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 areasSource: Horbert et al, 1982Climate parameters Characteristics In comparison tothe 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 islandSource: Oke, 1987Altered energy balance termsleading to positive thermalanomaly Features of urbanisationunderlying energy balancechanges 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
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.
Source: Horbert et al, 1982
Table 10.7 - Possible causes of the urban heat island
Source: Oke, 1987