Urban Climate

• Urban areas create 'microclimates' often called climatic domes. Have 2 layers|: below roof level is the urban canopy, and above is the urban boundary layer.
• Urban heat island - cities are warmer than surrounding countryside
• temperature inversion - an atmospheric condition in which temperature increases with height. as inversions are stable and do not allow convection, they trap pollution in the lower atmosphere.
• Photochemical pollution - exhaust fumes become trapped by temperature inversions, also associated with high pressure weather systems.
• Particulate air pollution - a form of air pollution caused by the release of particles and noxious gases into the atmosphere.
• Albedo - reflectivity of a surface. is the ratio between the amount of incoming insolation (suns rays) and amount of energy reflected back into atmosphere. Light surfaces reflect more than dark so have a greater albedo.
• Prevailing wind can make the urban boundary layer to extend over the down wind rural areas.
• Characteristics of urban microclimates: higher pollution levels; 5-15% more precipitations. more cloud and a lower relative humidity.
• Heat island: urban areas have a lower albedo. Dark surfaces absorb heat during the day and release slowly at night.
• Heat island: glass/steel reflect heat into surrounding streets. power stations/industry develop heat. buildings leak heat through poor insulation/air conditioning pumps out hot air.
• Heat island: less vegetation means less evapotranspiration; drains remove surface water quickly so reduced humidity. This means there is less heat energy evaporating the water.
• The temperature decline from the urban area to the rural area is called the thermal gradiant.
• Urban air pollution results in more condensation nuclei (help clouds form); mean more precipitation
• Fog/mist: are thicker/more persistent in urban areas. During the industrial revolution manufacturing centers experienced an increase in the number of days of winter fog. Today new dehli experiances dangerous fog.
• Urban areas have 25% more thunderstorms: heated air rises high in atmosphere cools and condenses quickly causing lightning.
• Urban canyons - narrow streets, bordered by high rise buildings funnel and concentrate winds.
• Venturi effect: a violent gust, caused in narrow gaps by air rushing to replace low pressure 'gaps' behind structures.
• Urban structures interfere with wind greatly. Architects have to consider the potential impacts of airflow.
• London tubes - urban heat island: get very warm. Air con is a challenge to install. Using groundwater to cool the stations, which will eventually cool the tunnels and the trains themselves.
• Albedo means reflected. A high albedo, will mean something reflects away the sunlight rather than absorbing it.
• heat island occurs as building materials absorb heat in day and release at night. Heat from industry, vehicles, air con release - anthropogenic heat.
• Heat island: air pollution can trap heat. Rain is removed quickly (drains) reducing evapotraspiration.
• Concerns with heat island: increased strain on energy (air con) and water supply. Chemical reactions producing smog are accelerated by high temperatures. Heat stroke increases - pressure on health services.
• Heat island can cause increased rates of temperature caused chemical weathering, increased risk of deterioration of historic monuments.
• Managing urban Heat Island Effect: sky view (openness between buildings in an urban area); Cool Cars (lighter colored car shell)
• Managing Heat Island: Cool Surfaces (those with a high albedo) - thety reflect short wave radiation; can reduce need for air con by 30%; must keep them clean to be effective. ALSO - dont absorb heat but reflect it out.
• Green Roofs - can reduce air con cost; reduces flooding; rediced air humidity; can insulate in winter; provides habitat and reduces air pollution.
• Managing Heat Island: Urban Trees - absorbtion of UV; improve water quality; reduce noise; improved well being; provides place for community gathering.
• Green roofs and urban trees are so good as they absorb CO2 and do evapotranspiration - decreasing the urban heat island effect.
• Channeling - wind is redirected down long streets, where there is less friction. Also called urban canyons
• Venturi effect - the squeezing of wind into an increasingly narrow gap resulting in a pressure decrease and velocity increase.
• Rainfall is higher in urban areas: air rises, cools, condenses into clouds and the urban heat island accelerates this. High rise buildings mean air rises, promotes increased vertical motion. - convectional rainfall.
• Precipitation - low pressure caused by rising air (air rises and leaves gap); surface winds are drawn in from the surrounding rural area. Air is then forced to rise over buildings.
• Pollution increases cloud formation: pollutants act as hydroscopic (water attracting) nuclei and assists in raindrop formation.
• Cities produce more water vapor from industrial sources - meaning more rainfall
• Fog - has increased with industrialisation, can cause smog which decreases air quality.
• Smog - mix of smoke and fog: common previously due to coal burning. Often cities are in river basins, air sinks and is static due to a lack of wind, Cooled air rolls downhill and is trapped by warming air above
• Smog - air is trapped in basin, pollutants mix with fog to create smog.
• Wind: uneven surface area causes a powerful frictional drag, meaning wind speed is lower. Wind is channelled in to canyons; Convectional processes can draw in strong winds from cooler surrounding areas.
• Thunderstorms: more frequent, produced by convectional uplift, Cumulonimbus clouds (cause thunder) may develop. Leads to more thunderstorms (all reasons air rises, cools, condenses apply here).