B Moisture

The absence of large bodies of standing water in urban areas and the rapid removal of surface runoff through drains reduces local evaporation. The lack of extensive vegetation cover eliminates much evapotranspiration, and this is an important source of augmenting urban heat. For these reasons, the air of mid-latitude cities has a tendency towards lower absolute humidity than that of their surroundings, especially under conditions of light winds and cloudy skies. During calm, clear weather, the streets trap warm air, which retains its moisture because less dew is deposited on the warm surfaces of the city. Humidity contrasts between urban and rural areas are most noticeable in the case of relative humidity, which can be as much as 30 per cent less in the city by night as a result of the higher temperatures.

Urban influences on precipitation (excluding fog) are much more difficult to quantify, partly because there are few rain gauges in cities and partly because turbulent flow makes their 'catch' unreliable. It is fairly certain, however, that urban areas in Europe and North America are responsible for local conditions that, in summer especially, can trigger off excesses of precipitation under marginal conditions. Such triggering involves both thermal effects and the increased frictional convergence of built-up areas. European and North American cities tend to record 6 to 7 per cent more days with rain per year than their surrounding regions, giving a 5 to 10 per cent increase in urban precipitation. Over southeast England between 1951 and 1960, summer thunderstorm rain (which comprised 5 to 15 per cent of the total precipitation) was especially concentrated in west, central and south London, and contrasted strikingly with the distribution of mean annual total rainfall. During this period, London's thunderstorm rain was 20 to 25 cm greater than that in rural southeast England. The effect is generally more marked in the cold season in North America, although urban areas in the Midwest

Figure 12.30 Anomalies of summer rainfall, rate of heavy rains, hail frequency and thunderstorm frequency downwind of the St Louis, MO, metropolitan area. Large arrows indicate the prevailing direction of motion of summer rain systems.

Source: After Changnon (1979). Reprinted by permission of the Journal of the American Planning Association.

Figure 12.30 Anomalies of summer rainfall, rate of heavy rains, hail frequency and thunderstorm frequency downwind of the St Louis, MO, metropolitan area. Large arrows indicate the prevailing direction of motion of summer rain systems.

Source: After Changnon (1979). Reprinted by permission of the Journal of the American Planning Association.

significantly increase summer convective activity. More frequent thunderstorms and hail occur for 30 to 40 km downwind of industrial areas of St Louis, compared with rural areas (Figure 12.30). The anomalies illustrated here are among the best-documented urban effects. Many of the urban effects here are based on case studies. Table 12.3 gives a summary of average climatic differences between cities and their surroundings.

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