3.2.3.4 Urban Climate

Urban Heat Island

Urban areas are significantly warmer than surrounding rural areas (8-10C in megacities), especially at night

Causes of UHI

Albedo, reduced evapotranspiration, urban geometry, heat release, air pollution

Albedo cause of UHI

Urban materials how low albedo so absorb more radiation and slowly release at night

Reduced evapotranspiration cause of UHI

Limited vegetation and water channelled into drains reduces evaporative cooling

Urban geometry cause of UHI

Tall buildings trap long wave radiation, reducing heat loss at night

Heat release cause of UHI

Heat from industry, vehicles and air conditioning increase temperatures by 1-2C

Air pollution cause of UHI

Pollutants act as a thermal blanket to absorb and re-radiate heat

Environmental concerns about UHI

Increased energy demand/co2 emissions, heat stress on urban ecosystems, increased ozone formation damages vegetation

Social concerns about UHI

Heat related illness (elderly vulnerable), sleep disruption reduces productivity, disproportionately affecting poor areas

Economic concerns about UHI

Increased cooling cost, infrastructure damage (road softens)

Strategies to combat UHI

Green infrastructure, Cool roofs/pavements, Water features, Urban planning

Green infrastructure to combat UHI

Green roofs, urban parks and street trees increase evapotranspiration and provide shade to reduce surface temperatures. Expensive

Cool roofs/pavements to combat UHI and negative evaluation

Higher albedo stores less radiation, retrofitting in old cities is difficult

Water features to combat UHI and evaluation

Lakes and fountains increase evaporative cooling. Water intensive

Urban planning to combat UHI

Lower building density improves ventilation corridors

Precipitation processes (causes 5-10% rainfall increase)

UHI generates evapotranspiration and convection. Condensation nuclei from pollution form raindrops. High rise buildings/varied heights cause turbulence and vertical motion.

Smog + example

Fog combined with industrial pollutants before UK Clean Air Act. London smog 1952

Thunderstorms creation

Convectional uplift (from UHI) in conditions of instability (building heights)

Features of urban wind

Reduced average speed, turbulence, venturi effect, channelling

Reduced average speed of wind

Buildings increase surface roughness and friction slows air flow

Turbulence of wind

Irregular building heights causes gusts due to frictional drag

Venturi effect

Wind accelerates through narrow streets due to pressure decrease

Chanelling

Wind redirected into urban canyons due to less friction

Particulate pollution (definition, causes and impacts)

Release of particles and noxious gases into the atmosphere. Originate in power stations and vehicle exhausts. Causes bronchitis and asthma which costs the NHS

Photochemical pollution (definition, causes and impacts)

Nitrogen oxides and volatile organic compounds react in sunlight to produce ozone. Formed by fossil fuel combustion and industrial emissions. Reduces photosynthesis, reduces human productivity through illness and reduces life expectancy

Pollution reduction policies

Congestion charges, Low emission zones, Clean air acts, Public transport, Urban greening

Effect of Congestion charges and evaluation

Reduced traffic volume and No2 levels, criticized for social inequality impacts

Low emission zones definition and evaluation

High polluting vehicles charged so particles reduced. Strong enforcement required

Clean air acts definition and evaluation

Shift from coal to cleaner fuels historically successful

Public transport methods and requirements for success

Park and ride, electric buses and cycling infrastructure. Long term behavioural changes required

Urban greening effect and evaluation

Trees absorb pollutants with 50% of London to be green by 2050, limited impact alone