contemporary urban environments - urban climate

what is urban climate?

climatic conditions of towns and cities differ to surrounding rural areas

what are the effects of urban areas on local climate?

temp - 0.5-0.3% annual increase (mean)

precipitation - 5%-10% increase but 5-010% decrease in snowfall

fog - 100% increase in winter and 30% in summer

wind - 10% less than average rural area due to friction

what is the urban canopy layer?

layer of air closest to the surface

what is the urban boundary layer?

forms a dome of warmer air that can extend as a plume downwind of the city

why does the urban heat island effect vary spatially?

• less in green areas - more evapotranspiration

• more in downtown areas - higher albedo

why does the UHIE effect exist in cities?

• designed to get rid of surface water quickly - reduces evapotranspiration and lack of vegetation

• surfaces in the city e.g. concrete has lower albedo so absorbs heat + buildings with large windows reflect the energy downwards onto streets below

• heat from industries e.g. air con releases heat

• air pollution from industries and vehicles creates a pollution dome - absorbs a large amount of outgoing radiation which traps heat

what are the impacts of the uhie?

health conditions e.g. deaths, asthma and heart conditions, heat exhaustion and sunstroke

→ more heating/cooling = more energy consumption

can worsen chronic conditions such as cardiovascular and respiratory diseases

can interfere with sleep, leading to fatigue, reduced cognitive performance, and general discomfort

outdoor environments uncomfortable, discouraging physical activities and outdoor social interactions.

how can the UHIE be mitigated?

• high albedo - reflects heat

• urban greening e.g. green walls and roofs

• cool surfaces - cool roofs with high albedo absorb less heat

• green roads - porous surfaces which allows vegetation to grow through

• green roofs - run-off + filters pollutants

• sky view factor - openness of urban area allows heat to escape

• Urban Parks & Green Corridors: Expanding parks and creating tree-lined streets can lower temperatures.

• Use Reflective & Cool Materials

  • Cool Roofs: Reflective roofing materials reduce heat absorption and lower indoor temperatures.

  • Cool Pavements: Using lighter-colored or permeable pavement materials minimizes heat retention.

  • White or Light-Colored Buildings: Reducing heat absorption by using reflective paint or coatings.

• Enhance Water-Based Cooling

  • Urban Water Bodies: Preserving and integrating lakes, ponds, and wetlands to create cooling effects.

  • Water Fountains & Mist Systems: Public misting stations and water features help reduce ambient temperatures.

  • Permeable Surfaces: Allowing water infiltration through porous pavement and green infrastructure to reduce heat storage.

why do cities have an increase in rainfall?

• UHIE causing low atmospheric pressures and convectional uplift - due to higher temps

• pollution increases number of hydroscopic nuclei present in the urban area

• UHIE generates convection - ground heated, rapid evapotransp - cumulus cloud + convectional weather patterns

• friction of buildings causes clouds to split and when they join again they converge to form rain clouds

how are winds changed in urban areas?

• lower speeds

• greater variability

• large-scale convection.

why do winds have lower speeds in urban areas?

due to the roughness of the land surface, which consists of buildings at a variety of heights, all helping to increase surface friction

why do winds have greater variability in urban areas?

layout of the buildings and building height. straight streets lined with tall buildings can produce ‘urban canyons’ - funnel wind down them, producing high velocities

• venturi effect, which is the effect of ‘squeezing’ the airflow → confined area which increased wind speed

• increased as buildings tend to get taller towards the city centre.

  turbulence is created by high-rise buildings disrupting the flow of air - highest pressure experienced in the upper part of the building with air flowing down the front and over the top

• behind the building there is a reverse circulation as suction occurs on the roof, walls and sides of the building - leads to turbulence.

