CUE: Urban Heat Island

Question 1

Why are temperatures higher in a city?

Answer 1

• Buildings & streets interfere with airflow
• Pollution
• The urban heat island

Question 2

What are the causes of urban heat island?

Answer 2

1. Lower albedo (‘reflectivity’): Dark surfaces (e.g. tarmac) absorb and store heat during the day and release it at night
2. Tall glass/steel buildings reflect heat, so reflecting energy downwards into the city.
3. Less vegetation less evapotranspiration the amount of moisture in the air (humidity) is reduced. Consequently, less heat energy is lost in evaporating it.
4. Buildings leak heat through poor insulation in winter and air conditioning pumps hot air into the streets in summer
5. Power stations, industries and vehicles (and people) all generate heat
6. Chemical pollution, soot and dust produced by vehicles and industry can ‘trap’ heat by creating cloud cover (creating a ‘pollution dome’) – this allows in short-wave radiation but absorbs and reflect back down much of the outgoing radiation.

Question 3

What’s the thermal gradient?

Answer 3

The temperature decline from the urban centre to the rural urban fringe is known as the thermal gradient.
- The difference can be more than 6 °C in late summer and 2 °C in winter. It’s usually strongest at night when additional cloud and dust acts as a thermal blanket – effectively operating as a locally enhanced greenhouse effect.

Question 4

Why is the urban heat island in London a concern?

Answer 4

centre of London is already up to 10DC warmer than surrounding areas
- asthma, stroke, etc.
- melting tarmac, car parks
- greater numbers of mice and rats
- more water shortages
- hay fever season extended
- more chemical weathering, more damage to historical buildings