Concrete Sector Sustainability Working Group (CSSWG)

 

 

 

Materials commonly used in urban areas, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) than the surrounding rural areas. This initiates a change in the energy balance of the urban area, often causing it to reach higher temperatures (measured both on the surface and in the air) than its surroundings. The energy balance is also affected by the lack of vegetation and standing water in urban areas, which inhibits cooling by evapotranspiration.

 

Other causes of a UHI are due to geometric effects. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the 'canyon effect'. Another effect of buildings is the blocking of wind, which also inhibits cooling by convection.

 

Some causes of a UHI are anthropogenic (i.e. man-made), though they are relatively minor in summer and generally in low- and mid-latitude areas. In winter and especially in high latitudes, when solar radiation is considerably smaller, these effects can contribute the majority of UHI. As urban areas are often inhabited by large numbers of people, heat generation by human activity also contributes to the UHI. Such activities include the operation of automobiles, air conditioning units, and various forms of industry. High levels of pollution in urban areas can also increase the UHI, as many forms of pollution can create a local greenhouse effect.

 

Heat islands form as vegetation is replaced by asphalt and concrete for roads, buildings and other structures necessary to accommodate growing populations. These surfaces absorb - rather than reflect - the sun's heat, causing surface temperatures and overall ambient temperatures to rise.

The lesser-used term heat island refers to any area, populated or not, which is consistently hotter than the surrounding area.

 

As UHIs are characterised by increased temperature, they can potentially increase the magnitude and duration of heat waves within cities. Research has found that the mortality rate during a heat wave increases exponentially with the maximum temperature, an effect that is exacerbated by the UHI.

 

Another consequence of urban heat islands is the increased energy required for air conditioning and refrigeration in cities that are in comparatively hot climates.

 

Aside from the obvious effect on temperature, UHIs can produce secondary effects on local meteorology, including the altering of local wind patterns, the development of clouds and fog, the number of lightning strikes and the rates of precipitation.

 

Climate change over the next few decades and beyond is likely to have a major impact on the both the frequency of occurrence and magnitude of extreme UHI events.

 

The heat island effect can be counteracted slightly by using white or reflective materials that have a high albedo such as concrete to build houses, pavements, and roads, thus increasing the overall albedo of the city. This is a long established practice in many countries. A second option is to increase the amount of evapotranspiration, by using concrete Sustainable Urban Drainage Systems and green roofs.

Evapotranspiration can be increased with the use of concrete sustainable urban drainage system.