• buildings are closer together - the airflow around one interferes with that around the next

how can the effects of wind in cities be managed?

using stilts (wind flow through base of building) and canopies may reduce the problem

• aligning main streets with prevailing wind - pollutants flushed out

• building on a podium - downdraughts do not reach ground level

• barriers built in potential venturi effect hotspots

why do winds have large scale convection in urban areas?

heat island effect also produces large-scale convection and uplift over the whole urban area, leading to lowering air pressure, and consequently air is drawn in from surrounding rural areas in much the same way as land or sea breezes are produced.

how does anthropogenic heat contribute to the UHIE?

heat released by human activity.

heat released from vehicles, central heating (alternatively, in some warmer areas, from air conditioning systems) and industrial activities is substantial.

^→ inject heat into the canopy layer and the upper boundary layer.

how does condensation nuclei cause more rainfall?

encourages cloud formation, since water vapour needs a surface onto which condensation can take place

why is non-pollution fog less common in cities?

due to higher temperatures – fog cannot form unless the air is cooled to its dewpoint - and the presence of more condensation nuclei means that there are more, smaller droplets, which tend to discourage the formation of thick obscuring fogs.

why are there pollutants in cities?

vehicle exhausts

industrial activities such as burning coal

construction work can produce coarse particles e.g. brick

what are some common pollutants in cities?

carbon monoxide

nitrogen oxide

particulate matter

sulphur dioxide

what is photochemical smog?

when particles react with UV light from the sun

nitrous oxides and carbons react with the sunlight to form ozone

what are the three ways pollution can be reduced?

• clean air acts

• vehicle control and public services

• zoning of industry

how do clean air acts reduce pollution?

1956: smoke-free zones

1990s: regulations on the levels of airborne pollution - local council are now required to monitor pollution to establish Air Quality Management Areas

2015: NOx emissions higher than recommended - cleaning up construction sites accounted for 12% lower emissions

how do vehicle control and public services reduce pollution?

• congestion charge in 2013

• creation of road schemes such as urban bypasses

• transit systems such is the Metro line in Manchester

• greater use of waterways for transport

how does zoning industry reduce pollution?

industry has been located downward in cities if possible and planning legislation has forced companies to build higher factory chimneys to emit pollutants above the immersion zone

how does Dubai utilize wind?

e.g. Burj Khalifa

aerodynamic structure - rounded edges to prevent it from forming whirlpools or vortexes

e.g. Masdar city

sun - solar energy

Simmens headquarters is positioned to face the directions of the prevailing wind, making use of the venturi tunnel underneath the building to maintain a cooling airflow

a breeze blows up to the roof through the buildings structure which cools public spaces

what air pollution strategies does London have?

ULEZ - daily charge

→ harmful nitrogen dioxide (NO2) concentrations have dropped by nearly 44% in the central ULEZ area since its introduction

congestion charge

low emission bus zones

^{→ s}trategically placed in areas with high pollution levels - Putney High Street, NO2 levels have been reduced by as much as 40% since the introduction of cleaner buses .

expanding cycling infrastructure - rotected bike lanes, bike-sharing programs, and cycling superhighways

increase in green spaces

what air pollution strategies does Mumbai have?

BMC's 'clean air Mumbai' initiative:

• planned to install air purifier at five most crowded areas - Dahisar toll naka, Mulund check naka, Mankhurd, Kalanagar junction and Haji Ali junction.
   

• the four major contributing factors to Mumbai’s deteriorating air quality were identified as: Road and construction dust, traffic congestion, Industries and Power sector and waste burning.

how did Mumbai reduce dust from construction sites?

• Provide dust screens along the external face of buildings where construction is in process

• Sprinkle water on these curtains and at ground floor in the open spaces

• Washing of tyres of all vehicles while exiting from site.

• Covering of all vehicles transporting debris or construction material

how did Mumbai reduce dust from roads?

• Mechanical/ e-power sweepers for dust cleaning

• Sprinklers to settle dust on roads and footpaths

• Using vehicle mounted air purification units and misting equipment

how did Mubai reduce traffic congestion?

