High temperatures and the Urban Heat Island

Our average summer temperatures are predicted to keep rising, such that by the middle of this century, we can expect what are now heatwave temperatures in most summers.

London also generates its own microclimate, known as the Urban Heath Island (UHI), which can result in the centre of London being up to 10°C warmer than the rural areas around London. This can aggravate the effects of hot weather.

Summer heatwaves may make our homes, workplaces and public transport uncomfortable, and can have affect on our health, particularly of vulnerable people.

The 2003 summer heatwave resulted in about 600 excess deaths in London. The hot temperatures in 2006 resulted in extremely high demands on London’s power supply network and subsequent ‘brown outs’, due to the high cooling demand. Future increases in electricity demand for cooling could affect London's sustainability.

Urban heat island

The Urban Heat Island (UHI) describes the increased temperature of urban air compared to the rural surroundings. The term ‘heat island’ is used because warmer city air lies in a ‘sea’ of cooler rural air.

The figure below shows a stylised heat island profile for a city, showing temperatures rising from the rural fringe and peaking in the city centre. The profile also demonstrates how temperatures can vary across a city depending on the nature of the land cover, such that urban parks and lakes are cooler than adjacent areas covered by buildings.

Sketch of an urban heat island

Source: The Met Office

The higher urban temperatures are caused by the increased capacity of the urban land surface (eg. roads, buildings, pavements) to absorb and trap heat.

This results in towns and cities remaining noticeably hotter than the surrounding countryside, particularly at night on calm, clear summer nights. The UHI can add 5-6°C to the nighttime temperatures experienced. During the summer heatwave of 2003, differences of up to 10°C between city and rural temperatures were measured in London. The GLA commissioned research into London’s UHI – ‘London’s Urban Heat Island: A Summary for Decision Makers’.

The UKCP09 climate change projections used a land surface scheme that does not include recognition of urban land surface that modifies the climate of cities through the UHI effect.

Beating the heat

Because we know that overheating is becoming an increasing problem in our urban environment, we have come up with the following solutions, which we’re working on with our partners:

• Research into the causes of damage to health by overheating in an urban environment
• An Urban Greening programme, helping to create a fairer society giving people access to cooling green spaces as well as cool our surroundings in hot weather.
• Supporting the Mayor’s targets to:
  - Increase tree cover by 5% by 2025
  - Increase greenery in the centre of London by 5% by 2030 and a further 5% by 2050
  - 100,000m2 of green roofs by 2012
  - Enhance 280ha of greenspace by 2012

• ‘Design summer year’ guidance for architects, to ensure we’re building the right types of building to cope with heat as well as cold
• Creating a network of weather stations across London to understand its microclimate
• Working with London Boroughs to identify and promote heatwave refuges, to ensure that we’re prepared in advance to protect London’s most vulnerable residents from the acute effect of heatwave.