Cities are where people have most transformed nature, replacing vegetation with roofed and paved surfaces, burying stream channels, creating indoor climates, and making huge artificial transfers of energy water and materials. Expanding cities transform hydrological relationships, changing the magnitude and frequency of flooding. Rising land prices often mean that homes are built on relatively unstable slopes or on the floodplains of rivers. The poor, especially in dense squatter settlements in third world cities, often have no choice but to occupy hazardous sites on steep slopes, close to rivers, or near polluting factories. All too often, their settlements are vulnerable to road collapse, water pipe breakages and sewer failures and to floods, landslides or subsidence.
Two aspects of this vulnerability are of special significance to geographers: the differing vulnerability of social groups and communities within the city; and the way in which expanding cities increase in vulnerability through time as they spread across more hazardous sites and occupy more unstable terrain. Knowledge of urban hydrology and urban geomorphology is not only a key to good urban planning but should also be available to every house purchaser. The home builder or buyer should ‘know the ground being built upon’.
Nature is widespread in cities. Only a small part of an urban area is completely paved and roofed. In European and North American cities, many suburban areas are dominated by green vegetation. Urban areas often have greater biodiversity and wildlife protection than adjacent intensively cropped farming country, 3000 different species having been recorded in a single suburban garden in Leicester, England.
Land prices and policies encouraging house building on existing urban land lead to denser occupation of urban land. Gardens are partially changed to impermeable, paved surfaces modifying natural air, water, materials and energy flows. Relatively little of the urban area is completely covered by roofs or paved surfaces and thus totally impervious. Satellite imagery showed only 0.9 km2 of the Bolton urban area in England to be entirely impervious, 6.5 km2 was 82 per cent impervious, while 25 km2 was 45 per cent impervious (Adi 1990).
Cities have little direct impact on the global radiation balance, but the internal urban climate produced by the absorption and subsequent reradiation of heat from the surfaces of the built enrivonment, and by the emission of artificial heat through combustion create an urban heat-island effect. Cities are warmer than the surrounding countryside at night and often,