Box 10E The urban heat island

The process of urbanisation produces radical changes in the nature of the surface and atmospheric properties of a region. It involves the transformation of the radiative, thermal, moisture and aerodynamic characteristics, and thereby dislocates the natural solar and hydrologic balances. For instance, the dense urban construction materials make the system store heat and waterproof the surface; the block-like geometry creates the possibility of radiation trapping, air stagnation or undesirable increased wind speed at pedestrian level depending on the height-to-width ratio of buildings. The seemingly inevitable increase of air pollution affects the radiation balance and supplies extra nuclei around which cloud droplets may form.

It has been estimated that the air above cities such as Berlin or Brussels is on the average 2 per cent drier than in the surroundings in winter, and 8 to 10 per cent drier in summer (Table 10.6). Compared with rural areas, clouds above these cities occur between 5 and 10 per cent more frequently, fog in winter twice as often, and in summer one third more. Rainfall is increased by 5 to 10 per cent, and there is 5 per cent less snow (Horbert et al, 1982).

The urban heat island, by which the air in the urban canopy is warmer than that in the surrounding countryside, is probably both the clearest and the best-documented example of inadvertent climate modification. The intensity of the urban heat island measured as the maximum temperature difference between city centre and the surrounding rural area is found to be proportional to the logarithmic of the urban population. In fact there is a better correspondence with the urban density expressed as the height-to-width ratio of the street canyons in the city centre. The commonly hypothesised causes of the urban heat island are illustrated in Table 10.7.


Table 10.6 - Average changes of climate parameters in built-up areas

Source: Horbert et al, 1982

Climate parameters Characteristics In comparison to
the surrounding area
Air pollution Condensation 10 times more
Gaseous pollution 5­15 times more
Solar radiation Global solar radiation 15­20% less
Ultraviolet radiation (winter) 30% less
Ultraviolet radiation (summer) 5% less
Duration of sunshine 5­15% less
Air temperature Annual mean average 0.5­1.5°C higher
On clear days 2­6°C higher
Wind speed Annual mean average 10­20% less
Calm winds 5­20% more
Relative humidity Winter 2% less
Summer 8­10% less
Clouds Overcast 5­10% more
Fog (winter) 100% more
Fog (summer) 30% more
Precipitation Total rainfall 5­10% more
Less than 5 mm rainfall 10% more
Daily snowfall 5% less


Table 10.7 - Possible causes of the urban heat island

Source: Oke, 1987

Altered energy balance terms
leading to positive thermal
anomaly
Features of urbanisation
underlying energy balance
changes
1 Increased absorption of solar radiation Canyon geometry ­ increased surface area and multiple reflection
2 Increased long-wave radiation from the sky Air pollution ­ greater absorption and re-emission
3 Decreased long-wave radiation loss Canyon geometry ­ reduction of sky view factor
4 Anthropogenic heat source Building and traffic heat losses
5 Increased sensible heat storage Construction materials ­ increased thermal admittance
6 Decreased evapo-transpiration Construction materials ­ increased 'water-proofing'
7 Decreased total turbulent heat transport Canyon geometry ­ reduction of wind speed