2.11. Urban Climate Flashcards
What is 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.
Light surfaces reflect more than dark surfaces.
What is a microclimate?
The small scale variations in temperature, precipitation, humidity, wind speed and evaporation that occur in a particular environment such as an urban area.
What is 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 the combustion of fossil fuels.
What is photochemical pollution?
A form of air pollution that occurs mainly in cities and can be dangerous to health.
Exhaust fumes become trapped by temp inversions and in the presence of sunlight, low level ozone forms.
It is associated with high pressure weather systems
What are temperature inversions?
An atmospheric condition in which temperature usually increases with height.
As inversions are extremely stable conditions and don’t allow convection, they trap pollution in the lower layer of the atmosphere.
What is an urban heat island?
The zone around and above an urban area which has higher temps than the surrounding rural areas.
What are the causes of urban climates?
- Less vegetation so less evapotranspiration
- Drains and sewers remove surface water quickly so reduce moisture in the air
- Large expanses of glass and steel reflect heat onto the surrounding streets
- Urban areas have lower albedo -> lots of dark surfaces like roofs and tarmac absorb heat
- Rising heat, water vapour from power stations and industry and condensation nuclei from power stations provide necessary conditions for precipitation
- Power stations, industries, vehicles and inhabitants generate their own heat
- Buildings leak heat through poor insulation in winter and air conditioners pump hot air into streets in summer
What is thermal gradient?
- Temp decline from urban centre to rural-urban fringe -> difference can be more than 5 degrees Celsius in late summer and 2 degrees Celsius in winter
- Usually strongest at night
Daytime: 0.6 degrees Celsius warmer
Night: 3-4 degrees Celsius warmer
Social concerns with urban climate
- As temps rise in summer months, conditions can become uncomfortable in buildings and on transport
- Heat stroke, asthma, organ damage and deaths increase
- Vulnerable groups e.g. babies and elderly are most likely to be affected
- Increased strain on supply of energy for cooling and air conditioning
Environmental concerns with urban climate
- Higher pollution levels -> lower wind speeds keep heat and pollution trapped in the city
- Higher evapotranspiration rate so greater extraction of water from the soil
- Earlier flowering times of vegetation and prolonged growing season
- Prolonged survival and increased reproduction rates of some animal and insects pests
Economic concerns with urban climate
- Increased heat leads to increased water consumption by residents and businesses -> extra strain on water supply and infrastructure-> can lead to water use restrictions
- Increased risk of deterioration of historical monuments and buildings through increased rates of temperature related chemical weathering
Management strategies: cool surfaces
- Cool roofs built from materials with high solar reflectance or albedo and store less solar energy during the day so aren’t major emitters of heat at night
- Cool pavements are also being trialled around the world
Management strategies: green roofs
- Consist of growing medium planted over a waterproof membrane, can reduce rooftop temps by 20-40 degrees Celsius on a sunny day
- Reduce rainwater run off, act as insulators and increase biodiversity
Management strategies: urban greening
- Planting trees and vegetation provide shade and can have a natural cooling effect
- Urban trees= carbon stores and can reduce flooding through interception and filter pollutants
