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A melting pot for different peoples, the arts, fashion and international business, London is one of the world鈥檚 largest and most populous cities. But it isn鈥檛 simply a cultural hotspot. As a result of what鈥檚 known as the 鈥渦rban heat island鈥� effect, summer night-time temperatures can be up to 7潞C higher than temperatures in rural communities just 20km away. This phenomenon of excess heat will be at its lowest in the afternoon, reaching its most intense overnight.

In a climate cooler than London鈥檚, an urban heat island may bring benefits such as increased thermal comfort for more months of the year. It also means, however, that the effects of climate change may be felt first and most severely in cities, exacerbating the effects of global temperature rises on health and energy consumption.

There are definite relationships between the factors that affect the urban heat island intensity and thermal comfort, such as air speed, humidity and air temperature (see Table 1). However, predicting future heat island intensities is a problem as it is difficult to forecast what these factors will be. Attempts to predict micro-scale climate effects are also limited in their ability to deal with very local effects.

Studies have, though, shown that typology can affect the intensity of the urban heat island. In one study, definitions of eight categories of urban geometry and 鈥済reenness鈥� were created (see Table 2). These took into account things such as the nature of the surfaces at the site and the openness of the site to the sky for energy receipt and re-emission, both critical factors.

Overall, the eight categories represent increasing urbanisation, from rural fields to narrow inner- city back streets. In general it was found that between one and eight miles from the centre, the lower- category sites were cooler while the higher categories returned higher temperatures. The categorisations show the degree to which small variations in urban design can affect significantly the local heat island intensity and, in turn, indoor comfort.

So what methods can be employed to improve comfort in the urban environment during higher-temperature periods?

Facades and albedo

One of the key factors determining the intensity of an urban heat island is the absorption of solar energy on roofs and in the street gorges (building facades, roads and pavements).

One adjustable aspect is the colour or albedo (reflectivity to solar radiation) of a facade. A study of two similarly sized, typical street gorges in central London evaluated the effect of surface colour on surface temperature during a sunny day.

Each facade had the same azimuth and was not overshadowed during the testing, and both buildings were about 13m high. One facade was brick painted a dark colour (reflectivity 3%). The other was divided into an upper dark brick surface (reflectivity 8%) and a lower rendered brick with a matt white finish (reflectivity 50%). Reflectivities are quoted for visible radiation only.

Figure 2 shows that by 1pm there was a clear divergence, with the three dark surfaces having similar temperatures and the white surface being some 6-10掳C cooler.

This surface temperature affects the warming of the air in the street gorge. In a separate test, simultaneous measurements of the air temperature and the various gorge surface temperatures showed there was a strong correlation between the mean gorge surface temperature and the air temperature (at half gorge height). This was true for both cloudy and sunny days (see Figure 3).

Taken together, this suggests that the peak gorge air temperature could have been reduced 3-4掳C on the sunny day if all surfaces had been a matt white colour rather than predominantly almost black. For the lowest surface temperature, and, therefore, air temperature, a facade needs high solar reflectivity and high infrared emissivity. Caution, however, is needed when choosing very reflective surface finishes as these may produce glare.

Not only is the air temperature reduced when using more reflective surfaces but the radiant temperature experienced by the occupants of a street will be significantly reduced, improving their thermal comfort.

Shading

Shading a street from direct solar radiation will have the combined effect of reducing the temperatures of the street surfaces and shielding the occupants from direct solar radiation. Awnings are used over streets in Spain, for example, to improve the summertime comfort of pedestrians. These help to mitigate the increase in air temperature and the radiant thermal environment. However, sunlight should not be excluded at all times as it has beneficial effects for the health and welfare of the street users. Window shading (using brise soleil, for example) also provides a seasonally selective way of reducing solar gain inside buildings. Shading should be designed to be reflective to avoid reducing a street鈥檚 mean albedo and shifting internally absorbed solar gain into the street.

Vegetation

Tall, mature trees provide shade and also reduce the air temperature in an urban environment because they convert some of the solar energy into latent heat energy through evaporation of water and they intercept solar radiation high above street level.

If deciduous trees are used, they have the advantage of naturally increasing solar access in the winter, when more heating is required.

Central London has numerous parks between otherwise built-up areas. Figure 4 shows a typical example from a one-day test along a transect through a London park from a residential area to another residential/shopping area on the other side.

The air temperature clearly reduces inside the park and, in line with the wind direction, the cooler air from the park appeared to be cooling the street gorge directly beyond the park (sites 9 and 10) for a distance of 200m to 400m. (Site 7 was anomalous and the reason was not clear.) Similar cooling effects of vegetation have been reported in many other cities, including Gothenburg and Mexico City.

All vegetation reduces the air temperature, but only as long as there is a supply of water to match the evapotranspiration rate. This also applies to鈥済reen roofs鈥�. But even dried grass reduces solar absorption compared with normal roof coverings. Vegetation can also provide shelter from winds.

Street gorge ventilation

Allowing a good flow of air through the urban environment by alignment of the street gorges with prevailing winds will reduce the overall heat island intensity and provide cooling air movement for the people in the streets.

This air flow will enhance the thermal comfort if it is not excessive and is not highly turbulent around corners of buildings or other obstructions. Clearly, this is more an option for new developments than existing ones, but care is needed to avoid impeding the air flow in existing street gorges through further building.

Impact on energy use

Attention is usually focused on the summertime effects of elevated temperatures, overlooking the potential benefits in the winter. In a temperate climate, the possible need to cool buildings artificially in the summer has to be balanced against reduced heating in the winter. A more intense urban heat island will undoubtedly lead to higher energy use for cooling.

However, one of the most important considerations is whether the total energy used over a whole year is greater or less. This has been addressed in London by modelling the performance of a standard air-conditioned office when located in different areas of the heat island. The greater degree of urban overshadowing (and thus protection from solar gain) of one building over another was included in the model.

Figure 5 shows how the urban heat island affects the annual heating and cooling load for an air-conditioned building in London according to its location. Cooling load is lowest at the category one sites (in more rural areas) and peaks at category five sites before reducing at generally more central sites. Heating load tends to vary inversely to this.

Figure 6 clarifies the effect of site on total load demand (heating and cooling) and shows a line of the mean values for each site category. This illustrates that the average total load demand peaks at category five sites and falls either side of this, and that there is residual variation unexplained by site categorisation alone. The other main factor is radial distance from the centre of a city.

For buildings that have air-conditioning, the annual balance of the effect of the heat island depends on the level of internal gains. Higher gains will reduce the annual need for heating and reduce the benefit of the heat island.

Overall, the annual cooling load at the centre of London was found to be 25% more than at a rural site. At the most, overshadowing reduces the cooling load to 14% more than at a rural site.

Heating load decreases towards the centre but, on balance, total annual load (for heating and cooling) rises towards the centre to 8.5% more than rural use, and then reduces at the most overshadowed sites.