Urban Climate

• Urban Microclimate = The small-scale variations in temperature, precipitation, humidity, wind speed and evaporation that occur in a particular environment such as an urban area
• The urban canopy layer is the layer of air closes to the surface, extending upwards the mean building height. Above this lies the urban boundary layer that may be over one km in daytime but just a few hundred metres at night
• Effects of urban areas on climate:

  • Average mean temperature in urban areas is 0.5 - 3 C higher
  • Precipitation in urban environments has a 5-10% increase in quantity
  • Relative humidity annually decreased by 6%
  • Fog in winter increases by 100%
  • Annual mean wind speed decreases by 20-30%
• The Urban Heat Island Effect

  • When an urban area has higher average temperatures compared to the surrounding rural areas
  • Surface urban heat islands are larger in the summer and are when the surface temperatures are increased
  • Urban temperatures peak over the CBD in mid-afternoon and secondary peaks occur over suburban residential areas
• The thermal gradient between urban areas and rural areas is often greatest at night due to the high heat storage capacity of building materials compared to vegetation
• Causes of the urban heat island effect:

  • Surfaces in urban areas have a much lower albedo, and therefore absorb large quantities of heat during the day, rural areas keep snow for a much longer period and have a greater albedo here also
  • Air pollution from industries creates a 'pollution zone' which allows radiation to enter but absorbs outgoing radiation
  • Urban areas designed to dispose of surface water fast, so less cooling by evaporation
  • Heat from industries and vehicles which all burn fuel
• Managing the urban heat island effect:

  • Cool Surfaces = cool roofs built from high albedo materials absorb and store less solar energy and emit less heat
  • Green Roads = roads with a more porous surface allowing water to seep in and grass to grow
  • Green Roof = a growing medium over a waterproof membrane, can reduce rooftop temperatures
  • Urban Greening = planting trees and vegetation provides share and can have a natural cooling effect
• Rainfall can be higher over urban areas, partly because of higher urban temperatures encouraging the development of lower pressure in cities, convection rainfall is heavier and more frequent.
• Reasons for more precipitation:

  • Urban heat island generates convection, rapid evapotranspiration takes place to form clouds
  • High rise buildings and mixed building height induces air turbulence and increased vertical motion
  • Low pressure causes surface winds to be drawn in, air then converges as it is forced to rise. Friction from the urban boundary created orographic process similar to a mountain barrier and as air comes back down, they converge and rise upwards to form rain clouds.
  • City pollutants increase condensation nuclei
• Rainfall downwind of major urban areas can be as much as 20% greater than it is in upwind areas.
• Fog

  • Fog increased with industrialisation
  • In London, in 1700s there would be 20 days of fog, but by the end of the 1800s, 50 days of fog
  • Pollutant particles acted as condensation nuclei and encouraged fog formation at night, usually under high-pressure weather conditions
  • UK Clean Air Act 1950 reduced fog as a result of reduced pollution
• Thunderstorms


  Develop in hot humid air and have violent and heavy precipitation associated with thunder and lightning
• Thunderstorm formation


  1. Convectional uplift in instable conditions

  2. Cumulonimbus clouds develop up to the height of the tropopause

  3. Inversion provides stability

  4. Updrafts of air through this cause rapid cooling

  5. Precipitation forms and condensing produces latent heat

  6. Latent heat fuels convectional uplift further

  7. Raindrops split in updraught

  8. Positive electrical charge builds up

  9. Discharge to overcome resistance in the cloud

  10. Rapid expansion of air forms a shock wave heard as thunder
• Main effects of wind in urban areas:

  • On clear nights, convectional processes draw in localised winds
  • Many cities have high rise buildings which have caused an increase in the channelling effect where wind at street level is increased or reduced depending on street orientation
  • Venturi effect intensifies wind speed as wind is funnelled through small openings
  • Individual building modifies air – on the windward side wide is deflected both upwards and over the building and downwards, descending flow can form a strong vortex
• How design impacts urban winds:

