Why Cities Are Hotter

Imagine looking at a thermal map of a large city at two o'clock in the morning, in the middle of a heatwave. The surrounding countryside appears in cool shades of blue and green, but the city itself glows orange and red, as though it were running a fever. Hours after sunset, its streets have scarcely cooled at all. The phenomenon is known as the urban heat island. It was first investigated in the 1810s by Luke Howard, an English chemist and amateur weather observer, who noticed that London was consistently warmer than the countryside around it, particularly at night.

The main culprit is the very material that cities are built from. Asphalt and concrete absorb enormous quantities of heat during the day and store it in their mass, much as a stone left in the sun stays warm long after dark. At night, they release that heat slowly back into the air, which is precisely why the difference between town and country tends to be greatest after sunset. Dark roofs and roads make matters considerably worse, since they absorb most of the sunlight that falls on them instead of reflecting it.

Then there is what cities lack. In the countryside, plants constantly release water through their leaves, and the evaporation of that water cools the surrounding air, in the same way that sweat cools the skin. Paved streets offer no such relief. Tall buildings, meanwhile, form narrow canyons that bounce sunlight from wall to wall and block the breezes that might otherwise carry the heat away. Add the waste heat pouring out of cars, factories and air conditioners, and the result is a city that can be several degrees hotter than fields only a few kilometres away.

It would be tempting to dismiss all this as a question of comfort, but in my view that would be a serious mistake. During a heatwave, it is the nights that make the heat truly dangerous. If the body cannot cool down while we sleep, it never fully recovers from the day, and elderly people in particular face a greater risk of heat exhaustion and heatstroke. Ironically, the most common response, switching on the air conditioning, merely pumps more heat into the street outside, so that the city grows hotter for everyone who cannot afford to do the same.

The encouraging news is that the remedies are neither mysterious nor particularly expensive. A roof painted white or covered with a reflective coating sends most of the sunlight that strikes it straight back into the sky, whereas an ordinary dark roof reflects only a small fraction. Trees do double duty, shading the pavement and cooling the air through their leaves. Were I responsible for any large city, I would start with two simple rules: every flat roof should be light-coloured, and every block should have trees. Neither would put an end to heatwaves, but together they would give the city, and the people trying to sleep in it, a chance to breathe at night.