Urban Climate Flashcards
Define urban heat island effect
The temperature difference between urban / suburban areas and the surrounding rural areas
What is the average temperature difference between an urban area and its surroundings?
1-3°C warmer in a city.
Clear, calm night -> 12°C warmer.
What are the factors that cause urban temperatures to vary?
Season
Weather conditions
Sun intensity
Ground cover
Give reasons for when UHI varies
Largest in summer
Decreases as size of city decreases
Give reasons why cities are warmer than rural areas
- Less reflective surfaces -> much lower albedo, absorb heat during day, slowly release heat at night.
- Air pollution from industry / vehicles -> increase cloud cover + ‘pollution domes’ absorb outgoing radiation.
- Disposal of water runoff -> changes urban moisture / heat budget.
- Heat emitted from industry / vehicles / buildings / people.
Why is UHI a concern?
- Hot temps make indoors / transport uncomfortable.
- Heat stroke, asthma, organ damage, deaths -> esp babies / elderly.
- UHI caused by hot, still, anticyclonic weather conditions -> more air pollution.
- Hot temps -> strain on energy for cooling / air con.
- Strain on water resources -> higher human consumption (drinking), more evapotranspiration, plants extract more from soil.
- Earlier flowering times / prolonged growing season -> allergies.
- Higher reproduction / lifespan of animals, insect pests.
- More algae in water.
- More deterioration of historical monuments / buildings -> more temp-related chemical weathering.
- Climate change -> increase intensity of UHI.
Name the policies for reducing pollution / managing UHI
Cool surfaces. Green roofs. Urban greening. Sky view factor. Cool cars.
What are ‘cool surfaces’ as a policy to reduce pollution / manage UHI
Eg cool roofs, cool pavements
-> made of materials with high solar reflectance -> absorb / store less solar energy during day -> emit less at night
What are ‘green roofs’ as a policy for reducing pollution / managing UHI?
Eg Plants on rooftops
- > lower roof temps by 20-40°C on a sunny day.
- > reduce rainwater runoff.
- > increase biodiversity.
- > insulators.
What is ‘urban greening’ as a policy for reducing pollution / managing UHI?
Eg Planting trees / vegetation
- > provides shade.
- > reduce peak temps by 5-20°C.
- > trees reduce urban flooding.
- > trees filter pollutants.
- > trees = Carbon store.
Define ‘sky view factor’
Relative openness between buildings
How does ‘sky view factor’ affect UHI?
Restricted sky view…
- > reduce escape of heat from surfaces -> heat builds up in ‘street canyons’.
- > reduce ventilation in UHIs if streets at 90° to prevailing winds.
What are ‘cool cars’ as a policy for reducing pollution / managing UHI?
Eg light colour car shells
- > reflect more sunlight -> reduce need for air conditioning.
- > cars are large contributors to UHI and pollution.
Which parts of local climate do urban areas affect?
Wind speed
Temperature
Clouds / precipitation
Pollution
What percentage of the world’s population is predicted to live in urban areas by 2025?
80%
How can urban areas affect wind?
Lower speeds
Greater variability
Large scale convection
Do urban areas have lower wind speed than outlying suburbs? Why?
Average 5% lower wind speed in city centre.
-> roughness of land surface -> increase surface friction
Why does wind speed vary in cities?
Due to layout of buildings / building height.
Where do ‘urban canyons’ form? How?
Straight streets lined with tall buildings -> urban canyons.
Buildings funnel winds -> Venturi effect -> high velocity
How is wind turbulence created?
High rise buildings disrupt the flow of air -> turbulence.
Any air not flowing in the general direction will cause turbulence
Give an advantage and disadvantage of wind turbulence / gustiness
Adv - turbulence ‘flushes out’ pollutants at street level.
Dis - unpleasant for pedestrians
What does ‘large scale convection’ caused by UHI lead to?
-> lowering air pressure -> air is drawn in from surrounding rural areas
Define ‘intensity’ of UHI
The maximum difference between the temperature of the rural area and peak urban temperature -> at night
Name the factors that influence the intensity of UHI
- Weather conditions -> high pressure systems with clear nights, still sunny days -> increase intensity.
- Season
Name the main factors that create UHI
- Anthropogenic heat
- Height / arrangement of buildings
- Nature of building materials
- Presence of water
- Pollutants
Define anthropogenic heat
Heat released by human activity
Eg vehicles, central heating (/air conditioning), industry
Give the reasons for rainfall patterns in urban areas
- UHI creates convection : rapid evapotranspiration -> clouds.
- High rise buildings / range of heights -> air turbulence + increased vertical motion.
- Surface winds drawn in from surrounding areas due to low pressure / prevailing winds move over city -> friction from urban boundary acts similar to mountain barrier -> air travels downwind of high rise buildings -> converge and rise upward -> rain clouds.
- City pollution -> pollutants are ‘water attracting’.
- Industry / power stations -> release water vapour -> clouds.
What percentage more rainfall is there downwind from major areas, compared to upwind?
20% more rainfall downwind from major areas
Why is there more rainfall downwind from major areas than upwind?
Heating of the surface / overlying air -> instability in atmosphere -> encourages air to rise.
Cools as it rises -> water vapour condenses into rain -> falls downwind of the city.
Why has the occurence of fog in urban areas increased?
Industrialisation
Eg London early 1700s - 20 days of fog per year
London late 1800s - over 50 days of fog per year
Why is there more fog in urban areas than rural areas?
Greater average number of particles in the are in urban areas.
Particles act as condensation nuclei -> encourage fog formation at night under high pressure conditions.
What strategies have been used to reduce the amount of fog in urban areas?
Clean Air Acts of 1950s
-> reduced smoke production, fewer particulate emissions in U.K. -> fewer foggy days
Give examples of places that experience high levels of fog.
What is the issue associated with this?
New Delhi, Beijing -> regular winter fogs.
‘Airpocalypse’ - fog traps pollutants -> toxic smog -> high death toll.
How is thunder created?
Extreme temperature -> rapid expansion of air -> shockwave -> thunder.
What is a reason thought to explain why there are more thunderstorms in urban areas?
Air pollution -> disturbs electrical charge of clouds -> lightning
How is lightning created?
By conventional heating and rapid uplift of air -> instability in atmosphere.
Uplifted air rapidly cools and condenses due to cold clouds -> releasing its heat -> further uplift.
Positive electrical charges build up in clouds -> lightning
