9.4 Urban Climate

• microclimate - climates of small areas, such as gardens, cities, lakes and forests. A microclimate is an expression of temperature, humidity and wind within a few metres of the ground.
• Factors affecting micro-climate:

  ocean proximity

  • frost free oceanic climate
  • trade winds islands (relief rain)
  • cold ocean vs hot land, frequent fog and low clouds at coast but clear sky's a few miles in land. Cold moist air gets trapped.
• Factors affecting micro-climate:

  Plant cover

  • forests can drop air temperature
  • cooling effect of evapotranspiration
  • trees shade from suns radiation
• Factors affecting micro-climate:

  Built environment

  • concrete absorb heat in urban areas
  • let out heat at night by radiation
  • urban areas are 7 degrees than rural areas
• Factors affecting micro-climate:

  Slope direction

  • areas facing towards the sun are significantly warmer than those facing the opposite direction
• Factors affecting micro-climate:

  Altitude

  • air cools as altitude rises
  • cold air will sit on an area since its more dense than warm air, causing extreme weather when in the mountains.
• Urban heat island - defines the urban area as a significantly warmer island surrounded by a rural sea of cooler temperatures
• albedo affect - level of absorbtion of a type of surface e.g concrete. At night the heat is given off.
• Albedo effect

  • use plants to absorb heat energy to take out heat from the air. The evapotranspiration can further decrease the air temperature. Building strategies that lessen the amount of heat energy let off.
• 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 combustion of fossil fuels.
• 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.
• 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.
• why is urban heat island a concern:

  • as temperatures rise in summer, conditions can become uncomfortable in buildings and on city transport systems.
  • the hot and still, anticlyonic weather conditions responsible for intense urban heat island events also produce higher air pollution. This is because the chemical reactions that produce ozone and smog are accelerated by high temperatures.
• why is urban heat island a concern:

  • excessive heat puts an increased strain on the supply of energy for cooling and air conditions
  • in warmer periods, added heat can lead to increased water consumption. This places strain on water supply. Evapotranspiration rates will also be higher, meaning trees will extract water at a greater rate.
• Strategies for managing urban heat island:

  • plant trees - provides shading, can also reduce urban flooding by intercepting rainfall and filter pollutants from the air
  • cool surfaces - built from materials with high albedo absorb so less solar energy during the day & thus not major emitters of heat into the atmosphere at night. Cool roads and pavements with reflective coats and seals.
  • sky view factor - narrow tall buildings trap heat, so if streets are angled perpendicular to the prevailing wind, this will reduce chances of ventilation & the removal of heat that accumulate.
• Precipitation in urban areas:

  • urban areas have 5-15% more rainfall than rural areas
  • urban air pollution results in more condensation nuclei, tiny particles essential for cloud droplets to form
  • air is warmer in cities leading to higher precipitation
  • air has to rise and cool below dew points and allow water vapour to condense buildings promote this uplift
  • cities may also produce large amounts of water vapour from industrial sources.
• Fog in urban areas:

  • the higher concentration of condensation nuclei over cities encourage formation of fog
  • fog tends to be thicker and persist longer in high pressure (anticlyonic), where wind is to weak to blow them away
  • during industrial revolution in the 19th century fog increased
• Thunder storms in urban areas:

  • develop in hot, humid air
  • produced by convectional uplift
  • rapid cooling and condensation, leading to the formation of water droplets, which charge the thundercloud and discharge as lightning
• Channeling - wind redirected down long, straight, canyon like streets where there is less friction. These are sometimes referred to as urban canyons
• Venturi effect - the phenomenon in which wind velocities increase where air flows through an increasingly narrow gap
• Wind in urban areas:

  • buildings increase the friction on urban winds and reduce wind speeds
  • on calm clear nights, when the urban heat island at its greatest, convectional processes draw in strong localised winds from cooler surrounding areas
  • in many cities, particular districts have become dominated by clusters of high rise buildings that has reshaped the sky line. This has caused an increase in the channeling affect
  • the venturi affect intensifies winds and can cause gusts at street level making walking difficult
• Coping with winds in urban areas:

  Burj Kalifa, Dubai

  • over 828m
  • withstands gusts of over 240km/h
  • softened edges to deflect wid and prevent forming whirlpool's
• Pollution reduction policies:

  • clean air acts
  • vehicle control and public transport (London ultra low emission zone, zero emission buses)
  • pedestrianisation
• Urban heat island - Mumbai

  • rising temperatures
  • Dharavi is typically 5degrees hotter than Matunga (immediate neighbour)
  • due to types of materials used, access to green spaces
  • green solutions such as tree plantations, rooftop gardens
  • water retentive pavements and reflective roofs
  • Dharavi is tightly compact with no open spaces/trees
• Air pollution in urban areas:

  • emission of particles caused by the combustion of fossil fuels
  • a combination of dust, soot and gases are produced from vehicles and industrial processes
  • the mixture of fog and smoke particles produces smog
  • increase in photochemical smog which can damage plants and a rage of discomforts to people
  • e.g Los Angeles has had problems with photochemical fog due to the high density vehicles, frequent sunshine and basin topography which traps photo-oxidant gases.