Sustainable and clean transport measures:

• 3000 electric buses are being implemented

• Fully Adaptive Traffic Control systems are already installed at 258 junctions and will be further expanded to balance 395 junctions

how did Mumbai reduce pollution from waste burning?

Sustainable waste management measures:

• Communication and awareness for sustainable segregation in households

• Monitoring and enforcement of the ban on waste burning

how can Mumbai’s reduction of waste burning be evaluated?

Banning waste burning reduces the amount of harmful pollutants released into the atmosphere which helps protect public health. However, bans may be ineffective in areas with limited resources which do not have other ways to dispose of waste and many governments may struggle to detect illegal burning.

how can Mumbai’s reduction dust from construction sites be evaluated?

The measures used to reduce dust from construction sites and roads prevent particulate matter which decreases the risk of health problems like heart disease, asthma, and low birth weight.

 

how can Mumbai’s reduction of traffic congestion be evaluated?

The Fully Adaptive Traffic Control system can help mitigate the amount of pollutants released and improve air quality by improving traffic flow and decreasing congestion. This prevents pollutants such as nitrous oxide and carbon monoxide being released.

However, as more control systems are implemented it may increase congestion if they are ineffective which will lead to more pollutants.

what is London’s ULEZ strategy?

ULEZ zones: where an emissions standard based charge is applied to non-compliant road vehicles.

• Since it launched in central London in 2019, it has helped cut the number of older, polluting vehicles on the road.

is London’s ULEZ effective?

97 per cent of vehicles seen driving in London on an average day now meet the ULEZ emission standards - Feb 2024

helped to reduce harmful roadside nitrogen dioxide (NO2) concentrations by 21% in outer London, 53% in central London and 24% in inner London → compared to a scenario without the ULEZ and its expansions in place.

Ulez and the pre-existing low-emission zone for reduced road traffic particulate matter emissions by 180 tonnes across London over three years

what is London’s greener transport?

Means of travel that does not increase emissions

• 9000 buses are all low or zero emission - electric, hydrogen, diesel and diesel-electric hybrid buses, all of which meet Euro VI emission standards

• All new licensed taxis in London must be zero-emission - more than 6,400 already operate

•  established over 300 rapid charge points for electric vehicles and opened rapid charging hub sites - these have six or more charge points in a single place.

is London’s greener transport strategy effective?

• Some buses are still diesel which produce carbon monoxide and nitrogen oxide

• TFL fares rising means people may be more inclined to drive or use taxi services

• London investing in green technologies and infrastructure e.g. electric vehicle charging points, hybrid buses, and cycling lanes. These initiatives have not only reduced emissions but have also encouraged the adoption of cleaner modes of transport, contributing to better air quality.

what is London’s healthier streets?

•  600 School Streets have been created In partnership with London boroughs - restrict vehicle access outside a school during drop off and pick up times.  This has helped to make it safer and easier for children to travel to and from school by foot, bike or scooter as well as improving air quality.

•  creating a growing network of new and upgraded Cycleways across London that everyone can use

• provide free cycle skills online training for all abilities to help you build your confidence and to cycle safely on London's roads

is London’s healthier streets strategy effective?

• Public perception - an increase in awareness and willingness to adopt sustainable transport options which suggests that the strategy is effectively changing attitudes towards car use and promoting different ways of travel.

• Improvements in air quality and increased physical activity due to walking and cycling may lead to better health

• However, it should be questioned whether the benefits of the Healthy Streets strategy are distributed equitably across different socioeconomic groups and neighbourhoods - ensuring that all residents have access to clean air and safe transportation options e.g. families can’t buy children scooters/bikes

how does albedo worsen UHIE?

Many urban materials (asphalt, dark rooftops, concrete) have low albedo, absorbing and retaining heat during the day and releasing it at night. This contributes to the intensification of UHIE, making cities significantly warmer than rural areas.

what is the daytime UHIE?

• Higher temperatures in urban areas due to heat absorption by buildings, roads, and pavements (low albedo surfaces).

