Why Cities Are Warmer

On a summer night, the center of a big city can be several degrees warmer than the countryside around it. Scientists call this an urban heat island. It happens in almost every large city, from London to Tokyo, and it is growing as cities grow.

The main cause is the material cities are built from. Roads, roofs and walls of stone and concrete soak up heat from the sun all day. After sunset they slowly give that heat back, so the air in the streets stays warm long into the night.

Cities also have fewer trees and less open ground. In the countryside, plants release water into the air, and this cools it, a little like sweat cools your skin. In a city, rain runs quickly into drains instead, and there is less water left to do this work.

Some cities are fighting back. Paris is turning school playgrounds into small parks with trees, and Los Angeles has painted some streets a pale color that reflects sunlight. These changes are small, but on a hot night a few degrees can matter a great deal.