Back in black

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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

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