Urban microclimates Flashcards
How much warmer are urban areas to the surrounding rural area?
0.5-0.8
What is general rule for the effects of buildings in urban areas?
The effects of buildings extend downwind by ten-times the height of the buildings
As a result of the general rule for the effects of buildings in urban areas, how much windier are urban areas to rural areas?
30%
What are the 3 urban air pollution reduction strategies?
Legalisation
Vehicle restriction
Technical innovations
How can legalisation reduce air pollution?
Strict controls on emissions
Smoke-free controls
Clean air act
Air quality targets are set and monitored by DEFRA
What are the advantages of legalisation?
People will be obliged to obey as consequences will be enforced if rules are broken - this will evidently reduce air pollution as people must follow
What are the disadvantages of legalisation?
Difficult to enforce - can take a long time
Expensive
Unfair of LIC
How can vehicle restrictions reduce air pollution?
It can reduce the number of vehicles in central urban areas
Park and ride schemes
Congestion charge
Development of GPS to monitor
What are the advantages of vehicle restrictions?
The reduction of vehicles on the road will bring down pollution in urban areas
Reduced crashes
Less traffic - quicker journeys
Congestion charge will encourage less people to drive
What are the disadvantages of vehicle restrictions?
Inconvenience
Contentious
Strain on public transport
How can technical innovations reduce air pollution?
Through industrial pollution controls using filters, lead-free petrol, catalytic converter, development of hybrid electric
What are the advantages of technical innovations?
By using filters it reduces toxins
Development of catalytic converters provides entrepreneur opportunities
What are the disadvantages of technical innovations?
Unfair of LICs
What does Albedo define as?
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.
What has a greater albedo lighter or darker surfaces?
Light surfaces as they reflect more
What is photochemical pollution?
A form of air pollution that occurs mainly in cities and can be dangerous to health.
How is photochemical pollution formed?
Exhaust fumes become trapped by temperature inversions and, in the presence of sunlight, low-level zones forms. It is associated with high-pressure weather systems
What is a temperature inversion?
An atmospheric conditions in which, usually, increases with height
Why do inversions trap pollution in the lower layers of the atmosphere?
Because they are extremely stable conditions and do not allow convection
What is the urban heat island?
The zone around and above an urban area, which has higher temperatures than surround rural areas.
How much of an increase in precipitation is there in urban areas to the surround rural area?
5-10% increase
How much of an increase in pollution is there in urban areas to surrounding rural areas?
1,000% increase
Why are cities warmer than rural areas?
Surfaces in the city tend to be much less reflective than those in rural areas
Why does the fact surfaces in the city being far less reflective than those in rural make the city warmer?
Building material have low albedo, and absorb large quantities of heat during the day which is slowly released at night
Air pollution from industries and vehicles increase cloud cover and creates a ‘pollution dome’
Impermeable surfaces dispose of water quickly
Heat is leaked from buildings
What is reflect in urban areas which does heat up the city?
Windows - they concentrate the heating effect in the surrounding streets by reflecting energy downwards
How does air pollution make the urban area warmer?
It creates more cloud cover which allows short-wave radiation but absorbs a large amount of the outraging radiation as well as reflecting it back to the surface
Why does the fact the surfaces are impermeable in urban areas make the area warmer?
Because it changes the urban moisture and heat budget - evapotranspiration means that more energy is available to heat the atmosphere
Why is the urban heat island a matter of concern?
Vulnerable groups like those who have asthma will be affected by the warmer temperatures
High pollution levels
Strain on supply of energy for air - conditioning
Increased water consumption because of heat - strain - water restrictions?
More plants - hay fever
Climate change
Why high pollution levels greater in urban areas?
Chemical reactions that produce ozone and smog are accelerated by high temp while the lower wind speeds keep heat and pollution trapped in city
What are the strategies for managing urban heat island?
Cool surfaces Green roofs Urban greening Sky view factor Cool cars
How does cool surfaces act as a strategy for managing the urban heat island?
Cool roofs built from materials with high albedo store less solar energy during the day - not major emitters of heat
How does green roods act as a strategy for managing the urban heat island?
Reduce rainwater run-off, act as an insulators and increase urban biodiversity by providing habitat spaces
How does urban greening act as a strategy for managing the urban heat island?
Planting trees and vegetation provides shade having a natural cooling effect
Trees are a carbon store, filter pollutants
Reduce flooding through intercepting rainfall
How does sky view factor act as a strategy for managing the urban heat island?
Lets heat escape
What is the sky view?
It describes the relative openness between buildings in an urban area - narrow streets have a restricted sky view
Why is restricted sky view not good for the urban heat island?
It reduces the escape of heat from street and building surfaces
How does cool cars act as a strategy for the urban heat island?
A lighter coloured car shell reflects more sunlight than a traditional dark shell - this cools the inside of the car and reduced the need for air conditioning
What did a recent study find about ‘car cooling’?
After parking in the sun for an hour, a silver Honda Civic had a cabin air temp about 5-16 degrees lower than an otherwise black shell car
Why is precipitation higher over urban areas than rural areas?
Urban heat island develops convection
High-rise buildings increases vertical motion of winds
City pollution can increase cloud formation (condensation nuclei)
Increased production of water vapour through industrial sources
Why does city pollution increase chance of rainfall?
Through pollutants which are hygroscopic (water attracting)
What is channelling?
Wind redirected down long straight canyon-like streets where there is less friction (sometimes known as urban canyons)
What is the Venturi effect?
The squeezing of wind into an increasingly narrow gap resulting a pressure decrease and velocity increase
What are the main effects which buildings have on wind?
Uneven surfaces cause turbulence as wind is forced around them
Exert a frictional drag through rigid surfaces - slower wind speed
What is case study of an area ‘coping with the wind’?
The Burj Khalifa in Dubai
What is the Burj Khalifa?
The tallest building in the world
At what speeds of wind is the Burj Khalifa built to withstand?
240km per hour
How has the Burj Khalifa been modified to cope with wind?
Softened edges deflect wind around
The structure prevents whirl whirlpools being formed
Its been orientated relative to the prevailing wind direction
What are the Amin strategies to reduce the number of cars in urban areas?
Mass transit system Park and ride Banning cars from driving on certain days and alternating between cars with licence plates that end in uneven or even numbers Greater provision for cyclists Road schemes like the urban bypass Bus lanes Congestion charge
What is an example of a mass transit system?
Metrolink in Manchester
What is an example of an area with greater provision for cyclists?
‘Snake’ bridge I’m Copenhagen
What is an example of an urban congestion charge?
London in 2003
