Urban Heat Island Effect [albedo effect]

The urban heat island effect is the difference in temperature between urban areas and the surrounding countryside. In large cities this can as much as a 5^oC difference between the city centre and the rural environs. Urban areas have large areas of hard reflective surfaces. This is referred to as the albedo effect. These surfaces absorb solar radiation and reflect this heat back into the atmosphere. Any reduction in this effect can have a positive effect on smog and airborne particles in the atmosphere.

Roof areas are a significant part of urban hard surfaces. Plants on green surfaces absorb heat and then use it through evapotranspiration. Green roofs therefore would play an important role in reducing urban temperatures, and subsequent improvements in air pollution/smog, as associated with the albedo effect.

In Tokyo the albedo effect increases the humidity within the city and this, with increased air pollution, is one of the main reasons for the growing tendency for very complex intensive green roofs on many buildings in that City.

In many parts of the USA there is growing interest in this benefit of green roofs. Research by NASA in Atlanta has compared temperatures of different surfaces. On a typical Atlanta day with maximum air temperature of 25°C (77°F) the following temperatures were recorded:

Tree shaded grass 28°C

Tree canopy 21°C

Asphalt in full sun 50°C

Membrane roof surface 52°C

Research at Trent University has found on a typical day with a temperature of 18.4^oC a normal roof surface temperature was 32^oC while that of a green roof was 15^oC.

The reduction in the protection of photochemical smog and subsequent improvements in air quality needs to be recognised as a powerful planning ‘tool’ and potential mitigation for polluting developments. Local Authorities may include green roof plans as part of their commitment to Air Quality Management Areas [AQMA].

A study in Toronto to demonstrate the viability of green roofs. THe study focusses on a number of areas particularly air quality, energy and urban heat island.