a voice. Our policy platform is clear: this means shifting power from Whitehall to local government, but it also means looking beyond the town hall. Co-operative approaches are about placing power directly in the hands of people and communities.


There are many great examples of Co-operative councillors and local communities taking the lead on this. Co-operative councils like Oldham and Plymouth have pioneered new working relationships with residents, underpinned by a genuine commitment to working with communities rather than merely doing things to them.

Building a fairer future is, by definition, a bottom-up endeavour. Oldham, Plymouth and examples like the Elephant Project in Greater Manchester, where people with experience of disadvantage are involved in decision-making, or buses in Witney run by Co-operative councillors and the local community – are the building blocks of creating a better politics and a fairer economy.

This thread runs through our work over the last few years on community wealth building too – keeping wealth circulating in local economies through growing the local co-operative sector. Worker-owned businesses thriving at the expense of global corporate giants and private outsourcers. Assets owned by communities – from pubs to post offices to rooftop solar panels.

And it runs through our work in Westminster too – with Co-operative MPs and peers calling for parents, not private business, to own and run nurseries; for the stewards of our countryside to be farmers rather than big landowners; and for workers to have a stake in their workplaces and a share of the profit.

Far from being ignored, as suggested in last week’s article on community power, our work has never been more relevant and our co-operative voice is louder than ever.

Anna Birley is policy offer at the Co-operative party.