Urban Heat Island (UHI) effect

• UHI effect:


  In winter the difference between urban and rural temperatures can be up to 12° higher

  • Rural areas do not store as much energy and release the heat quicker than urban areas
  • Temp can increase during periods of 'anticyclonic weather' (high pressure) > produce clear skies and low winds > greater isolation to reach urban surface > low winds prevent warm air dispersion
  • Temperature sinks ~ found above green space and water e.g. parks, lakes
  • Temperature plateaus ~ occur in areas with same land use e.g. industrial areas
  • Temperature cliffs ~ when temp changes rapidly

  
• Compare the daytime surface temperature and air temperature:

  Surface temp higher than air temp

  > except around areas of water where air temp higher than surface

  
• Compare the night time surface and air temperatures: 


  More similar

  • Night surface and air temp closer
  • Little difference but does fluctuate
  • Higher surface temp at night than air temp

  
• Compare diurnal surface temperatures (daily):

  Overall higher surface temp at day except at night over water where its higher but still lower than the plummeted surface temp

  • Lots of fluctuations but peaks over CBD

  
• Compare diurnal air temperatures:

  Closer, but air temp always higher in day, but comes close over CBD and less fluctuation

  
• Why are urban areas warmer than rural?

  1. Lower albedos
  2. Reflecting energy downwards (e.g. Walkie Talkie building, London)
  3. Greater variety of heat sources
  4. Little surface water
  5. Air pollution 'dome'
• 1. Lower albedos


  Albedo (reflectivity) of a surface is very important in urban areas as darker surfaces have low albedos and therefore more heat

  • Urban environments have lower albedos (darker coloured) so are therefore more hot e.g. Asphalt roads
  • Vegetation has a low albedo but evaporation helps to cool the surface, the most dense areas of veg are the coolest due to evaporation
  • Cities have less veg > less evapotranspiration > reduces cooling effect

  
• 2. Reflecting energy downwards


  Some urban surfaces e.g. buildings with large windows have a high reflective capacity and multi-storey building tend to concentrate heating effect in surrounding streets by reflecting energy downwards

  • Heat slowly released at night
• 3. Greater variety of heat sources:

  • The heat from industries/ transport/ buildings all burn fuel,
  • People generate the heat (cities have high population densities and therefore hotter)
  • Air conditioners release hot air into the atmosphere, heating homes etc
• 4. Little surface water:

  In urban areas, moisture often 'disposed of' quickly when it reaches the surface via drainage systems.

  • Water has a high heat capacity which means it maintains a constant temp over 24 hours
  • Urban areas that are coastal or with large lakes/ rivers > tend to be less hot with less pronounced UHI effect
• 5. Air pollution 'dome'

  • Air pollution form industries and vehicles increase cloud cover > creates 'pollution dome' > short-wave radiation but absorbs large amounts of outgoing radiation > trap heat and reflect it back to the surface
• UHI - spatial and temporal variations:

  In summer the effects of the UHI tend to be greatest

  • More intense heat > more heat absorbed by building and emitted at night

  At night UHI effects are greatest:

  • Building emit the heat stored in the day as low albedo

  Proximity to water: UHI effects are lowest closer to water

  • Water helps maintain steady heat over a day > water warms slowly > holds heat longer > cools slower > stays cool longer > heat lost in evaporation
• UHI - spatial and temporal variations:

  UHI effects are greatest during anticyclonic (high-pressure) weather conditions

  • High pressure > sinking air > clear blue skies > decrease air dispersion as less wind

  Intensity of sunlight: tropical cities likely to suffer more from UHI effects

  • More intense heat > concentrated on a smaller area of Earth's surface at higher latitudes

  Ground cover: More green areas = less UHI

  • Heat energy lost in evapotranspiration and evaporation > cooling
• What problems are associated with the UHI effect?

  1. Photochemical smog
  2. Suffering/ discomfort/ allergies
  3. Chemical weathering of historic buildings
  4. Algal blooms
  5. Increased water consumption
  6. Greater demand for cooling
• 1. Photochemical smog

  • High temp accelerate chemical reactions that produce tropospheric ozone and photochemical smog
  • Low winds keep heat/ pollution trapped
• 2. Suffering/ discomfort/ allergies:

  • Causes heat stroke/ asthma/ organ damage/ death
  • Vulnerable groups e.g. babies and elderly
  • E.g. Black Monday, Paris, 2003 > 3000 deaths in one night during a heat wave
• 3. Chemical weathering of historic buildings:

  • High temps combined with acidic atmospheric pollution > historic building degrade more rapidly due to chemical weathering
• 4. Algal blooms

  • More likely in urban water course
  • Saps water of oxygen and deprives aquatic life of oxygen
• 5. increased water consumption:

  • By residents and businesses
  • Could lead to water conservation strategies being enforced
• 6. Greater demand for cooling:

  • Air conditioning > increase costs and further heat release/ noise
• Managing/ reducing UHI:


  Cities around the world are attempting to tackle the factors leading to the development of UHI. In the planning of urban areas, 'sky view factor' (SVF)should be considered as urban morphology that influences the development of UHI's
• What is the sky view factor?

  The proportion of visible sky that can be seen from any point on/ at ground level

  • Tall building in close proximity to other building create an urban canyon where heat is stopped and trapped
  • Measured on a scale of 0-1 (0 = complete shut out of light from street level)
• Other strategies to reduce the UHI:

  1. Cool roofs
  2. Green roofs
  3. Urban greening
  4. Light roads and pavements
• 1. Cool roofs: 


  What are they?

  • Light coloured surfaces to reflect heat, higher albedos

  Benefits:

  • Cost-effective
  • Reflects energy back into space > decrease greenhouse gas effect

  Problems:

  • Glare (especially on roads for driving)
  • Air traffic control
  • 1% roof area in NYC
• 2. Green roofs


  What are they?

  • Layers of material for drainage with a rainwater catchment system that absorbs up to 80% of annual rainfall
  • Greenery on roofs e.g. plants, grass, moss

  Benefits:

  • Create habitats
  • Decrease temp > cooling effect (evaporation)
  • Noise and acid rain decrease (absorb atmospheric pollutants)
  • Recycled materials for soil > lighter

  Problems:

  • Large scale? only 1% roof area in NYC
  • May not work on some roof shapes
  • Takes time
  • Not always successful
• 3. Urban greening:


  What is it?

  • Incorporating green areas in urban environments e.g. roofs, streets, cities e.g. Olympic Park, Bosco Verticale, Milan

  Benefits:

  • Produce oxygen and increase humidity
  • Decrease CO2 and dust particles > decrease smog
  • Visually appealing
  • Vertical = more effective e.g. plants on flats

  Problems:

  • Rely on people for maintenance
  • Expensive to maintain
  • Carefully manage growth
  • How liveable are flats? Less light and more insects
• 4. Light roads and pavements:

  What are they?

  • Higher albedo surfaced roads and pavements, lighter colours to reflect heat e.g. Alaska > prevent melting and thawing

  Benefits:

  • Pavements 30 degrees cooler
  • Could save $100 million in LA/ year from energy
  • Recycled materials

  Problems:

  • Safety of coatings?
  • Scale > surfaces not applicable everywhere
  • Glare > accidents?
  • Decreased evaporation of moisture due to higher albedo which reflects heat energy
• UHI 


  Sinks, cliffs, plateau