Science Puzzle
Why Are Cities Warmer Than the Countryside?
On a still summer night, the centre of London is often about 4 °C warmer than the fields outside it. In the heatwave of 2003 the gap reached 10 °C. The city isn't any nearer the Sun, and the same weather passes over both.
Why are cities warmer than the countryside?
The Answer
A city soaks up the day's sunshine in its brick, concrete and tarmac and gives the heat back all night. It also loses less heat by evaporation and makes heat of its own.
Dark roads and roofs absorb most of the sunlight that falls on them, and heavy materials like concrete and brick store a lot of heat. After sunset that stored heat seeps back out for hours. In the countryside, soil and plants store far less, and the open fields cool quickly by giving off heat to a clear sky. In a city, tall buildings block much of the sky, so the heat they give off is partly caught by the buildings opposite.
The countryside also has a built-in cooling system. Plants and damp soil lose water by evaporation, and evaporation takes heat with it. In a city, rain runs straight into the drains, so there's much less water to evaporate. On top of that, cars, air conditioners, factories and millions of people all add heat.
The effect is called an urban heat island. Luke Howard, the chemist who named the clouds, noticed it in London more than 200 years ago and wrote about it in 1818. Planting trees, adding green roofs and painting surfaces pale all help cities cool down.
The principle: Urban heat islands. Cities store the day's heat in concrete, brick and tarmac, lose less of it to evaporation and add heat of their own, so they stay warmer, especially at night.