Urban Microclimates (DP IB Geography)

Revision Note

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

Illustration showing wind flow from rural to urban areas, depicting urban boundary and canopy layers, and urban plume development downwind. Areas labeled rural, suburban, urban.
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

A graph showing increasing temperature from rural to urban areas, with labels: temperature plateau (suburban), cliff (industrial), peak (CBD), and sink (inner city).
Urban heat island effect
Heat map of London's "Heat Island" in mid-May with light winds and clear skies, indicating temperature in degrees Celsius. Areas and key locations such as the City, River Thames, and Epping Forest are marked.
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

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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