D Heat islands

The net effect of urban thermal processes is to make city temperatures in mid-latitudes generally higher than in the surrounding rural areas. This is most pronounced after sunset during calm, clear weather, when cooling rates in the rural areas greatly exceed those in the urban areas. The energy balance differences that cause this effect depend on the radiation geometry and thermal

Table 12.2 Energy budget figures (W m-2) for the Cincinnati region during the summer of 1968.


Central business district

Surrounding country



I300 2000




Short-wave, incoming (Q + q)


763 -




Short-wave, reflected [(Q + q)a]






Net long-wave radiation (Ln)


-I00 -98




Net radiation (Rn)


543 -98




Heat produced by human activity


29 26i




Notes: 'Pollution peak.

fAn urban surface reflects less than agricultural land, and a rough skyscraper complex can absorb up to six times more incoming radiation.

^Replaces more than 25 per cent of the long-wave radiation loss in the evening. Source: From Bach and Patterson (1966).

Cooling tower

Nuclear power plant (1500 MW)

■JRefinery (6m t/yr) Cumulus cell

Steel mill


, Moscow

^Montreal *

Budapest. Tokyo WashingtonQ 0 Chicago


Sheffield Munich




Boston-Washington N Donetz Basin o Earlier l97os • Later l97os

N. America g

Extraterrestrial solar radiation

Net radiation " at earth's surface

Available potential energy

- Photosynthesis

Geothermal heat lo-3 lo-2 lo-1 1

lo lo2 lo3 lo4 lo5 lo6 lo7 lo8

Figure 12.26 A comparison of natural and artificial heat sources in the global climate system on small, meso- and synoptic scales. Generalized regressions are given for artificial heat releases in the 1970s (early 1970s circles, late 1970s dots), together with predictions for 2050.

Sources: Modified after Pankrath (1980) and Bach (1979).

Manhattan properties of the surface. It is thought that the canyon geometry effect dominates in the urban canopy layer, whereas the sensible heat input from urban surfaces determines the boundary layer heating. By day, the urban boundary layer is heated by increased absorption of short-wave radiation due to the pollution, as well as by sensible heat transferred from below and entrained by turbulence from above.

The heat island effect may result in minimum urban temperatures being 5 to 6°C greater than those of the surrounding countryside. These differences may reach 6 to 8°C in the early hours of calm, clear nights in large cities, when the heat stored by urban surfaces during the day (augmented by combustion heating) is released. Because this is a relative phenomenon, the heat island effect also depends on the rate of rural cooling, which is influenced by the magnitude of the regional environmental lapse rate.

For the period 1931 to 1960, the centre of London had a mean annual temperature of 11.0°C, compared with 10.3°C for the suburbs, and 9.6°C for the surrounding countryside. Calculations for London in the 1950s indicated that domestic fuel consumption gave rise to a 0.6°C warming in the city in winter and this accounted for one-third to one-half of the average city heat excess compared with adjacent rural areas. Differences are most evident during still air conditions, especially at night under a regional inversion (Figure 12.27). For the heat island effect to operate effectively there must be wind speeds of less than 5 to 6 m s-1. It is especially apparent on calm nights during summer and early autumn, when it has steep cliff-like margins on the upwind edge of the city and the highest temperatures are associated with the highest density of urban dwellings. In the absence of regional winds, a well-developed heat island may generate its own inward local wind circulation at the surface. Thus the thermal contrasts of a city, like many of its climatic features, depend on its topographic situation and are greatest for sheltered sites with light winds. The fact that urban-rural temperature differences are greatest for London in summer, when direct heat combustion and atmospheric pollution are at a minimum, indicates that heat loss from buildings by radiation is the most important single factor contributing to the heat island effect. Seasonal differences are not necessarily the same, however, in other macroclimatic zones.

The effects on minimum temperatures are especially marked. For central Moscow, winter extremes below -28°C occurred only eleven times during 1950 to 1989 compared with twenty-three cases at Nemchinovka west of the city. Cologne, Germany, has an average of 34 per cent fewer days with minima below 0°C than its surrounding area. In London, Kew has an average of seventy-two more days with frost-free screen temperatures than rural Wisley. Precipitation characteristics are also affected; incidences of rural snowfall are often associated with either sleet or rain in the city centre.

Figure 12.27 Distribution of minimum temperatures (°C) in London on 14 May 1959, showing the relationship between the 'urban heat island' and the built-up area.

Source: After Chandler (1965).

Heat Island Chandler

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  • ofelia
    Are urban heat islands causing globe, warming?
    2 years ago
  • lena
    What is a heat island weather and climate?
    3 years ago