Urban Climates

The Urban Heat Island Effect

Artificially created ‘Heat Islands’ are becoming increasingly common through the rise in urbanization. Approximately half of the world’s population now live in urban areas. For example, in central London the urban heat island enhances summer night time temperatures by 5-6 °C.

Contributing factors:

Anthropogenic heat release - Heat derived from industry and domestic central heating radiates from cities. Especially in high latitude cities and to a lesser extent in mid-latitude cities like Montereal in Canada, which reached a record maximum of 12°C warmer than surroundings in 1971.

The fabric of a city - Heat retaining (non-reflective) and emitting properties of building materials means that they absorb much of the incident radiation, which in turn is released as heat. This is called the albedo effect.

The lack of vegetation within towns and cities means that less light is intercepted, less evapotranspiration, thus less vapour is released (which naturally cools air surrounding vegetation).

Canyon Geometry - The close arrangement of high-rise buildings form urban street canyons found in most cities also contribute to the trapping of heat energy. The dynamics leading to this process is due to the reduced sky view factor that inhibits the escape of reflected radiation back into the atmosphere, causing the radiation to be absorbed by neighbouring structures.

Within the urban canopy layer (from ground level to roof top) turbulence caused by air flow constricted to urban street canyons, even in light wind conditions, is sufficient to thoroughly mix atmosphere around and above a city, thus affecting the dispersion of air pollution. However, the layout of a city and the characteristics of its buildings can have a profound effect upon circulation. Funnelling of wind between buildings causes localised increase in wind velocity of up to 3 times greater than aloft (otherwise known as the Venturi effect) and clusters of buildings can impede airflow, entrapping pollution leading to potentially harmful concentrations accumulating. Overall wind flow is retarded by the surface roughness of a city, although there is a higher prevalence of gusts brought about by the Venturi effect.

The urban heat island effect is most detectable at night when the urban atmosphere cools slower than surrounding rural areas.

Encapsulates all of the processes within the urban heat island.

Innovations to counteract the heat island effect:

 Ø   The drive to create living roofs in a number of global cities, including New York, where a tax rebate is offered for home owners who install green roofs in metropolitan areas.

Visit Living Roofs for further details: 

http://livingroofs.org/2010030566/green-roof-benefits/heatisland.html

Ø     Use materials that have a higher solar reflectance (albedo).

Visit Concrete Thinking for a Sustainable World for an analysis of how concrete can reduce the heat island effect through increasing surface albedo:

http://concretethinker.com/solutions/Heat-Island-Reduction.aspx