Urban climates

• Microclimate


  where there are small scale variations in temperature, humidity, wind speed and environment

  * urban areas create their own climate
• Urban heat island effect

  urban areas are significantly warmer 'islands' surrounded by a rural 'sea' of cooler air

  increased temperature in urban areas compared to surrounding rural areas
• Impacts of UHI


  UK* London- 600 heat related deaths (2003)* 9 degrees warmer than surrounding areas* mortality increase with high temperatures, warm night temperatures, age etc
• Precipitation

  1. more buildings in urban areas means that lots of warm air is released
  2. increased warm air released, rises due to low pressure
  3. cloud formation happens at an abundance/fatser rate
  4. air cools and condenses
  5. clouds precipitate and rainfall occurs UHI increases convection which leads to more showers and low pressure draws moist air into the city from the countryside* increased cloud formation means increased risk of rain* urban areas receive up to 10% more frequent cloud cover than rural
• Thunderstorms


  higher temperatures in urban areas means hot humid air

  * hot, humid air rises, cools and condenses forming clouds

  * formation of towering cumolonimbus clouds

  * rapid cooling means water droplets, hail and ice form

  * raindrops are split in the uplift of air and positive electric charge released

  * electric charge builds up until high enough to overcome resistance in cloud, released to areas of negative charge on ground or cloud leading to lighning

  * thunder is formed due to the sudden increase in pressure and temperature from lighning, rapid expansion of clouds
• Fog


  intensity, length + frequency of fog are greater in urban areas

  * the further away from urban area, towards rural area, fog density decreases

  * larger the city + greater quantity of urban structures + materials the greater the impacts of these microclimate changes
• Wind


  urban landscapes increase/ decrease wind speed and alter wind direction

  * wind velocity is reduced by friction and windbreak effect caused by buildings

  * open spaces (countryside) = more wind

  * cities experience less wind than surrounding rural areas because there are obstacles (buildings) in the way

  * wind velocity can be 30% lower in cities

  * overall pattern in city with skyscraper is the formation of urban canyon winds as buildings increase the street level winds which separate and travel down various street
• Why do cities have higher temperatures?

  1. Albedo effect- lots of urban surfaces (concrete, tarmac, bricks) absorb more of the solar radiation + store it

  2. Higher population in cities- lots of anthropogenic heat sources give off heat enegy e.g cars

  3. Reflections in insolations

  4. Heat from urban areas is released at night
• Strategies to reduce UHI

  1. Cool surfaces

  2. Green roofs

  3. Urban greening

  4. Sky view factor

  5. Cool cars
• Cool surfaces


  cool roofs

  * materials/coatings that significantly reflect sun and heat away from buildings

  * reduces roof temperatures

  * increases comfort of occupants
• Urban greening


  maintaining and enhancing urban green spaces

  * helps to counteract the warming effect of UHI

  * plants are effective at absorbing particulate matter and nitrogen oxides