Urban Microclimates (DP IB Geography)

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,

• 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