  • Main street aligning with prevailing wind = Pollutants flushed out, reduced UHI effect
  • Building on stilts = Allows wind to flow through the base of a building
  • Building on a podium = Downdraughts do not reach ground level
  • Barriers in potential Venturi effect hotspots = Reduction in high-speed gusts between buildings.
• Particulate air pollution is caused by the release of particles and noxious gases into the atmosphere. Emissions of particles are largely caused by the combustion of fossil fuels and a combination of dust, soot and gases produced from vehicles
• Carbon Monoxide

  • A colourless, tasteless, odourless and poisonous gas produced by incomplete combustion
  • Estimated that road transport is responsible for 90% of all carbon monoxide emissions in the UK
  • Carbon Monoxide affects the transport of oxygen around the body by the blood, breathing in low levels can result in headaches, nausea and fatigue
• Nitrogen Oxides (NOx)

  • Forms when exposed to sunlight, react with hydrocarbons to produce the components of petrochemical smog – a mixture of ozone, nitric acid, aldehydes and secondary pollutants
  • Road transport is estimated to be responsible for about 50% of total nitrogen oxide emissions
  • Nitrogen oxides can inflame the lining of the lung and impacts are more pronounced in people with asthma, can cause acid rain
• Particulate Matter (PM)

  • Tiny bits of solids or liquids suspended in the air
  • Particles originate mainly from power stations and vehicles exhausts, other matter includes small bits of metal and rubber from engine wear, dust, ash etc.
  • Particles smaller than about 10 micrometres are referred to as PM10s and can settle in the airway and deep in the lungs, causing health problems.
• Sulfur Dioxide (SO2)

  • Colourless gas witha sharp odour, reacts easily with other substances to form harmful compounds such as sulphuric acid
  • In the UK, the major contributors are coal and oil burning by industry
  • Short term exposure may cause coughing, tightening of the chest and narrowing of the airways. May also produce haze, acid rain and damage lichens and plants
• Concentration of pollutants may increase five or six times in witner because of temperaure inversions
• Temperature Inversions = Cool moist air trapped by the 'lid' of warmed air above, trapping in pollutants
• Mixture of fog and smoke produces smog, this was common in the European cities during the Industrial Revolution because of high incidence of coal burning. Smog in London in December 1952 was responsible for over 4000 deaths
• Photochemical smog is formed when sunlight reacts with nitrogen oxides and hydrocrabons to produce ozone and peroxyacetly nitrate (PAN)
• Los Angeles has a problem of photochemical smog because of high densities of vehicles and lots of sunshine
• Chemical PAN linked to vehicle emissions has been deemed particularly hazardous and linked to the Los Angeles 'eye-sting'
• Photochemical smog is a particular hazard during anticyclonic conditions because once air has descended, it is static because of the absence of wind, so can remain for weeks during summer
• Clean Air Acts

  • After London smog of 1952, British government created legislation
  • Clean air act of 1956 introduced smoke free zones which began to clean up the air
  • Reinforced in 1990s with regulation on levels of PM10s, montiored by local councils and can establish Air Quality Management Areas
  • In London, construction sites are responsible for 12% of the city's NOx production, these are trying to be reduced
• Urban Transport Solutions

  • London Congestion Charge in 2003
  • Park and ride
  • Metrolink in Manchester
  • More cycling provision e.g. 'snake' bridge in Copenhagen
  • Bus and car-pooling lanes
• Air pollution reduction in London

  • 280,000 trees have been planted
  • First Ultra Low Emission Zone in 2019
  • Hybrid and zero emission buses
  • 1500 electric vehicle charging points by 2020
  • A £48 million scrappage fund for polluting cars
• Before the first pollution measures in London in 2016, the air exceeded hourly legal limit for nitrogen dioxide for more than 4000 hours, in 2019 this had fallen to just above 100 hours, a reduction of 97%
• Cities are also zoning industry to reduce pollution, and there is legislation for higher factory chimneys to emit pollution above the inversion layer