• Peak UHIE intensity is lower during the day because both urban and rural areas are heated by direct sunlight.

• Vegetation and shading (e.g., trees, green spaces) can help reduce daytime UHIE by providing shade and cooling through evapotranspiration.

what is the nightime UHIE?

• Stronger UHIE effect at night because urban materials (asphalt, concrete) release the heat they stored during the day, keeping cities warmer.

• Rural areas cool down faster, as they have more vegetation and soil, which lose heat quickly.

• Lack of natural cooling in urban areas leads to higher nighttime temperatures, increasing heat stress and energy demand for cooling.

how else can pollution lead to increased precipitation?

Hygroscopic particles, also known as are tiny airborne particles that attract water vapor. These particles can come from natural sources (e.g., sea salt, dust, volcanic ash) or human activities (e.g., pollution, vehicle emissions).

2. Role in Cloud Formation

• Water vapor in the atmosphere cannot condense into water droplets unless it has a surface to adhere to.

• Hygroscopic particles absorb moisture and encourage condensation, allowing cloud droplets to form around them.

• When more hygroscopic particles are present, more water droplets form, leading to denser clouds.

• As these droplets coalesce (combine), they grow larger and become heavy enough to fall as precipitation.

what are some examples of strategies used in cities to reduce the effects of UHIE?

Whitewashing Streets & Rooftops – Athens and New York have adopted white or light-colored coatings on buildings and pavements to reflect sunlight.

• Urban Water Bodies & Fountains – Madrid and Singapore integrate lakes, fountains, and artificial water bodies to cool the surroundings.

• Blue-Green Infrastructure – Rotterdam’s water squares double as recreational areas and rainwater collectors to cool the city.

• Cool Pavements – Phoenix and Tokyo use reflective or permeable pavements to lower surface temperatures.

what is the impact of urban geometry on wind patterns?

• When wind flows between tall buildings, it accelerates due to the Venturi effect (like air passing through a narrow nozzle).

• This creates wind tunnels, increasing wind speeds at street level, which can be uncomfortable for pedestrians.

• Example: In cities like Hong Kong and Chicago, wind speeds increase in narrow streets between skyscrapers, sometimes leading to hazardous conditions.

• Streets lined with tall buildings create urban canyons, which trap heat and pollutants while disrupting natural airflow.

• Deep, narrow canyons (with high building-to-street width ratios) reduce ventilation, leading to stagnant air and worsening the Urban Heat Island Effect (UHIE).

Properly aligned streets and gaps between buildings can create ventilation corridors, allowing fresh air to circulate and dissipate heat.

• Tall buildings can force high-altitude winds downward, creating downdrafts at street level.

• Example: London’s Walkie-Talkie building caused severe wind acceleration and temperature reflection, leading to pedestrian discomfort.

how do urban climates impact human health and wellbeing?

• Urban Heat Island Effect (UHIE) raises city temperatures, increasing the risk of heat exhaustion, heatstroke, and dehydration—especially among vulnerable groups like the elderly and outdoor workers.

• Nighttime heat retention disrupts sleep, leading to fatigue, stress, and cardiovascular risks.

• Example: The 2003 European heatwave caused over 70,000 deaths, many in densely built urban areas with poor cooling infrastructure.

• Cities often have poor air quality due to vehicle emissions, industrial pollution, and construction dust.

• High levels of PM2.5 and NO₂ increase the risk of asthma, lung cancer, and heart disease.

• Example: Beijing and Delhi frequently experience hazardous air pollution levels, leading to increased hospital admissions for respiratory problems.

• Hot and polluted environments discourage outdoor activities like walking, running, and cycling.

• Lack of pedestrian-friendly infrastructure leads to sedentary lifestyles, increasing obesity and diabetes rates.

• Example: Cities with high air pollution (e.g., Los Angeles, Mexico City) see lower outdoor activity levels, impacting overall fitness.