Urban Climate Flashcards
What is the urban heat island effect
Urban areas being warmer then rural areas
What are urban heat islands
Urban areas w higher air temperatures than the surrounding rural areas eg in London
Where are the highest temperatures found
Industrial areas and the most built up eg the CBD
What are temperature sinks
There are pockets of cool air above parks and bodies of water (eg rivers or ponds)
What are temperature plateaus
Areas within the city with the same land use (eg industry) generally have the same temperature
What are temperature cliffs
Temperature can change rapidly when land use changes (eg from inner city housing to CBD, high rise buildings)
Rapid changes are referred to as temperature cliffs
6 ways in which urban climate issues can be managed
Green roofs t
Urban greening t
Cool cars t
Building design w
Laws
Public transport
How do green roofs manage urban climate
Not dark so doesn’t absorb heat
Vegetation stores water so when it transpires it takes energy out of the air
Consists of a growing medium planted over a waterproof membrane
It can reduce rainwater run off
Can increase urban biodiversity by providing habitat space for birds and small animals
How does urban greening manage urban climate
Planting trees and vegetation gives shade- surface peak temp reductions of 5-2 degrees may be possible
It can have a national cooling effect as seen by lower temp in urban parks around the world
Urban trees act as urban stores and can reduce urban flooding by intercepting rainfall and filters pollutants from the air
How do cool cars manage urban climate
Lighter coloured car shell reflects more sunlight then dark car shades
This cools the inside and reduces need for air conditioning
Cars in cities contribute significantly to higher temp and pollution levels experienced there
SO use of cool cars benefit city and driver
How do building designs- the Burj Khalifa Tower
828m tallest building in the world
Concern- windstress and wind vortexes pulling building side to side
Has to withstand gusts of over 240km/hour
Series of aerodynamics improvements had to be made to ensure building can withstand this
Soften edges of building deflect wind around the structure and prevents forming whirlpools/vortexes
The entire tower orientated relative to prevailing wind directions
Sways slowly back and forth about 2m at very top- but doesn’t suffer like skyscrapers in the past
How do laws manage urban climates
Clean air acts
Introduced smoke free zones into urban areas and this policy slowly began to clean up the air
1990- tough regulations imposed on levels of airborne pollution, particularity on PM10s
Local councils in UK now require to monitor pollution in their areas and to establish air quality management
Areas where= likely to be exceeded
London air quality standards improved
2015- NOx emissions still higher then UK and a European law recommended
Measures introduced to clean up 12% of NOx emmisions
More dust suppressants at industrial sites
How does public transport manage urban climate
ULEZ (ultra low emission zone) introduced 2020
Exhaust emission standards set and daily non compliance charge introduced to encourage cleaner vehicles to drive around London
Double decker buses operating in central London=hydbrid and all single deck- 0 emission
What are the 4 main causes of the UHI effect (these are greater in urban areas)
Absorption of heat by urban surfaces (urban- darker)
Air pollution
Heat from human activity
Less evapotranspiration
Buildings
Industry
Car friction
Rapid urban drainage
Explain how absorption of heat by urban surfaces is a cause of UHI
Concrete brick and tarmac surfaces absorb and store heat from the sun during the day
They slowly release the heat as long wave radiation
This is most noticeable at night when it warms the air
Urban surfaces also have low albedo- they absorb more energy rather then reflecting it
How does air pollution cause UHI
Air pollution from cars and factories increased cloud cover over the city
It also creates a ‘pollution dome’ - a layer of pollution over the city
Both these things trap ongoing heat radiation and reflect it back to the surface
How does heat from human activity cause UHI
Cars factories offices central heating etc and people all release heat
How does less evapotranspiration cause UHI
When it rains the water is quickly removed by drainage systems so there’s little surface water to evaporate
Also there isn’t much vegetation so there’s little transpiration
Evapotranspiration uses heat energy, so less evapotranspiration means higher temperatures
How does the UHI effect vary between day and night
The UHI effect is stronger at night
Urban day time temperatures are on average 0.6 degrees Celsius warmer then the surrounding rural areas, but at night time the temperatures can be 3-4 degrees warmer
This is because rural areas cool down at night but urban areas don’t cool as much because urban surfaces continue to release heat that they’ve absorbed during the day
How does the UHI effect vary seasonally
It’s stronger in Summer (in madlutitude cities like Lon)
Average winter temperature can be up to 2 degrees warmer but in summer up to 5 degrees warmer
This is because there is more solar radiation in summer, so urban areas absorb more heat
How do anti cyclones affect the UHI effect
It’s stronger when there’s an anticyclone
They cause clear skies and low winds
If there are no clouds, more solar radiation reaches and heats the ground
Low winds means warm air isn’t blown away
Why is average wind speed lower in cities then rural areas
Tall buildings create friction that slows down the moving air
Where are wind speeds O
Areas that are totally sheltered from wind by buildings
Explain wind vortexes
You get turbulence around buildings
This happens when wind hurts the face of a building- some it’s deflected down, some around the sides and some over the top
When these winds hit other buildings or the ground they cause vortexes (bodies of swirling air)
What is the canyon effect
You gg powerful gusts of wind when wind is channeled down streets
This is known as the canyon effect and affects the planning of buildings
Where does it rain more and more intensely
Urban
Why does it rain more and more intensely in urban areas (2)
The UHI effect means the air in urban areas is warm and warm air can hold more water
The warm moist air rises- this is called convection uplift
As it rises, it cools, the water vapour condensed and it rains- convection rainfall
Urban areas generate huge amounts of dust and pollution.
Particles of dust and pollution floating about in the air acts as condensation nuclei (the trigger water to condense around them)
This encourages clouds to form, rather than allowing the warm, moist air to disperse
V rapid warmer n and rising leads to formation of thunderstorms
Where is their more fog and why
Urban areas
As there’s a higher concentration of condensation nuclei
Where is there more snow and fog and why
More in rural areas and lesss in urban areas
Snow melts faster in urban area
This is because it is warmer due to UHU effect
Also why fewer days of frost
How does wind speed differ in rural and urban areas
Wind on average 30% lower in urban places
However, there are heat variations in wind speeds within an urban environment caused by the alignment, height and distribution of urban buildings
What is the wind break effect
Some urban places are v sheltered from effects of the wind
What is the ventuin effect
Violent gusting as air is forced through narrow gaps and air pressure is reduced
How many more clouds are in urban places
10%
Percentage more of pollutants in urban places
10X more nitrous dioxide- causing respiration problems and acid rain
200X more sulphur dioxide (same as above)
2X more Co2- contributes to climate change
10X more hydro carbons cause heart problems, headaches and tiredness
More diesal particulates cause respiratory problems and dirty buildings
