Urban Microclimates (DP IB Geography)
Revision Note
Written by: Bridgette Barrett
Reviewed by: Jenna Quinn
Urban Microclimate Modification
Cities create their own microclimate domes
This means that they have unique:
Temperature ranges
Wind patterns
Clouds and precipitation rates
Pollution
Urban microclimates vary according to urban areas’ size, shape and location
There are several causes, some of which include:
Changes to land surfaces: concrete, brick and tarmac
Cities have fewer trees than surrounding rural areas
Trees shade the ground, preventing heat from the sun from being absorbed
Dark rooftops and dark pavement absorb more solar radiation
Tall buildings reflect and absorb sunlight
Cars engines and factory exhaust produce heat
Fewer plants in urban settings mean that less evapo-transpiration occurs (a process that cools the air)
Poor building insulation means the release of heat at night
Within these microclimate domes, there are two levels:
Urban canopy - processes act in the spaces between buildings below roof level
Urban boundary - processes acting above roof level and extends downwind as a plume into the surrounding rural areas
Patterns of precipitation and air quality are extended to immediate areas via the prevailing winds
A diagram of the microclimate dome
Urban climates have the following characteristics:
Lower relative humidity
2-3 weeks fewer frosts
Greater diurnal temperature range: higher max and min temperatures as compared to rural areas
Pollution levels are higher
Fog and photochemical smog are likely
5–10% more cloud
5–15% more precipitation
Increased thunderstorms
Lower wind speeds, with the exception of the tunnelling effect
Varying pressure gradients
Urban Heat Island Effect
The air in urban areas can be 2 - 5°C warmer than nearby rural areas
This is known as the urban heat island (UHI) effect
The UHI is most noticeable when there is little wind
The highest temperatures are found in densely built-up areas and industrial areas, where activities generate more heat
Temperature sinks (where temperatures fall) are found above green spaces and water - e.g. parks and lakes
Temperature plateaus (where temperatures remain the same) occur in areas with the same land use - e.g. industrial areas
Temperature cliffs (where temperatures increase) occur when temperatures change rapidly from one land use to another - e.g. suburban housing to high-rise inner city buildings
Illustration of the urban heat island effect
Causes of the urban heat island effect
The main causes of the UHI effect are:
Air pollution: pollution from cars, industry, etc. increases cloud cover and produces a pollution dome. Both of these trap outgoing heat and reflect it to the surface
Heat from human activities: air conditioning units, heating homes and offices, etc.—all release heat into the surrounding area
Absorption of heat by urban surfaces: urban surfaces have a low albedo. Tall buildings reflect and absorb sunlight
Less evapotranspiration: removal of green spaces and trees reduces the cooling effect of evapotranspiration
Diurnal and seasonal temperatures
There is a larger range between daytime and night-time temperatures (diurnal range), compared to rural differences
In urban areas, daytime temperatures are approximately 0.6°C warmer and night-time temperatures can be up to 4°C warmer
Rural areas do not store as much energy and release heat quicker than urban areas
Average urban summertime temperatures can be as much as 5°C warmer in mid-latitude cities, with average winter temperatures of 2°C warmer
Temperatures can increase during periods of anticyclonic weather (high pressures)
These produce clear skies and low winds
Which allow greater insolation to reach urban surfaces
The low winds prevent warm air from being dispersed
Urban Microclimate Management
Strategies to reduce the urban heat island (UHI) effect
Strategies are aimed at reducing the causes of the UHI effect, which are:
Air pollution
Heat from human activities
Absorption of heat
Removal of green spaces
Reduction of air pollution
Reducing emissions by introducing clean air zones and congestion charges
Modifications to buildings and planning
Changes to building design, such as using reflective materials, can reduce the absorption of heat
Adding gardens to rooftops increases the amount of vegetation as well as decreasing heat absorption
Making buildings more energy efficient to reduce the loss of heat
Increasing green spaces
Parks, gardens and vegetation increase evapotranspiration, which has a cooling effect
The increased vegetation cover also improves air quality
Air Pollution Patterns & Management
Air pollution patterns
The amount of particulates in urban areas is greater than in rural regions
Sources include:
Vehicle exhausts
Burning wood, coal, cigarettes, rubbish, etc. releases fine and coarse particulates
Construction, mining and quarrying
Plants and moulds generate coarse particulates such as pollen and mould spores
Poor urban air quality has several effects, including:
Respiratory problems such as asthma
Increasing haze through increased emissions of sulphur dioxides and nitrous oxides
An increase in carbon dioxide, adding to the enhanced greenhouse effect and global warming
Increased particulates in the atmosphere attack building facades
Photochemical oxidants cause eye irritations and headaches
Smog
Smog happens when smoke particulates, sulphur oxides, hydrocarbons, etc. mix with fog
The London smogs of the 1940s were caused by the sinking of cold air trapping air, pollutants in a pollution dome
Today smog is more likely due to photochemical reasons:
Sunlight reacts with the chemical pollutants in the atmosphere
UV light causes them to break down into secondary, harmful chemicals to form photochemical fog
Photochemical fog is a major problem in large cities like Los Angeles, Mexico City, Beijing, etc.
