Urban Climate

• What effect does the urban heat island have on precipitation?

  It generates convection and increases precipitation
• What happens to ground surfaces in urban areas that contributes to precipitation?

  They heat up, causing rapid evapotranspiration
• What type of clouds can result from rapid evapotranspiration?

  Cumulus clouds
• How do high-rise buildings affect air movement in urban areas?

  They induce air turbulence and promote vertical motion
• What causes surface winds to be drawn into urban areas?

  Low pressure from rising air
• What happens to air as it is forced to rise over the urban canopy?

  It converges and rises, forming rain clouds
• How does friction from the urban boundary affect air movement?

  It creates an orographic process similar to mountains
• What occurs to moving air due to barriers created by high-rise buildings?

  It may split apart and then converge again
• What role does city pollution play in precipitation?

  It increases cloud formation and rainfall
• How do pollutants assist in raindrop formation?

  They act as hygroscopic nuclei
• What is suggested about city pollution and lightning?

  It enhances the chance of lightning
• What sources produce large amounts of water vapor in cities?

  Industrial sources and power stations
• How much greater can rainfall be downwind of major urban areas compared to upwind areas?

  Up to 20% greater
• What atmospheric condition is necessary for thunderstorms to develop?

  Hot humid air
• When is the likelihood of thunderstorms increased in urban areas?

  Late afternoon and early evening in summer
• What causes thunderstorms to be produced?

  Convectional uplift under extreme instability
• What happens to air in the central area of a towering cloud?

  It causes rapid cooling and condensation
• What is released during condensation that fuels convection uplift?

  Latent heat
• What causes the rapid expansion of air during a thunderstorm?

  Extreme temperatures generated by lightning
• Why is fog more likely in urban areas?

  It was discovered in the 1950s that the average number of particles in city air in the more developed world was much greater than in rural areas. The particles acted as condensation nuclei and encouraged fog formation at night, usually under high-pressure weather conditions.

  In the UK the Clean Air Acts of the 1950s resulted in a dramatic reduction in smoke production and particulate emissions, and a decrease in the number of foggy days. In contrast cities undergoing more recent industrialisation events are experiencing more fog.
• What are condensation nuclei?


  Small particles in the air that allow water vapor to condense into droplets, forming clouds, fog, or haze
• How does air quality in urban areas compare to rural areas?

  It is often poorer in urban areas
• What causes particulate air pollution?

  Release of particles and noxious gas
• What gas is primarily caused by road traffic?

  Carbon monoxide
• What do nitrogen oxides produce when exposed to sunlight?

  Components of petrochemical smog
• What environmental issues are caused by nitrogen oxides?

  Accelerated weathering and acid rain
• What causes particulate matter in the air?

  Power stations and vehicle exhausts
• What is sulphur dioxide released from?

  Coal and oil burning
• What are the health impacts of sulphur dioxide?

  Coughing and chest tightening
• What environmental issue is caused by sulphur dioxide?

  Acid rain and building corrosion
• What do condensation nuclei help form?

  Clouds, fog, and precipitation
• What is photochemical smog associated with?

  Damage to plants and discomfort to people
• Why is photochemical smog a hazard during anticyclonic conditions?

  Air is static due to lack of wind
• What are the four main effects of wind on urban areas?

  Channeling effect, venturi effect, vortex, airflow spacing
• What happens to the wind on calm and clear nights in urban areas?

  Strong localised winds are drawn in
• How do high-rise buildings affect wind at street level?

  They increase or reduce the channeling effect
• What is a consequence of the channeling effect in urban areas?

  It reduces pollution but causes pedestrian discomfort
• What is the venturi effect in relation to wind?

  It intensifies wind speeds through small openings
• What difficulties can the venturi effect cause for pedestrians?

  It may make walking difficult due to severe gusts
• What happens when wind reaches the ground after descending from buildings?

  It can create a vortex around windward corners