Urban areas have climates which are noticeably different from surrounding rural areas, and which in turn affect the type of flora and fauna that may flourish. The climatic differences increase with increasing size of the urban area, and show a gradient from the suburbs to the city centre.
Average temperatures are higher throughout the year, particularly at night, due to the conductive property of hard surfaces and buildings, and heat generation by industry, transport and other activities within the city. Precipitation is drained away artificially, with less absorption into the ground, giving generally drier soils. The reduced evapotranspiration means low humidity, resulting in a climate which has similarities with the Mediterranean.
This has several effects:
- The period of winter inactivity is generally shortened. Both introduced and native species of plants may flower earlier in spring, and stay in growth later into the autumn. Winter-flowering species flower well. These in turn support invertebrates, birds and animals, which may thus survive in relatively high numbers and be less liable to ‘crash’ in periods of severe weather. Cold winter temperatures still occur in urban areas, but in general winters are less harsh than in the rural surrounds.
- Particular species of plants and animals are able to survive further north than in rural areas, though periods of frost may cause losses. In general these species are seen as a bonus to the variety of wildlife to be found in towns.
- Periods of hot, dry weather in summer, with very low humidity and rapid drying out of soils means that habitats typical of rural areas may struggle to survive, and be difficult to create. Woodlands may fail to develop the full range of organisms which flourish in the cooler and more humid climate in rural areas. The urban flora which appears naturally on urban waste land, with its colourful burst of growth in early summer from annuals and biennials, typical pioneer shrub and tree species, and rather barren winter appearance, may indicate the type of habitat best suited to the urban climate.
