Heat in the city: the urban heat island

Concrete, asphalt, stone and brick can store a lot of heat. During the day they absorb the sun's energy, and in the evening and at night they release it back into the air. Dark surfaces such as asphalt and tar roofs get especially hot. The US Environmental Protection Agency EPA describes that the difference from the surrounding countryside is therefore felt most in the evening and at night.

Plants cool in two ways: they give shade, and they evaporate water through their leaves, which draws heat from the surroundings. Densely built, sealed districts lack both. Narrow street canyons also slow the wind that would otherwise bring in cooler air. Waste heat from cars, machines and air conditioning adds to it.

For the body, the lack of cooling at night is the real problem. People who cope with heat during the day need recovery at night. If the flat stays warm, sleep is poor, and the strain adds up over several days of a heatwave.

The simple rule is: closed by day, open at night. Keep windows shut while it is warmer outside than inside, and shade them from the outside where possible with shutters, blinds or awnings. Sunlight that has already passed through the glass has already heated the room – that is why curtains on the inside help less.

Late in the evening and early in the morning, in the coolest hours, ventilate as strongly as possible, ideally right across the flat. A thermometer indoors and a look at the outside temperature show you when airing is worthwhile. Use appliances that produce heat, such as the oven or tumble dryer, as little as possible on hot days.

Top-floor flats and rooms facing south or west heat up most. During the hottest period, use the coolest room in the home, for example for sleeping. A fan moves the air and can feel pleasant, but it does not cool the air itself. In very high heat a fan alone is often not enough.

The World Health Organization WHO describes that older people, young children, pregnant women, people with chronic illness and people who live alone are particularly at risk in heat. Older people often feel thirst less and notice overheating later. During a heatwave, phone or visit relatives and neighbours who live alone every day, and remind them to drink.

Some medicines can affect how the body copes with heat. Anyone who takes medication regularly should ask their doctor or pharmacist before summer what to look out for – and not change anything on their own.

Confusion, reduced consciousness or a very high body temperature in heat are an emergency: call the local emergency number (112 in the EU) at once. Until help arrives, move the person to a cooler place. And a parked car heats up very quickly in the sun – children and animals never stay inside, not even for a quick errand.

Heat warnings from the weather service or health authorities give you a head start. If you know several hot days are coming, you can move shopping, sport and heavy work into the cool morning hours. Midday and early afternoon are the worst times for exertion.

Many cities have cool public places: libraries, churches, museums, shopping centres or specially set-up cooling centres. Find out before summer which ones are near you. Outdoors, shade, a hat, light, airy clothing and drinking regularly all help.

If the power fails during heat, fans and air conditioning stop. Then shading, airing at night and cool places matter all the more. A small stock of drinking water and a battery-powered lamp help bridge the time.

In the long term, trees, green spaces, light-coloured surfaces and green facades noticeably cool cities. Some of this can be imitated on a small scale: plants on the balcony, light-coloured shading outside, shaded windows. Many of the most effective measures against heat cost nothing but a little planning.