Urban Microclimates Temperature Wind Pollution On average 1-2% warmer than the surrounding rural areas Heat island uneven- peaks in city centre and industry and troughs in parks/lakes Heat island effect produces large scale convection by lowering air pressure so that air is drawn in from surrounding urban areas 5% slower than suburbs due to roughness of land surface- buildings at different heights- increase surface friction Large Scale Convection Heat Islands are a result of: Anthropogenic heat Height and arrangement of buildings Building materials Presence of water Pollutants Urban Heat Island Lower Speeds Temperature Wind Wind speeds more variable Around some buildings- gusty and strong Straight streets with tall buildings create ‘urban canyons’ which funnel wind to create a high velocity Turbulence created by high rise buildings disrupting flow of air Behind buildings- reverse circulation as suction occurs on roof, walls and sides of buildings Shape and spacing of buildings affect wind Urban microclimates are dirtier than rural areas Urban Microclimates Pollution Greater Variability Fossil Fuels Clouds and Precipitation In 19th century large amounts of soot was produced from coal-fired sources Up to 80% of winter sun was lost Situation worse in winter as more coal used for heating 1950s London only had 50% sun compared to rural areas Reduced coal-fired heating, trains and power stations Rain Fog Precipitation Cloud Greater cloud coverage in urban areas 5-10% more Due to a greater concentration of condensation- about 100 times more than rural areas Needs cool surface- condenses on buildings Rain falls more frequently Cities designed to remove surface water so humidity in day is lower At night urban humidity is higher than rural surroundings Often associated with cities Due to higher temperatures- cannot form until air has cooled to its dewpoint- more condensation More common in cities 5-30% more than rural areas Not only are cities wetter but are also prone to more thunderstorms