The urban heat island effect; the impact of urban areas on precipitation, fog and wind; urban air quality and pollution; and policies to reduce urban air pollution.

What this dot point is asking

AQA section 3.2.3 wants you to explain how cities create their own climate: the urban heat island effect, the impact of urban areas on precipitation, fog and wind, urban air quality and pollution, and the policies used to reduce air pollution. It applies physical-geography energy and atmosphere ideas to the city.

The urban heat island effect

The causes are:

• Surface materials: concrete, brick and tarmac have a low albedo (absorb more radiation) and high heat capacity, storing heat by day and releasing it slowly at night.
• Urban geometry: the urban canyon of tall buildings traps radiation by multiple reflection and reduces the sky view that allows night-time cooling.
• Anthropogenic heat: heating, vehicles, industry and air conditioning release waste heat.
• Reduced evapotranspiration: little vegetation or open water means less cooling by evaporation.
• Pollution dome: aerosols can trap outgoing longwave radiation.

Precipitation, fog and wind

Cities also alter the rest of the local climate. Precipitation tends to be higher over and downwind of cities, because the UHI drives extra convection and the abundant condensation nuclei (pollution) promote cloud and rain. Fog can be more frequent where humidity and pollution combine (historically the lethal smogs). Wind is modified: buildings create turbulence and gusting, while the urban canyon can channel and accelerate wind down streets, even as overall surface friction reduces average wind speed.

Urban air quality and pollution

Policies to reduce air pollution

Cities use several approaches:

• Vehicle control: congestion charging, low-emission / clean-air zones, restrictions on the most polluting vehicles, and incentives for electric vehicles.
• Public-transport improvement: investing in buses, trams, cycling and rail to reduce car dependence.
• Planning and regulation: emission standards, locating polluting industry away from residential areas, and adding green space to absorb pollutants and cool the city.

Effectiveness varies: congestion and clean-air zones have measurably cut pollution in cities such as London, but can be regressive, displace traffic and meet opposition. The most effective approach combines the measures and sustains them, with strong enforcement.

Try this

Q1. Define the urban heat island effect. [2 marks]

• Cue. The tendency for urban areas to be warmer than surrounding rural areas, greatest at night and under calm, clear conditions.

Q2. Explain why dark urban surfaces contribute to the UHI. [3 marks]

• Cue. Concrete, brick and tarmac have a low albedo, absorbing and storing solar heat by day and releasing it slowly at night.

Q3. Outline one policy to reduce urban air pollution and one limitation. [3 marks]

• Cue. A clean-air or congestion zone cuts polluting traffic; limitation: it can be regressive, displace traffic and face opposition, and needs enforcement.