Urban climate

• What are the causes of the urban heat island?

  -Lower absorption of heat
• What is the urban heat island

  The air is 2-5 degrees hotter in urban areas this is known as the urban heat island effect it is most noticeable when there is little winds. Cities heat also fluctuates more than the surrounding areas this is most apparent in London, where the highest temperatures are  recorded high density areas and in industrial locations
• What are the causes of the urban heat island effect

  1.Absorption of heat by ubran surfaces

  2. Air pollution

  3.Heat from Human activity

  4)Less evapotransiation
• Why is the less absorption of heat

  less in urban areas as Urban surfaces have a lower albedo (fraction of light the surface reflects) due to being many tarmacs instead of green open space which means that more solar energy will be absorbed and stored. Also Large glass and steel reflect heat into surrounding (Walkie talking building, postmodern building melts local cars in London due to the untraditional material)
• What is the imapct of human activities

  There is also wasted heat from urban activities such as traffic congestion, heating homes and air conditioning, factories more homes and business, these all generate their own heat and this leaks out at night due to poor insulation.( This leads to heat at night being much warmer than the surrounding locations).
• Why is their more air pollution?

  There is increased pollution from urban activities, cars, industry leads to increased cloud cover as it produces increased concentration nuclei and a pollution dome. These trap heat and reflect it back to surfaces.
• Why is there less evapotranspiration

  Evapotranspiration uses heat energy

  When it rains in urban areas moisture is moved rapidly so there is little water to evaporate, also there isless vegetation
• What is an urban microclimate?


  The term 'urban microclimate' refers to the small-scale variations in temperature, precipitation, humidity, wind speed and evaporation that occur in an urban area that is different to that of the surrounding rural area.
• What is the urban canopy and the urban boundary layer?


  The urban canopy layer (UCL) is the layer of air closest to the surface, extending upwards to the mean building height. Above that lies the urban boundary layer (UBL) which forms a dorm of warmer air that can extend as a plume downwind of the city; it can be over 1km in height during daytime or just a few hundred metres at night.
• What is the urban heat island?


  The zone around and above an urban area, which has higher temperatures than the surrounding rural areas. Temperatures will fluctuate in urban areas depending upon season, weather conditions, sun intensity and ground cover. Surface urban heat islands are typically largest in the summer. Larger cities have stronger urban heat island effects. There is very little variation in surface temperatures over areas of water.
• Why is there little variation in surface temperatures over areas of water?

  Air over water is relatively cool; a combination of the absorption of heat by the water itself and the evaporation at its surface, lowers the temperature.
• Why do cities tend to be warmer than rural areas?


  Surfaces tend to be much less reflective than those in rural areas; building materials such as concrete, bricks and tarmac have a much lower albedo and act like bare rock surfaces, absorbing large quantities of heat during the day and slowly releasing it at night.

  Air pollution increases cloud cover and creates a 'pollution dome', which increases heat.

  Urban areas are designed to dispose of surface water quickly; reduced potential for cooling by evaporation.

  Heat comes from industries, buildings and vehicles, which burn fuel. Although they regulate the temperature indoors, air conditioning units release hot air into the atmosphere.

  Even people generate heat and cities contain large populations in a small space.
• Why is the urban heat island a matter of concern? / what are the impacts of the urban heat island?


  - conditions become uncomfortable in buildings & on city transport systems --> cases of heatstroke & asthma increase.

  - the anticyclonic weather conditions responsible for intense heat events produce higher air pollution levels.

  - excessive heat puts strain on supply of energy for cooling and air conditioning.

  - leads to increased water consumption, placing an extra strain on the water supply infrastructure.

  - can cause a longer allergy season for those who have hayfever.

  - greater potential for algal blooms in water courses as a result of rising temperatures.
• What are the strategies to manage the urban heat island (UHI)?


  - Cool surfaces (roofs built from materials with high albedo absorb & store less solar energy during the day, so are not major emitters of heat into the urban atmosphere at night). cool roads, pavements etc.

  - Green roads: roads with a porous surface that allows water to seep in and grass to grow through - reduces heat absorbed by road surface.

  - Green roofs: reduce rooftop temperatures, reduce rainwater run-off, act as insulators, increase urban biodiversity.

  - Urban greening: planting trees & vegetation provides shade and has a natural cooling effect.

  - Sky view factor: a restricted sky view (narrow streets & tall buildings) will reduce the escape of heat from street and building surfaces. Otherwise, accumulation of heat leads to increased air temperatures.
• Outline precipitation in relation to the urban climate.


