Urban Microclimates

Structure Of Air Above Urban Area Greater amounts of dust mean increasing concentrations of hygroscopic particles. There is less water vapour because water is removed quickly by drainage and sewers. There is less vegetation to take in water and release it later, but more CO2 and higher proportions of noxious fumes owing to combustion of imported fuels and discharge of waste gases by industry.

Structure Of The Urban Surface More heat retaining materials with lower albedo (reflectivity). Better radiation-absorbing properties. Rougher surfaces, with a great variety of perpendicular slopes facing different aspects. Tall buildings can be very exposed, and the deep streets are sheltered and shaded.

Resultant Processes Radiation and Sunshine. Clouds and Fogs. Temperatures. Pressure and Winds. Humidity. Precipitation.

Radiation and Sunshine Short-wave radiation is scattered by dust. There is a higher absorption of longer waves owing to surfaces and CO2. Tall buildings in shaded, narrow streets diffuse sky radiation and screen it. Industrial haze means reduced visibility.

Clouds and Fogs Higher incidence of thicker cloud covers in summer and radiation fogs or smogs in winter because of increased convection and air pollution. Concentrations of hygroscopic particles accelerate the onset of condensation. Day temperatures on average are 0.6 degrees warmer.

Temperatures Stronger heat energy retention and release, including fuel combustion, gives significant temperature increases from suburbs into the centre of built-up areas creating heat “islands”. These can be up to 8 degrees warmer during winter nights. Snow in rural areas increases albedo, thereby increasing the differences between urban and rural. Heating from below increases air mass instability overhead, notably during summer afternoons and evenings. Big local contrasts between sunny and shiny surfaces, especially in the spring.

Pressure and Winds Severe gusting and turbulence around tall buildings, causing strong local pressure gradients from walls. Deep narrow streets much calmer unless aligned with prevailing winds to funnel flows along them (the Canyon Effect).

Humidity Decreases in relative humidity occur in inner cities owing to lack of available moisture and higher temperatures there. Partly countered in very cold, stable conditions by early onset of condensation in low-lying districts and industrial zones.

Precipitation Perceptibly more intense storms, particularly during hot summer evenings and nights owing to greater instability and stronger convection above built-up areas. Probably higher incidence of thunder in appropriate locations. Less snow cover in urban areas even when left uncleared.

Urban Heat Island Urban areas are generally warmer than those of the surrounding countryside. Temperatures are on average 2-4 degrees higher in urban areas. This creates an urban heat island due to heat and pollution. Low wind speeds due to high buildings and urban surface roughness. Buildings have high heat conductivity and a reduction in heat diffusion.

Activity Using the information: What do you understand by the term Heat Island? Describe the rate of temperature change with distance out of the city centre. Using examples, explain how human activities affect local climates.