Reasons for the Urban Microclimate
- Buildings absorb more heat than natural surfaces & can store heat
- Create additional particulate matter: dust and hygroscopic nuclei - which affect the amount of sunlight reaching the city
- Affect the amount of moisture in the air
- Tall buildings affect the way air moves - and so wind velocity is reduced
Climate: average weather, usually take over a period of 30 years for a particular region and time period.
Urban Area: an area characterisied by high population density and vast human features in comparison to the area surrond it. This includes cities, towns and conurbations.
In this case larger urban areas have a greater effect on the local micro climate
Temperature: The Urban Heat Island Effect
- Artifical materials used in buildings & roads have a higher thermal capacity then natural surface.
- They store heat during the day and release it slowly at night when air is cooling as a result of radiation.
- Further heating of the air is provided by human activity: e.g. power stations, factories and traffic.
- Diferences in sumer: up to 5 degrees celcius
- Difference in winter: ~ 2 degrees celcius
- Most notable when wind speeds are low as they tend to reduce temeprature differences by dispersing build up of warm air or cloud cover.
- Rediction in the amount of sunlight reccieved is determined by cloud cover & seasons.
- In the UK winter: cloud and dust can abosrb up to 50% of available insolation.
- Conversly over areas of water, for example the River Thames and Lea in London, urban temperatures decrease rapidly.
Precipitation: Frequency & Intensity
- As air is usually warmer in cities it can hold more moisture & relative humidity levels are up to 6% lower.
- There is less vegetation and exposed bodies of water - evapotranspiration is reduced.
- Cloud cover: more frequent and thicker then surronding rural areas, mainly as a result of convection currents & higher concentrations of cloud forming nuclei in the air,
- Higher levels of cloud explain why precipitation is on average 5-15% greater in terms of mean annual totals in large urban areas.
- Monthly rainfall is about 28% higher 30-60km downwind of cities, compared with upwind precipitation.
Precipitation: Hail, Snow & Fog
- 400% greater incidence of hailsotrms, as strong heating, especially in summer, gives rise to very strong convection - 25% greater chance of thunderstorms.
- If precipitation falls as snow, it tends to lie on the ground for less time due to higher temperatures, leading to rapid snow melt.
- Fogs & MIsts tend to be thicker and persist for long - particuarly when anticyclonic conditions mean winds are too weak to blow them away.
- greater concentration of condensation nuclei encourages condensation and enables mist/fog to form more easily
- and poor insolation due to cloud and radiation by dust particles means they tend to persist for longer before being burnt off
- Quality of air in urban areas is invariably poorer than in the surronding countryside.
- Cities may experiance up to 7 times more dust particles in their local atmosphere.
- The dominant factors is the burning of fossil fuels particuarly from veichles and industrial sources.
- The gaseous and solid impurities resulting from these fossil fuels mean that, compared to rural areas, urban areas have as much as:
- 200x more sulphur dioxide
- 10x more nitrogen dioxide
- 10x more hydrocarbons
- 2x more carbon dioxide
- These human made pollutants tend to increase cloud cover and precipitation & cause photo-chemical smog leading to higher temperatures and reduced sunlight.
- In particular sulphur dioxide and nitrogen dioxide are major contributors to acid rain.
Air Quality: Pollutants
- Sulphur Oxdies: esepcially sulphur dioxide are emitted from the burning of coals and oils.
- Nitrogen Oxides: are emitted from high temperature combustion. They can be seen as a brown haze above, or downwind of cities.
- Carbon Monoxide [CO]: a colourless, odourless, non-irritating but poisonous gas. It is a product of incomplete combustion in fuels - mainly from veichles.
- Carbon Dioxide: a green house gas emitted form combustion.
- Volatile Organic Compounds [VOCs]: such as hydrocarbon fuel vapours and solvents.
- Particulate Matter [PM]: is measured as smoke and dust.
Air Quality: Smog [Case Study: London 1952]
- A term used to describe the mixture of smoke and fog.
- More likley to originate from vehicles and industrial emissions that are acted on it the atmosphere by sunlight to form secondary pollutants. These can also combined with primary emissions to form photo-chemical smog.
- Formed when an anticyclone setteled over Southern England & remained static for around five days in early December.
- Thames basin is prone to radiation fog: sheltered from westerly winds.
- Temperature inversion trapped polluted air & a combination of dense fog and sooty black coal smoke produced a dense, persistant smog.
- The extreme reduction in visibility was accompanied by an increase in criminal activity as well as transportation delays & a virtual shut down of the city.
- During the 4 day period of the fog: 12,000 people are believed to have been killed as a result of respiratory problems.
Air Quality: Photochemical Smog
- Forms when sunlight hits various pollutants in the air.
- Produces airborne particulate matter and ground level ozone.
- As these compounds are seen as the by-prdocuts of industrial society, photochemical smog is considered as a problem of modern industralisation.
- It is present in all modern cities, but is most common in cities with sunny, warm, dry cliamtes.
- E.g. Los Angeles, California, USA.
Pollution Reduction Policies
- In the UK air quality targets are set by the department fro Environement, Food and Rural Afairs [DEFRA].
- A network of air quality monitoring centres record and publish the levels of key air pollutants.
- Localised peaks are often in cities, but average values are also important for human health.
- Created a daily warning system made public via media to indicate air pollution levels.
- Urban Background particulate matter levels have shown an overall decreasing trend since 1993, as have roadside particulate levels.
- While rural ozone levels have shown no clear long-term trend, urban background levels have shown an overall rise long term since 1993: mainly due to the reduction in urban emissions of nitrogen oxide, which destorys ozone close to their emission source.
Wind Speed: direction & frequency
- Wind speed is reduced in cities by as much as 30% because there is increase frictional drag as the wind is obstructed by buildings.
- Conversely, very talls buildings can channel wind inbetween narrow spaces creating a canyon effect where, locally, wind speeds are higher,
- This effect, combined with turbulance, can prove a hazard to people at street level.
Conclusion: urban areas do show a significant micr
- Urban areas demonstarate distinctive climatic characteristics that differr from the norm of that region.
- The reasons for this are due to the man made structures that reflect and abdorb temepratures differently to natural surrondings.
- Urban areas also generate more dust, and condensation nuclei.
- Less energy needed for evapotranspiration, due to concrete surfaces, artifical drainage and reduced wind speeds.
- Pollution and smog retain energy.
- Excessive amounts of fossil fuels burnt for industry and business, can be greater than inputs form the sun.
- Buildings retain and condust heat easily.
- Wind speeds are lower as a result of building height and roughness of urban surfaces.