Back in black

In Helen Czerski’s feature (March, p70) she explains how black clothes worn in hot weather absorb more heat than white clothes. But also that since black clothing also radiates more heat than white, black is okay to wear in the heat.

Helen’s right, black does radiate as much heat as it absorbs – but it does so at a higher temperature. A beekeeper in Britain demonstrated this by painting some panels various colours, leaving them out under the Sun and then taking their temperatures. He decided his beehives should be painted white because the other coloured panels were all hotter and the black panel was 10°C hotter than the one painted white. (Although, interestingly, the beekeeper reported that a dark red panel ended up being hotter than the black one.)

Similarly, a black car that’s parked in the hot sunshine will radiate as much heat as it absorbs at a temperature of 30°C, for example, but a white car will be radiating the heat it absorbs at 20°C – quite a difference. Also, on p96 of the March issue you answer the question ‘Why do astronauts always wear white suits?’ And the reason is to counter the extremes of heat and cold in space – white best reflects heat from the Sun and radiates the least heat from the astronaut.

Donovan Gericke, Stilfontein, South Africa