  Rainfall higher over urban areas.

  Urban heat island generates convection; as ground surfaces are heated, rapid evapotranspiration takes place and can result in cumulus cloud and low pressure, which means increased rainfall.

  Mix of building heights induces air turbulence and promotes vertical motion --> encourages low pressure.

  City pollution can increase cloud formation and rainfall.

  Cities may also produce large amounts of water vapour from industrial sources and power stations.

  The heating of the surface and the overlying air creates instability in the atmosphere that encourages air to rise. As it rises, it cools and water vapour condenses into rain that falls.
• Outline fog in relation to the urban microclimate.


  Fog has increased alongside deindustrialisation.

  Particles in city area act as condensation nuclei and encourage fog formation at night, usually under high-pressure weather conditions.

  In the UK, the Clean Air Acts of the 1950s resulted in a reduction in smoke production and particulate emissions, thus a decrease in the number of foggy days. However, cities, such as New Delhi and Beijing, recent industrialisation has caused more fog.
• Outline thunderstorms in relation to the urban microclimate.


  They develop in hot humid air and are characterised by violent, heavy precipitation associated with thunder and lightning. Urban areas have more thunderstorms.
• What is the Albedo?


  The reflectivity of a surface. It is the ratio between the amount of incoming insolation and the amount of energy reflected back into the atmosphere. Light surfaces reflect more than dark surfaces and so have a greater albedo.
• What are the three main effects of wind in urban areas?

  - Lower wind speeds due to roughness of land surface (buildings at different heights), increasing surface friction.

  - Greater variability due to building height & layout; straight streets lined with tall buildings can produce 'urban canyons', which funnel wind down them quickly, thanks to the Venturi effect (the phenomenon in which wind velocities increase when air flows through an increasingly narrow gap). Turbulence is created by high-rise buildings disrupting the flow of air.

  - Large-scale convection: UHI produces large-scale convection over the whole urban area, leading to lower air pressure, so air is drawn in from surrounding rural areas in the same way as sea breezes are produced.
• What is channelling?


  Wind redirected down long, straight, canyon-like streets where there is less friction. These are sometimes referred to as urban canyons.
• What is the Venturi effect?


  The phenomenon in which wind velocities increase when air flows through an increasingly narrow gap e.g. between 2 buildings.
• What is particulate air pollution?


  A form of air pollution caused by the release of particles and noxious gases into the atmosphere. Emissions of particles can occur naturally but they are largely caused by the combustion of fossil fuels.
• What is photochemical pollution?


  A form of air pollution that occurs mainly in cities and can be dangerous to health. Exhaust fumes become trapped by temperature inversions and, in the presence of sunlight, low-level ozone and other noxious gases form. It is associated with high-pressure weather systems.
• What is temperature inversion?


  An atmospheric condition in which temperature, unusually, increases with height. As temperature inversions do not allow convection, they trap pollution in the lower layer of the atmosphere.
• What is being done to manage the problem of the urban microclimate in London?

  Clean Air Act of 1956 which has been reinforced by later legislation.

  280,000 trees have been planted to absorb pollution.

  Ultra Low Emission Zone came into being in 2019.

  Increased use of hybrid and zero emission engine buses.

  Increase in number of electric vehicle (EV) charging points.
• Outline zoning of industry as a way to manage issues of the urban microclimate.


  Industry has been located downwind in cities if at all possible. Planning legislation has forced companies to build higher factory chimneys to emit pollutants above the inversion layer.
• Why is it hotter in urban areas compared to rural areas?


  Urban areas have surfaces that are less reflective and absorb more heat.
• What materials contribute to the lower albedo in urban areas?


  Building materials such as concrete, bricks, and tarmac.
• How do urban surfaces behave in terms of heat absorption and release?


  They absorb large quantities of heat during the day and slowly release it at night.
• What effect does air pollution have on urban temperatures?


  It increases cloud cover and creates a 'pollution dome', which raises heat levels.
• What is a 'pollution dome'?


  A layer of air pollution that traps heat in urban areas.
• How does urban design affect cooling by evaporation?


  Urban areas are designed to dispose of surface water quickly, reducing cooling potential.
• What are the sources of heat in urban areas?


  Industries, buildings, and vehicles that burn fuel.
• How do air conditioning units contribute to urban heat?


  They regulate indoor temperatures but release hot air into the atmosphere.