🌆3.2.3.4 - Urban Climate Flashcards
What aspects of the climate can urban areas modify?
Wind speeds
Temperature
Clouds and precipitation
Pollution
In what ways can winds be modified by urban areas?
Lower speeds
Greater variability
Large-scale convection
Why are wind speeds generally lower in urban areas?
The roughness of the land surface, consisting of buildings at a variety of heights, creates greater surface friction
What is an urban canyon?
A street with tall buildings either side but a space in between
What is the Venturi effect?
The effect of ‘squeezing’ the airflow, increased as buildings get taller towards the city centre
Where is the highest air pressure?
In the upper part of the building with air flowing down the front and over the top
How does convection occur over urban areas?
Convection and uplift over the area means that air pressure is lowered and air is drawn in from surrounding rural areas
What is the urban heat island?
Urban areas are on average 1-2 degrees warmer than their surrounding rural areas
What is albedo?
The degree of warming of a surface depends on the amount of insolation absorbed by the surface, which in turn depends on how much is reflected. The reflection is albedo.
What is sky view factor?
The amount of sky you can see without your view being impeded by tall buildings
What is specific heat capacity?
The amount of energy required to raise the temperature of 1kg of a material by 1 Kelvin
What is the intensity of an urban heat island?
The maximum difference between the temperature of the rural area and the peak urban temperature
How can weather conditions impact the intensity of the urban heat island?
High pressure systems with clear nights and sunny days increase the intensity
What are the five main factors causing urban heat islands?
Anthropogenic heat Height and arrangement of buildings Nature of building materials Presence of water Pollutants
What is sensible heat?
Heat energy which can be felt, as opposed to latent heat which must be measured
How does cloud cover differ in urban areas?
It is greater
Why is cloud cover greater in urban areas?
There are more condensation nuclei, so cloud formation is greater
Why are there more thunder storms in cities?
Enhanced convectional uplift from the heat island can lead to instability in thermals, creating more storms
How do precipitation levels differ in urban areas?
They are higher
What is the speed effect?
Wind is slowed by friction from the building, friction with obstacles
What is the downdraught effect?
Turbulence is created around the top of the building, downdraught forms at the front
What is a downwind eddy?
A circular motion of wind behind the building and turbulence near the top of the building occurs as the wind is forced upwards
What can photochemical pollution lead to?
Smog - consisiting of ozone and peroxyacetyl nitrate (PAN)
When were London’s ‘Pea Soupers’?
December 1952
How did the smog form in London?
Windless, cold climate and an anticyclone (little wind, high pressure, still air) nothing moved and the particulates were trapped
What did the smog cause?
Pneumonia, TB, heart failure, bronchitis
How many died in the London smog?
4000+
When was the clean air act introduced?
1956
What is urban greening?
Involves growing plants wherever possible in urban areas
What are the benefits of urban greening?
Improved air cooling, making towns and cities more bearable
Improving air quality by absorbing gases such as CO2
Improved respiratory health
What is zoning of industry?
The process of dividing land into zones, and moving industry to the outskirts
What does zoning of industry lead to?
Less particulates being trapped over the city in the cool, moist air above
What are transport solutions?
Bus only lanes introduced to encourage the use of public transport in cities, congestion charges introduced
How does temperature vary in cities?
Pockets of cool air above parks and water bodies (temperature sinks)
Highest temperature in industrial areas and densely populated areas
Causes of Urban Island Effect
Absorption of heat by urban surfaces
Air pollution
Heat from human activity
Less evapotranspiration
Absorption of heat by urban surfaces
. Urban surfaces have low albedoes- absorb heat in day, release it at night
. Windows reflect heat
Air pollution
. Increased cloud cover
. ‘Pollution dome’
. Reflect outgoing heat into the city
Heat from human activity
Air conditioning
Central heating
Cars
Less evapotranspiration
. Little vegetation- less heat energy used, increased temp
. Water quickly removed through drainage systems, less to evaporate
Issues with urban heat island effect
. Heat stroke
. Pressure on energy supplies (air con)
. Increased water consumption in warmer weather
. Increased evapotranspiration rates
. Plants flower longer + earlier (discomfort for allergy sufferers)
. Increases rates of temperature related chemical weathering
Strategies to reduce urban heat island effect
Green roofs
Urban greening
Light cars
Discourage urban canyons
How do green roofs reduce urban heat island effect?
Decreased roof temp 20-40C
Increased urban biodiversity
How does urban greening reduce urban heat island effect?
Shade = cooling
How do light cars reduce urban heat island effect?
Reflect heat
Cooler inside of car
Less air con needed
How does discouraging urban canyons reduce urban heat island effect?
. Urban canyons decrease ventilation if perpendicular to prevailing winds
. Slows removal of heat
Why are wind speeds overall slower in urban areas?
Tall buildings = friction
Venturi effect
. Air squeezed between streets of tall buildings
. Creates high velocity winds
How does urban heat island effect worsen urban winds?
. Urban heat island effect greatest at night
. Air drawn in from cooler surrounding areas
. Creates strong, localised win
How can urban wind be reduced?
Buildings on stilts
Decreases pressure difference
Causes of increased rain, fog, and thunderstorms in urban areas?
. Pollutants- condensation nuclei
. Urban heat island
How does urban heat island worsen urban rain, fog, and thunderstorms?
. Warm moist air holds more water
. Rises (conventional uplift)
. Condenses, rain (conventional rain)
Why is there less snow and frost in urban areas?
. Increased temperatures
. Melts quickly
Causes of particulate pollution
. Vehicle exhausts (80%)
. Burning (cigarettes and refuse)
. Construction, mining, quarrying (cement dust)
. Plants, moulds (pollen, mould spores)
Causes of photochemical pollution
Burning fossil fuels
How can air pollution be reduced?
Transport
Clean Air Acts
Industrial Zoning
Urban Greening
Ways to reduce air pollution: transport
Congestion charges
Pedestrianisation
Public transport
Alternative fuels
When was the Clean Air Act introduced in London?
1956
Ways to reduce air pollution: industrial zoning
. Too late in HIC’s
. LIC cities designed to have industry downwind
. Taller chimneys
. Force factories to reduce gas and particulate emissions (filters)
Ways to reduce air pollution: Urban Greening
. Plants filter pollutants from air
. Plants in urban canyons- decreased NO2 by 40% and particulate matter by 60%
Walkie Talkie
. Its curved face is channeling gusts strong enough to knock people over
. Reflects light intense enough to melt cars
. This is the downdraught effect when wind is forced downwards - most forceful when facing prevailing wind
. To solve problem protective fins have been added
