Urban Temperatures

Temperatures measured at street level in large cities can be several degrees higher than those in the surrounding countryside (Figure 3.17). Isotherms on a map of an isolated city and its environs look like the height contours of an island, which leads to the clumsy notion of an 'urban heat-island effect'; a simpler term is urban heating. It is evident in some cities during the day but is particularly apparent at night, being often greatest at about 9 p.m., because city surfaces cool more slowly than those in the suburbs. The consequence is that the daily minimum is raised more than the maximum.

Towns of only 10,000 in the USA show discernible urban heating. It can be considerable in larger places; the centre of Johannesburg may be as much as 11 K warmer than in suburban valleys on a dry winter's night. Measurements in central Sydney have shown temperatures 3 K higher than on the outskirts, and similar urban heating has been measured in Melbourne. The city of Nairobi (1°S) cools about 2 K less than outside during the night, and becomes about 0.3 K hotter in the day (Table 3.6).

There have been many studies of urban heating in the USA. Extensive studies in St Louis in Missouri (with a population of 600,000 within 1,150 km2) have shown urban heating of up to 3 K on summer afternoons. Urban heating in Chicago is most at 9 p.m. in August, least at 1 p.m. in April and October, and averages 1.9 K, or 2.8 K in the absence of wind or cloud. Overall, weather-station temperatures in the USA have an urban bias of +0.1 K for the monthly maximum temperatures and +0.4 K for the minima. The latter is the average of a range of

January: maximum

Figure 3 17 The distribution of (a) July minimum and (b) January maximum temperatures, in the vicinity of Brisbane, Australia.

Table 3.6 Urban heating, i.e. differences between temperatures inside Nairobi (1°S and 1,829 m elevation), and outside at the airport, during the dry month of January or the cloudy month of July, at the times of either minimum or maximum temperature, i.e. at about 4 a.m. and between 8 a.m. and noon, respectively

Table 3.6 Urban heating, i.e. differences between temperatures inside Nairobi (1°S and 1,829 m elevation), and outside at the airport, during the dry month of January or the cloudy month of July, at the times of either minimum or maximum temperature, i.e. at about 4 a.m. and between 8 a.m. and noon, respectively

+2.4 K to -1.1 K, for a large city and rural circumstances, respectively. It appears that a tenfold increase of city population raises the maximum urban heating (relative to the outskirts) by 2 K, on average.

Urban heating is greater on weekdays than at weekends because cities are busiest during the week. This may explain an observation that global temperatures of the lower atmosphere, observed by satellite over fourteen years, are higher during weekdays than at the weekend by a few hundredths of a degree.

The growth of cities has led to an increase of the temperatures measured at weather stations within them. In general, the daily minimum in cities in the USA rose between 1901 and 1984 by about 0.13 K, while the maximum remained steady. This effect in cities has to be borne in mind when examining past records for evidence of global warming.

Causes

Several factors are responsible for urban heating:

1 Artificial heat may be appreciable, especially in the largest industrialised cities. An astonishing 640 W/m2 has been quoted for Manhattan. Even in Sydney, where the rate of artificial heat generation is only a tenth as much, it is comparable with wintertime netradiation fluxes (Figure 2.19) and can equal half the incoming solar radiation.

2 The construction materials used in modern cities: concrete, brick, rock and bitumen all readily absorb the daytime heat and release it slowly to the atmosphere at night. The effect of this is to reduce slightly the daytime maximum temperature, which, in combination with the nocturnal urban heating, reduces the daily range.

3 The drainage of water from a city and the lack of evaporation from soil and plants prevent evaporative cooling, so that daytime temperatures are raised.

4 There may be a reduced albedo (Section 2.5) because of the relative absence or removal of snow, the replacement of vegetation by low-albedo material such as roadway bitumen, and because of the canyon structure of built-up areas, which trap radiation. As a result, solar heating of the city is increased.

One side-issue of urban heating may be an aggravation of the incidence of mob violence. Riots are rare in the USA when temperatures are below freezing, but the likelihood at 31°C apears to be twice that at 20°C (Figure 3.18). Why this should be so is for sociologists to explain.

Daytime urban heating in temperate climates can be reduced by two or three degrees if there are many trees in the streets and parks, causing evaporative cooling as well as shade. Places in desert areas may be cooled by evaporation from irrigated gardens and crops. However, vegetation has no effect in the evening, when transpiration has stopped.

Thus we end consideration of the manner in which some of the incident radiant energy is used in heating the ground and then the adjacent atmosphere. Much of the remaining net radiation is often used in evaporating water, which is discussed in the next chapter.

o 30

o 20

"O

daily maximum temperature: °C

Figure 318 The effect of the daily maximum temperature on the likelihood of riots in seventy-nine cities in the USA during the period 1967—71.

NOTES

3.A The transfer of sensible heat

3.B Effects of latitude and elevation on mean temperature 3.C High temperatures and human mortality 3.D Acclimatisation and adaptation 3.E Windchill 3.F Temperature and crops 3.G The annual range of monthly mean temperatures 3.H Cold nights

3.I Growing-degree-days and agriculture 3.J Degree-days and comfort 3.K The conduction of heat 3.L The thermal belt

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