Smog is more common in warm, sunnier cities, as these places tend to suffer from temperature inversion fog (a layer of warm air is trapped below dense, cooler air)
This keeps the pollutants at the surface level
Air pollution management
Rapidly developing countries have some of the highest rates of air pollution and reducing urban air pollution globally, is a challenge
Strategies include:
Technical innovations
Vehicle restrictions
Government legislation
Technical innovations
Filters
Fitted to industrial gas and particulate exhausts, filters carbon out of the gases released during industrial processes
Catalytic converters fitted to vehicle exhausts remove harmful pollutants before being released
Photo-catalytic materials (smog-eating material)
Façades are retrofitted to the front of old buildings or new buildings are constructed with photocatalytic concrete
Special tiles are coated with titanium dioxide, which is a pigment that acts as a catalyst and is also used in sunscreen
When UV rays hit the tiles, a reaction occurs, converting mono-nitrogen oxides (smog-producing substances) into less harmful calcium nitrate and water
Self-cleaning concrete (Tiocem)
This is photocatalytic concrete that has titanium dioxide mixed in
When sunlight strikes a building, nitric and nitrogen oxides will be able to break down
Greening the urban area
Improve air quality by planting trees and vegetation
Vertical gardens: around concrete columns and on the sides of buildings
Roof gardens
Urban agriculture using open and derelict spaces
Air purification towers
Dutch designed “Smog Free Tower”, which is an air purifying tower that sucks in pollution and expels clean air
The first tower was installed in Rotterdam and cleans 3.5 million cubic metres of air per day
Self-driving cars
Studies have estimated that self-driving vehicles could improve fuel efficiency by 15–40%, which would reduce local emissions of pollutants as well as global greenhouse gases
Hydrogen fuel additives
Additives improve fuel combustion and reduce emissions in existing vehicles
UK-developed 'ezero1' technology, feeds small amounts of hydrogen into the vehicle's air intake, creating a more efficient burn
Alternative fuels
Electric vehicles
LPG
Dual fuel or bi-fuel vehicles that can switch between LPG and petrol
Synthetic “gas to liquid” (GTL)
Reduced nitrogen oxide (NOx) emissions of 5–37% and particulate matter (PM) emissions of 10–38%.
Natural gas can also be converted into dimethyl ether (DME) as another alternative to diesel
Reduces NOx emissions by around 25%, and PM emissions are virtually eliminated
Vehicle restrictions
Congestion charge
Charges for using vehicles in certain places at certain times (e.g. London's congestion charge)
This reduces pollution through a reduction in road traffic (London's emissions dropped 15% in its first year)
However, it can increase fringe/outer zone traffic and emissions as people try and avoid the charge by using alternative routes
Selective bans
Certain days and times are designated as no travel times for vehicles
Pedestrianisation
Vehicles are restricted from entering certain places at certain times
This reduces emissions by reducing road traffic
Park and ride
Local authorities provide buses at the urban periphery, and they charge a flat rate for all-day parking and transportation from the parking area to the urban centre
Improvements to public transport
Improved bus services make accessing areas cheaper, faster and more efficient
Trams and light railway services run on lines that avoid congestion
Car sharing/pooling
Many urban centres have designated lanes for cars with two or more people in them
This keeps the flow of traffic moving and reduces journey times and emissions
Government legislation
Legislation can be local or global
However, according to the UN,
One in three countries in the world lack any legally mandated standards for outdoor air quality - UNEP 2021
Laws aim to reduce pollution by limiting emissions from industry, private and public facilities and vehicles
Industries are are regulated under Integrated Pollution Prevention and Control (IPPC), set up under the Pollution Prevention and Control Act of 1999
Factories are not allowed to emit 'dark' smoke under the Clean Air Act of 1993, except in unavoidable circumstances (e.g. starting up)
The amount of dirt and dust emitted is also strictly monitored/controlled
Chimneys must have up-to date modern filters/scrubbers fitted
The Air Quality Standards Regulations of 2010 in the UK regulate significant air pollutants
Laws set air quality standards such as:
UK Clean Air Acts of 1956 and 1968 reduced domestic pollution through the introduction of smoke free zones
Industrial pollution was reduced by introducing tall chimneys, thereby dispersing pollutants higher into the atmosphere
The introduction of the MOT emissions test by the Road Vehicles Regulations means all vehicles have to pass an emissions test to be allowed on UK roads
In Scotland, roadside emissions tests are carried out and fines issued if the vehicle fails
Local authorities in the UK can issue fines to people leaving their engines running unnecessarily
Case Study: New Delhi
In 2023, New Delhi was identified as the most polluted capital city in the world
During 2022, schools and colleges were closed for several days at a time due to the levels of pollution
In November 2023, the air pollution level was 100 times the World Health Organisation's healthy limit
Causes of pollution in New Delhi
Over 19% of the pollution is the result of the over 3,000 industries in the area
New Delhi is inland so there is often little wind to move the pollution away
The areas around New Delhi are agricultural and crop burning in winter adds to the pollution levels
The use of diesel-powered irrigation pumps also contributes to emissions from agriculture
There are 11 coal-fired power stations in the area surrounding New Delhi
In 2023, all but one of the power stations exceeded the allowed emission levels
Many residents still use wood and biofuel for heating and cooking
Between 1988 and 2020, the number of cars in New Delhi increased by 3.1 million
Burning of waste and landfill fires
In April 2022, the Bhalswa landfill site caught fire and burned for twenty days
Methane emissions from the landfill sites
Since 2020, there have been 37 major methane leaks from the Ghazipur landfill site
Impacts of pollution in New Delhi
A study by Greenpeace and IQAir estimated that 54,000 premature deaths in New Delhi were caused by air polllution
Doctors during times of high pollution report increased numbers of patients with:
Breathing problems
Irritated eyes and throats
Asthma
Lung cancer
The University of Chicago energy policy institute estimates that life expectancy in New Delhi is reduced by 11.9 years due to air pollution
Schools and colleges are frequently closed during winter due to air pollution levels
It is estimated that 50% of children in New Delhi have irreversible lung damage
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