CUE - Urban Climates Flashcards
Urban climate
A set of climatic conditions that prevail in a large metropolitan area which differ from the climate of the rural surroundings
Albedo
The reflectivity of a surface- the amount of incoming insolation that is absorbed and reflected back
Darker surfaces have a lower albedo
Insolation
Solar radiation
Anticyclonic
High atmospheric pressure - dry, calm conditions
Summer - heatwave
Winter - cold and clear
Condensation nuclei
Water-attracting particles e.g dust in the atmosphere around which raindrops can form condensation nuclei
Micoclimate
Small-scale variations in temperature, precipitation, humity, wind speed and evapouration that occur in a particular environment
Urban dome
The urban microclimate within which the weather is different fom the surrounding rural area
Two levels to the dome, the Urban Canopy and the Urban Boundary
Particulates
Microscopic matter in the air e.g PM10 - exhaust fumes, ash, cement dust
Photochemical pollution
A form of air pollution that occurs mainly in cities - can be dangerous to health
Exhaust fues become trapped by temperature inversions and react with sunlight to form low-level ozone
Temperature inversions
An atmospheric condition where a layer of warm air lays on top of colder air underneath
This does not allow convection so pollution becomes trapped in the lower layer of the atmosphere around the city
The urban heat island effect
The zone around and above cities that has higher temperatures than the surrounding rural areas
Isotherm
A line joining places wth equal temperatures
Chanelling
Wind redirected down long, straight canyon-like streets where there is less friction
Sometimes referred to as urban canyons
What causes the urban heat island effect?
Urban surfaces have a lower albedo so absorb more insolation which is released slowly when the air cools at night
Effective urban drainage removes surface water quickly so less evapouration takes place (has a cooling effect)
There is less vegetation for evapotranspiration (has a cooling effect)
Anthropogenic heat sources e.g heating systems, machines, cars and industrial processes
Why are precipitation and storms more frequent and intense in urban areas?
Urban areas have up to 10% more rainfall than surrounding areas
The urban heat island effect causes low atmospheric pressure and convectional uplift -> convectional rainfall
Particulate pollution means more condensation nuclei present in the urban air
Why is humidity less frequent in urban areas?
More rapid runoff of water
Less evapotranspiration
Less bodies of water
Why is fog more frequent in urban areas?
Greater concentration of airborn particulates that act as condensation nuclei
Why are thunderstorms more frequent and intense in urban areas?
In urban areas the liklihood of thunderstorms is 25%
Low pressure caused by convectional uplift form tall cumulonimbus clouds
Rising heat, water vapour and particulates creates intense rainfall and thunderstorms
During condensation, latent heat is released that fuels convectional uplift. Raindrops are split in the uplift creating a positive electrical charge which forms lightning
What are the wind speeds like in urban areas?
Average wind speeds are lower in cities than surrounding areas as the buildings exert a powerful frictional drag on the moving air creating turbulence (rapid changes in wind speed and direction)
Average wind speeds can also be high as high rise buildings can channel air into the ‘canyons’ between them to create the Venturi effect so gusts of wind are stronger
How can a single building modify the flow of air passing over it?
Air is displaced upwards and around the sides of the building
the descending flow causes a vortex when it reaches the ground
How can a group of buildings modify an airflow passing over them?
If the buildings are widely spaced the airflow ill be similar to that of a single building
If the buildings are close together the airflow is interrupted by the next strcuture so produces an unpredictable pattern of airflow
What is air quality like in urban areas?
Key atmospheric pollutants are ozone, nitrogen dioxide, sulphur dioxide and particulate matter
Photochemical smog can be very harmful to people’s health e.g headaches, eye irritation and chest pain
Give an atmospheric measure in urban areas
UK Clean Air Acts of 1956 and 1968 reduced domestic pollution by introducing smoke control areas and reduced industrial pollution with taller chimneys to disperse pollutants higher in the atmosphere
Give a policy for vehicle control
Congestion charges in central London - people are charged if they use their vehicle in certain areas at certain times
Reduced traffic and emissions in the congestion zone by 15% in the first year
Give a policy for public transport systems
Encouraging people to use public transport services instead of their cars to reduce pollution e.g improved bus services and park and ride schemes
e.g the Metrolink in Manchester which took over 2.5 millon cars off of the road
How is photochemical smog formed?
Pollutants e.g nitrous oxides and hydrocarbons trapped in the lower atmosphere by temperature inversions comes into contact with sunlight and the UV rays break them down into ozone which can produce harmful photochemical smog
