Urban Heat Island

Designing for the urban climate

How do you factor in something as inherently complex as the urban environment into building design? Dane Virk, an Engineering Doctorate student at UCL, working with CIBSE, outlines some of the issues surrounding the topic.

Increasing urbanisation combined with projected changes in climate have the potential to exacerbate the negative impact of overheating within buildings. As a result, the relationship between developments and urban climates will become increasingly important. To factor these effects into urban design, planners need to understand how the physical properties of urban environments contribute to this warming, and what tools and data are available to them. Unfortunately, the physics of urban environments is highly complex and the effects vary from building to street to neighbourhood and on to city scales.

Urban Heat Island effect

One of the best documented phenomena of urban warming is the nocturnal Urban Heat Island (UHI) effect. UHIs occur when urban areas have higher temperatures than the rural surroundings, the difference being the ‘UHI intensity’. This is a result of the form and fabric that make up urban landscapes. In London, the nocturnal UHI has been shown to reach up to 9°C.

The materials used in buildings have a greater capacity to store heat than greener rural areas. They absorb greater amounts of incoming solar radiation during the day and then release it at night. Buildings, pavements and roads usually have darker surfaces compared to rural landscapes, increasing their ability to absorb heat. The formation of street canyons and increased building density in cities also helps trap solar radiation, whilst reducing the rate of cooling at night. This spatial density reduces wind speeds, and limits the amount of heat transported away from the urban environment.

Urban areas also have less vegetation and vegetated surfaces, which reduces the amount of heat lost from surfaces by evapotranspiration. This explains why larger urban parks and bodies of water often have cooler air temperatures than the surrounding built up area. In addition, heat and moisture emissions are produced by transport, buildings and humans, and contribute to this urban warming.

At the city scale, larger, densely populated cities have been shown to result in higher heat intensities due to the net impact of these factors. However, at a more local scale this intensity will vary across an urban area. At these smaller scales, microclimatic factors become increasingly relevant. There is a good correlation between the radial distance of a site from an urban centre and its air temperature; the closer to the centre, the more the temperature rises.

In the not too distant future…

Modelling the impact of various design interventions and mitigations strategies, such as urban greening to abate urban warming should become part of the design process. The outputs from models that simulate urban climates can be used as inputs into building simulation models, to test how these interventions could impact building performance. Buildings and their surrounding environments can be designed to perform better under future warmer climate scenarios. This can only be achieved if designers understand the physical effects of possible abatement measures at different spatial and temporal scales and have accessible decision support tools and data available to them.

Technical detail explained – simulating conditions to design better buildings

Dynamic thermal simulation tools can be used to simulate overheating within buildings. These tools require external weather data as inputs. The Chartered Institution of Building Service Engineers (CIBSE) provides a series of Design Summer Year (DSY) weather files, available for 14 UK locations that can be used in overheating analysis. In order to factor in the UHI in London, CIBSE has developed new DSYs for London that give designers the choice of three different locations within the city for three years of varying heatwave severity. The London Weather Centre weather file represents a central urban location, Heathrow represents a semi-urban location and Gatwick represents a rural location. CIBSE is in the process of selecting and developing new weather files for other UK cities using the same methodology. Research has shown that both Birmingham and Manchester  have significant UHIs and early indications have shown that there are suitable sites for new weather files.

These weather files have been shown to successfully represent the impact the UHI effect has on overheating. However, at more local level, microclimatic factors could have a greater impact. There is software available in the form of urban models that can simulate these effects, which vary in complexity and technical knowledge needed. These models can dynamically simulate variations in urban environments, using the physical properties of land use types and topography as inputs and outputting a range of variables such as air temperature and wind speeds. Analysis can range from pedestrian thermal comfort in a street, to the impact of neighbourhood scale land use changes on air temperatures. A review of some these models is outlined in a forthcoming Zero Carbon Hub report on overheating.

Using the tools and data outlined, the city scale UHI effect can now be usefully modelled. However, modelling how neighbourhood scale developments such as Kings Cross, Nine Elms or the proposed redevelopment of Old Oak Common impact the local environment will require use of more complex urban models. Another issue is that current regulations do not require quantitative evidence of these impacts. Certification schemes such as BREEAM Communities do encourage evidence based planning, but their needs to be stricter regulations in order to improve urban design across the whole city.

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