On a sunny day, cars can become lethal demonstrations of the greenhouse effect. Visible light passes through the windows, and some of it is converted into heat. Because the car is closed, the hot air does not rise and escape, and infrared wavelengths do not pass easily though window glass. Heat is trapped inside the vehicle and builds up rapidly.
The effect can be dramatic even in relatively mild weather conditions. When it is 22C (72F) outside, a car can reach 47C (117F) in one hour; when it is 30C the car reaches 47C in less than half an hour. Leaving a window open a crack makes little difference, and even two open windows with a through draught will not bring the temperature down to a safe level.
Children and animals have proportionately more surface area than adults, absorbing heat more rapidly. They can reach dangerous temperatures and die within minutes. In the UK, the RSPCA advises anyone who sees a dog in distress in a hot car to make an emergency call to 999 immediately.
In the US, where temperatures are often higher, an average of 37 children have died each year since 1998 after being left in hot cars.
In many cases, a distracted driver simply forgets having left a dog or toddler in the back. One easy solution is taking off a shoe and giving it to the child at the start of the journey, providing an an automatic reminder when you leave the car. The same trick also works with dogs – though your footwear may suffer.
• This article was amended on 9 July 2015 to correct the draft/draught homophone.
