Urban Microclimates (SL IB Geography)

• 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

A 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

Urban heat island effect

London's urban heat island - note how the isotherms curve around open areas such as Epping Forest, indicating large areas of reduced temperature. Central London shows a greater concentration of heat due to high density land use

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

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

• 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

• 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