Tropical urban climates

A striking feature of recent and projected world population growth is the relative increase in the tropics and subtropics. Today there are thirty-four world cities with more than five million people, twenty-one of which are in the less-developed countries. By ad 2025 it is predicted that, of the thirteen cities that will have populations in the twenty to thirty-million range, eleven will be in less-developed countries (Mexico City, Sao Paulo, Lagos, Cairo, Karachi, Delhi, Bombay, Calcutta, Dhaka, Shanghai and Jakarta).

Despite the difficulties in extrapolating knowledge of urban climates from one region to another, the ubiquitous high-technology architecture of most modem city centres and multi-storey residential areas will tend to impose similar influences on their differing background climates. Nevertheless, most tropical urban built land differs from that in higher latitudes; it is commonly composed of high-density, single-storey buildings with few open spaces and poor drainage. In such a setting, the composition of roofs is more important than that of walls in terms of thermal energy exchanges, and the production of anthropogenic heat is more uniformly distributed spatially and is less intense than in European and North American cities. In the dry tropics, buildings have a relatively high thermal mass to delay heat penetration, and this, combined with the low soil moisture in the surrounding rural areas, makes the ratio of urban to rural thermal admittance greater than in temperate regions. However, it is difficult to generalize about the thermal role of cities in the dry tropics where urban vegetation can lead to 'oasis' effects. Building construction in the humid tropics is characteristically lightweight to promote essential ventilation. These cities differ greatly from temperate ones in that the thermal admittance is greater in rural

Table 12.3 Average urban climatic conditions compared with those of surrounding rural areas.

Atmospheric composition carbon dioxide X2

sulphur dioxide X200

nitrogen oxides X 10

carbon monoxide X200(+)

total hydrocarbons X20

particulate matter X3 to 7

Radiation global solar -15 to 20%

ultraviolet (winter) -30%

sunshine duration -5 to 15%

Temperature winter minimum (average) + 1 to 2°C

heating degree days -10%

Wind speed annual mean -20 to 30%

number of calms +5 to 20%

Fog winter +100%

summer +30%

Precipitation total +5 to 10%

Source: Partly after World Meteorological Organization (1970).

than in urban areas due to high rural soil moisture levels and high urban albedos.

Tropical heat island tendencies are rather similar to those of temperate cities but are usually weaker, with different timings for temperature maxima, and with complications introduced by the effects of afternoon and evening convective rainstorms and by diurnal breezes. The thermal characteristics of tropical cities differ from those in mid-latitudes because of dissimilar urban morphology (e.g. building density, materials, geometry, green areas) and because they have fewer sources of anthropogenic heat. Urban areas in the tropics tend to have slower rates of cooling and warming than do the surrounding rural areas, and this causes the major nocturnal heat island effect to develop later than in mid-latitudes (i.e. around sunrise (Figure 12.31A)). Urban climates in the subtropics are well illustrated by four cities in Mexico (Table 12.4). The heat island effect is, as expected, greater for larger cities and best exemplified at night during the dry season (November to April), when anticyclonic conditions, clear skies and inversions are most common (Figure 12.31B). It is of note that in some tropical coastal cities (e.g. Veracruz; Figure 12.31A), afternoon urban heating may produce instability that reinforces the sea breeze effect to the point where there is a 'cool island' urban effect. Elevation may play a significant thermal role (Table 12.4), as in Mexico City, where the urban heat island may be accentuated by rapid nocturnal cooling of the surrounding countryside. Quito, Ecuador (2851 m) shows a maximum heat island effect by day (as much as 4°C) and weaker night-time effects, probably due to the nocturnal drainage of cold air from the nearby volcano Pichincha.

Ibadan, Nigeria (population over one million; elevation 210 m), at 7°N, records higher rural than urban temperatures in the morning and higher urban temperatures in the afternoon, especially in the dry season (November to mid-March). In December, the harmattan dust haze tends to reduce city maximum temperatures. During this season, mean monthly minimum temperatures are significantly greater in the urban heat island than in rural areas (March + 12°C, but December only +2°C due to the atmospheric dust effect). In

hours months

Figure 12.31 Diurnal (A) and seasonal (B) heat island intensity variations (i.e. urban minus rural or suburban temperature differences) for four Mexican cities (see Table 12.4).

hours months

Figure 12.31 Diurnal (A) and seasonal (B) heat island intensity variations (i.e. urban minus rural or suburban temperature differences) for four Mexican cities (see Table 12.4).

