urban climate Flashcards
what is an urban heat island and when is it highest
when urban areas have higher tempertarue than surrounding rural area
can be 1-3 degrees warmer on average
and at night can be 12 degrees difference
highest = mid afternoon over CBD
2nd peaks = suburbs
diff largest = night due to buildings store heat
how do UHI happen
- surfaces less reflective so absorb more heat and then release it at night
- large buildings refelct heat onto street
- air pollution creates cloud cover which reflects radiation back down
- drains remove water so no cooling effect by evapotranspiration
- heat from industry
problem with UHI
high temp causes heat stroke etc
in anticyconic conditions UHI increased
warmer temps low level ozone and smog production is accelerated so more polluted air
more water consumption
drier soils
manage UHI
- build cool surfaces with high albedo to reflect energy
- “green roads” porous so water in and grass can grow so less energy absorbed
- urban greening to provide shade reducing temp by up to 15 degrees
- more trees more carbon stored and more interception and less pollutants
- streets parallel to wind so ventilated and pollutants escape
precipitation in urb area
higher rainfall as hotter so more convectional rainfall
rain downwind is 20% greater upwind urb area
why does it rain more
- heated ground so rapid evapotranspiration forming cumulous cloud
- pollutants form clouds and provide condensation nuclei for raindrop formation
- industry releases water vapour
- low pa draws surface wind which rises over higher urb canopy and is spit over buildings when it converges it rises form raincloud
fog in urban areas
greater due to particulates acting as condensation nuclei encouraging fog formation over night
industrialisation increased it
1700s 20 days
1800s 50 days
reduction of fog
clean air act 1950s
reduced smoke production and particulates released
saw less fog
impact of heavy fog
new dehli winter fog trapped pollutants forming toxic fog
deaths
thunderstorms in urban areas
event that happens in hot humid conditions
heavy rain/thunder/lightning
common in urban areas
how do thunderstorms form
1) cumulonimbus cloud due to convection
2) up draught air through cloud cause cool and condense forming water droplets releasing latent heat fueling uplift
3) raindrops split forming positive charge which builds and overcomes cloud and charge is discharged to negative areas on earth - lightning
4) the high temp causes rapid expansion air developing shockwave known as thunder
wind in urban area
wind speed is slower due to friction from buildings
urban wind patterns
1) clearn night, UHI greatest, convetional processes draw strong winds
2) high rise, CHANNELING, street level fast, removes particulates
VENTURI EFFECT wind funnelled through narrow gaps forming very strong gusts making it hard to walk
3) descending flow on windward side forms VORTEX
on lee side low pressure draws air down forming complex vortices
how to buildings impact wind
far apart = each act as individual
close = wakes interact
very close = skimming, dead air where pollution build
mitigation urban wind issues
city planning
streets parallel prev wind to ventilate them and remove particulate, will lower the UHI aswell
barriers in venturi hotspots to reduce high speed gusts
air quality in urban areas
poorer than rural due to high particulate emissions from industry and vehicles
types of pollutants
co from cars - nausea headache
NOx from cars - inflame lungs + acid rain
PM from exhausts and industry - settles in lungs
SO2 from coal burn - tight chest + acid rain
how does pollution get bad
natural
combust fossil fuels
5x worse winter due to temperature inversions
what is smog
when fog and smoke particulate combines
1952 in LDN 4000 died as so thick
photochemical smog is associated with plant damage and eye irritation (LA eye sting)
sunlight causes NOx from exhausts to form low level ozone
pollution reduction
Clean Air Act 1952
- smoke free zones
- particulate regulations
- local councils monitor
- clean up construction sites (12% LNDs NOx)
Cars
- ban on certain days
- better public transport
- regulations on polluter vehicles
London
280,000 trees
Congestion charge 2003
ULEZ 2019
zero emission buses
1500 EV charge points
2016 exceeded houly legal limit particulate by 4000hrs
2019 just 100hrs (97% reduction)
city planning
- place industry downwind
- chimneys high above inversion zone
