urban

• Climatic dome


  The distinctive climate and weather features that can be found surrounding a urban area
• Microclimate


  Small scale/local scale variations in temperature precipitation, humidity, weed speed and evaporation that occur in a particular environment such as an urban area
• Fog

  Visible cloud water droplets or ice crystals suspended in the air at or near the Earth's surface
• Relative humidity


  How much water vapor is in the air, compared to how much it could hold at that temperature
• Urban plume


  Dome of warmer air that extends downwind of the city. Wind often changes the dome to a plume shape
• Urban areas create their own climate and weather or microclimate/local climate. This is sometimes referred to a 'climatic dome' within which the weather is different from that of the surrounding rural area in terms of temperature, relative humidity, precipitation, visibility (fog), air quality, and winds speed
• Climatic dome


  • May extend upwards up to 250-300m and its influence may extend tens of kilometres down wind
• Levels within the urban zone


  • Below roof level there is an urban canopy with processes acting in the space between buildings (canyons)
  • Above this is the urban boundary layer whose characteristics are governed by the urban surface
• The dome extends downwind as a 'urban plume' into the surrounding rural area
• Effects of urban areas on local climate

  • Temperature
  • Relative humidity
  • Precipitation
  • Visibility (fog)
  • Air quality
  • Winds speed
• There will also be seasonal variations, diurnal variation, land use variation, and geographical variation in the effects of urban areas on local climate
• Urban heat island effect (UHIE)


  The zone around and above an urban area which has higher temperatures than the surrounding rural area
• Anticyclonic conditions


  Clear skies, dry, hot weather in summer, cold weather in winter, low wind speeds, high pressure weather systems
• 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
• Temperature inversion


  An atmospheric condition in which temperature unusually increases with height. Extremely stable conditions that do not allow convection, trapping pollution in the lower layer of the atmosphere
• The urban heat island effect (UHIE) is the zone around and above an urban area which has higher temperatures than the surrounding rural area. Quite frequently the centre of the city will be on average 2-4˚C warmer than the surrounding countryside, sometimes as much as 10˚C
• The edge of the urban heat island is well defined because temperatures change abruptly from urban to rural areas – referred to as a 'cliff' in temperatures
• Temporal changes to urban temperatures

  • Diurnal changes
  • Seasonal changes
  • City size
• Typically the urban area temperatures are at their highest in the mid-afternoon over the CBD. The temperature range from rural to the city centre is often greatest at night due to the high heat storage capacity of building materials compared to vegetation
• There is very little variation in the surface temperature over areas of water. This is because water maintains a fairly constant temperature over a 24-hour period due to its high heat capacity
• The effects of the urban heat island are stronger when there is an anticyclone – clear skies mean more solar radiation and low winds mean air isn't blown away. Temperature differences are most marked during anti-cyclonic weather during winter
• The effect of the urban heat island decreases as city size decreases
• Spatial changes to urban temperatures

  • Landuse changes
  • Micro-hot spots and micro-cold spots
• Micro-hot spots and micro-cold spots

  • Shopping centres and industrial premises (hot spots)
  • Parks, fields, and water bodies (cold spots)
• Vegetation cools the surface through evaporation of water, so the mostly densely vegetated areas are the coolest areas
• Where development is most dense the land surface temperature is near 30°C
• Why are urban areas warmer than rural areas?

  • Reflectivity of surface (albedo)
  • Air pollution from industries and vehicles
  • Reduced evapotranspiration
  • Heat produced by people (anthropogenic)
  • Sky view factor
• Surfaces in the city 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
• In winter, rural areas keep snow for a much longer period and therefore have a greater albedo ranging from 0.86-0.95
• Air pollution from industries and vehicles increases cloud cover and creates a 'pollution dome' which allows in short wave radiation but absorbs a large amount of the outgoing radiation as well as reflecting it back to the surface
• In urban areas, water falling on to the surface is disposed of as quickly as possible. This changes the urban moisture and heat budget – reduce evapotranspiration means that more energy is available to heat the atmosphere
• Heat escapes from houses, shops, offices and even people. Cars and other vehicles circulate in streets and emit heat. Factories and power stations release heat into the atmosphere by emissions from chimneys and into streams by expelling cooling water
• A restricted sky view, as found for narrow streets and tall buildings, will reduce the escape of heat from street and building surfaces. This can contribute to the accumulation of heat within 'street canyons' and lead to an increase of air temperatures
• If streets are angled perpendicular to the prevailing wind, during intense urban heat island events this will reduce the chance of ventilation and removal of heat and pollutants that accumulate between the buildings
• The isotherm map of London shows the pattern of temperature for the Urban Climate London (UCL) in mid-May during calm atmospheric conditions. Clear skies and light winds provide the best conditions for a more pronounced heat island effect. The general pattern shows temperatures at their highest in the city core, reaching 10 -11°C, which contrasts with the rural fringe where temperatures dropped to 5°C
• The map of London shows a positive correlation between high surface temperatures and high density urban areas. The relatively cool areas to the southwest coincide with the large open and green spaces of Richmond Park
• Impacts of the Urban Heat Island

  • High summer temperatures making conditions unbearable
  • Increased health issues like heat stroke, asthma, organ damage, and death
  • Delays recovery from stress, leading to aggressive behaviour, murder rates and suicide
  • Higher pollution levels
  • Energy stress
  • Water stress
  • Deterioration of historical monuments and buildings
  • Earlier flowering times of plants and trees
  • Prolonged survival and higher reproduction rates of some animals and insect pests
  • Greater potential for algae blooms in water courses
• The effects of the 2003 heat wave were greatest in London in terms of the number of deaths per head of population, especially amongst the elderly
• In New Taipei city, Taiwan research in 2016 reveal the intensity of the UHI phenomenon to be statistically significant when considering the suicide rate among older residents
• Chemical reactions that produce ozone and smog are accelerated by high temperature while lower wind speeds keep the heat and pollution trapped in the city