Source: Jauregui (1987). Copyright © Erdkunde. Published by permission.

general, urban-rural minimum temperature differences vary between -2° and + 15°C. Two other tropical cities exhibiting urban heat islands are Nairobi, Kenya ( + 3.5°C for minimum temperatures and +1.6°C for maximum temperatures) and Delhi, India (+3 to 5°C for minimum temperatures and +2 to 4°C for maximum temperatures).

Despite insufficient data, there seems to be some urban precipitation enhancement in the tropics, which is maintained for more of the year than that associated with summer convection in mid-latitudes.

Table 12.4 Population (1990) and elevation for four Mexican cities.

Population (millions)

Elevation (metres)

Mexico City (I9°25'N)

15.05

2380

Guadalajara (20°40'N)

1.65

1525

Monterey (25°49'N)

1.07

538

Veracruz (I9°II'N)

0.33

SL

Source: Jauregui (1987).

Source: Jauregui (1987).

SUMMARY

Small-scale climates are determined largely by the relative importance of the surface energy budget components, which vary in amount and sign depending on time of day and season. Bare land surfaces may have wide temperature variations controlled by H and G, whereas those of surface water bodies are strongly conditioned by LE and advective flows. Snow and ice surfaces have small energy transfers in winter with net outgoing radiation offset by transfers of H and G towards the surface. After snow melt, the net radiation is large and positive, balanced by turbulent energy losses. Vegetated surfaces have more complex exchanges usually dominated by LE; this may account for >50 per cent of the incoming radiation, especially where there is an ample water supply (including irrigation). Forests have a lower albedo (<0.10 for conifers) than most other vegetated surfaces (0.20 to 0.25). Their vertical structure produces a number of distinct microclimatic layers, particularly in tropical rainforests. Wind speeds are characteristically low in forests and trees form important shelter belts. Unlike short vegetation, various types of tree exhibit a variety of rates of evapotranspiration and thereby differentially affect local temperatures and forest humidity. Forests may have a marginal topographic effect on precipitation under convective conditions in temperate regions, but fog drip is more significant in foggy/cloudy areas. The disposition of forest moisture is very much affected by canopy interception and evaporation, but forested catchments appear to have greater evapotranspiration losses than those with a grass cover. Forest microclimates have lower temperatures and smaller diurnal ranges than their surroundings.

Urban climates are dominated by the geometry and composition of built-up surfaces and by the effects of human urban activities. The composition of the urban atmosphere is modified by the addition of aerosols, producing smoke pollution and fogs, by industrial gases such as sulphur dioxide, and by a chain of chemical reactions, initiated by automobile exhaust fumes, which causes smog and inhibits both incoming and outgoing radiation. Pollution domes and plumes are produced around cities under appropriate conditions of vertical temperature structure and wind velocity. H and G dominate the urban heat budget, except in city parks, and as much as 70 to 80 per cent of incoming radiation may become sensible heat, which is distributed very variably between the complex urban built forms. Urban influences combine to give generally higher temperatures than in the surrounding countryside, not least because of the growing importance of heat production by human activities. These factors lead to the urban heat island, which may be 6 to 8°C warmer than surrounding areas in the early hours of calm, clear nights, when heat stored by urban surfaces is being released. The urban-rural temperature difference under calm conditions is related statistically to the city population size; the urban canyon geometry and sky view factors are major controlling factors. The heat island may be a few hundred metres deep, depending on the building configuration. Urban wind speeds are generally lower than in rural areas by day, but the wind flow is complex, depending on the geometry of city structures. Cities tend to be less humid than rural areas, but their topography, roughness and thermal qualities can intensify summer convective activity over and downwind of the urban area, giving more thunderstorms and heavier storm rainfall. Tropical cities have heat islands, but the diurnal phase tends to be delayed relative to mid-latitude ones. The temperature amplitude is largest during dry season conditions.

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