1. 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