Urban Climate

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  • Created by: 8cburton
  • Created on: 27-05-15 14:22

Basics

  • Cities create their own climate called 'climatic dome'
  • Urbans are usually warmer than surrounding rural areas... this is called the heat island effect
  • The highest temp are found in densely built up areas and industrial areas.
  • Albedo= reflectivity of a surface
  • Microclimate= small-scale variations in temp, precipitation, humidity, wind and evporation
  • Photochemical smog= Exhausts fumes become trapped by temperature inversions and in the prescence of sunlight low level ozone forms
  • Temperature inversion= Atmospheric condition in which temperature increases with height. Extremely stable conditions and do not allow convection, they trap pollution in lower atmosphere.
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Heat island effect

Areas within the city with the same land use generally have same temperature- these are called temperature plateaus. Temperature can rapidly change when land use changes resulting in temperature cliffs. Pockets of cool are above parks and bodies water are called temperature sinks.

4 main causes of the heat island effect:-

  • Absorption of heat by urban surfaces- concrete, brick and tarmac surfaces absorbs and store heat from the sun during the day then slowly releases it at night as long wave ratiation.
  • Air pollution- cars and factories release pollutants  creating a 'pollution dome' . This traps outgoing heat and refects it back to the surface.
  • Heat from human activity- cars, factories, offices, central heating, air conditioning until and people themselves all release heat.
  • Less evaporation- Drainage system lead to less surafe water to evaporate and little vegetation so little transpiration. Evapotranspiration uses heat energy so less evaportanspiration means higher temperatures.  
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Heat island effect varies seasonally and duirnally

  • The heat island effect is stronger at night. This is because rural areas cool down at night but urban areas dont as much because the surfaces continue to release heat they absorbed in the day.
  • UHI stronger during summer in mid latitude cities like London because there is more solar radiation to be absorbed.
  • Stronger whens theres an anticylcone as they cause clear skies and low wind speeds (more solar radiation and warm air not blown away)
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Winds and rain in urban areas

WIND

  • Average winds lover due to tall buildings causing friction
  • Some areas are completly protected by buildings
  • Tubulence around buildings- wind hits the face of the building, some deflected down, some round the sides and some over top. When these winds hit other buildings of the ground they cause voritices (bodies of swirling air).
  • You get powerful gusts of wind when it is channelled down streets- canyon effect.

RAIN

  • Higher urban temp is able to hold more water. The moist warm air rises- convectional uplift/ convectional rainfall
  • Presence of high-rise buildings and a mixture of building heights induces air turbulence which promotes vertical motion
  • Cities produce large amounts or water vapour from industrial sites and power stations
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Fog, thunderstorms and snow

Fog- Increased with industriaisation due to more condenstion nuclei 

Thunderstorms- In urban areas the chance of thunders in increased particularly during the late afternoon and early evening in the summer months. Developed by convectional uplift.

  • Cumulonimbus clouds may develop up to the height of the tropopause where the inversion produces stability 
  • updraft through central area of cloud causes rapidly cooling and condensation leading to the formation of water droplets
  • During condensation laten heat is released, further fueling convection uplift
  • As raindrops are spilt in the updraft postive electrical charge builds up in the clouds (lightening)
  • When the charge is high enough a discharge occurs to negative areas of the could to earth. (thunder) 

Less snow and frost due to the heat island effect

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Urban air quality

Suspended particular matter

Solid matter in urban areas (mainly from power stations + vehical exhausts that float in the air) are respondible for smog, repiratory and may contain carcinogens 

  • Vehical exhaust produce very fine particulates (80%) 
  • Burining of refuse, cigarettes and fuel produce fine and coarse particulates 
  • Construction, mining and quarrying produce coarse particulates
  • Plants and moulds generate coarse particulates 

Sulfur dioxide

Produce haze, acid rain, respiratory problems, damage to lichens and corrosion of buildings

Oxides of nitrogen 

Cause accelerated weathering of buildings, photochemical reactions, resp probs, acid rain and haze

Photochemical oxidants 

Breathing difficulties, eye irritation, coughs, chest pain, damage to plants and smog

Carbon and monoxide

Heart problems, headaches and fatigue

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Photochemical smog

  • Dec 1952 smog killed 4000 people 'pea- souper' = due to high incidence of buring of burning coal on domestic fires
  • Sunlight on NOx and hydrocarbons in vehical exhusts gases causes a chemical reaction producing low level ozone 
  • Photochemical smog is particularly hazardous during anticyclonic conditions because once the air has descended it is static and can persist for weeks. Many cities located in rive basins (mexico city) to the refief insures a sheltered location, perfect for establishing photochemical smog.
  • Bad at rush hour (jnitrogen dioxide in London)
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Congestion charges and Pedestrianisation

CONGESTION CHARGES

  • People are charged if they use their vehicles in certain places at certain times
  • In central London congestion charge has reduced traffic and emissions in the congestion zone by up to 15% since 2003
  • However people travel around the edge of zones to avoid being charged which increases traffic in those areas
  • Difficult to enforce the charge because the volume of traffic is so large its hard to process all the fine correctly

PEDESTRIANISATION

  • Vehicals are restricted from entering certain places at certain times
  • Athens declared an area of about 2.5km traffic free
  • can lead to shops receiving fewer custormers because people cna ony get to them on foot
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Public transport improvement + Legalisation

PUBLIC TRANSPORT IMPROVEMENTS

  • Improved bus services
  • Bus only lanes
  • Park and ride schemes
  • Tram and light railway services which run on line so they dont get caught in traffic. they also pollute less than buses. However they are expensive to construct (metrolink cost over £1 billion)
  • New schemes can cause problems eg.park and ride schemes can shift traffic problems to the rural areas

LEGALISATION

  • The UK clean air act reduced domestic pollution by introducind smoke-free zones where only smokeless fuels could be burned and reduce industrial pollution by introducing tall chimneys.
  • The road vehicles regulations reduce exhaust emissions by ensuring cars pass an emissions test in the MOT
  • Throughout the UK local authorities can issue fines to people who leave their engines running unnecessarily
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Alternative fuels

  • Biofuels are produced by plants. Have lower particulate emission
  • However growing the crops reduces biodiversity
  • Liquefied petroleum gas is gas produced by fossil fuels that has lower emissions than petrol and diesel 
  • However cars have to be converted to use  LPG and service stations adapted to distribute it
  • Electric cars have lower emissions because they run off batteries 
  • However electric vehicles need recharge points and producing and disposing of the batteries can cause environmental problems
  • Lead-free petrol has also reduced pollution in the UK
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