Those cool Bedouins all dressed in black - how did they get to be so cool?

Improbable research
Dressed in the dark
How Bedouins know that black is cool
By Marc Abrahams (from the good old Guardian)

Why do Bedouins wear black robes in hot deserts? The question so intrigued four scientists - all non-Bedouins - that they did an experiment. Their study, called Why Do Bedouins Wear Black Robes in Hot Deserts?, was published in the journal Nature a quarter of a century ago.

"It seems likely," the scientists wrote, "that the present inhabitants of the Sinai, the Bedouins, would have optimised their solutions for desert survival during their long tenure in this desert.

"Yet, one may have doubts on first encountering Bedouins wearing black robes and herding black goats. We have therefore investigated whether black robes help the Bedouins to minimise solar heat loads in a hot desert."

The research team - C Richard Taylor and Virginia Finch of Harvard University, and Amiram Shkolnik and Arieh Borut of Tel Aviv University, quickly discovered that, as one might suspect, a black robe does convey more heat inward than a white robe does. But they doubted that this was the whole story.

They found inspiration and guidance in a 1969 report about cattle. John Hutchinson and Graham Brown, of the Ian Clunies Ross Animal Research Laboratory, who were working with Friesien dairy cows, discovered that light and heat penetrate deeper into white cattle hair than into black. The saving grace for the cattle is that even a tiny amount of wind whisks away that extra heat.

However, cattle are not people. So, what of man? Taylor, Finch, Shkolnik and Borut measured the overall heat gain and loss suffered by a brave volunteer. They described him as "a man standing facing the sun in the desert at mid-day while he wore: 1, a black Bedouin robe; 2, a similar robe that was white; 3, a tan army uniform; and 4, shorts (that is, he was semi-nude)."

Each of the test sessions (black-robed, white-robed, uniformed, and half-naked) lasted 30 minutes.

It was hot there in the Negev Desert at the bottom of the rift valley between the Dead Sea and the Gulf of Elat. The volunteer stood in temperatures that ranged from a sultry 35C to a character-building 46C. Though he is now nameless, this was his day in the sun.

The results were clear. As the report put it: "The amount of heat gained by a Bedouin exposed to the hot desert is the same whether he wears a black or a white robe. The additional heat absorbed by the black robe was lost before it reached the skin."

Bedouin robes, the scientists noted, are worn loose. Inside, the cooling happens by convection - either through a bellows action, as the robes flow in the wind, or by a chimney sort of effect, as air rises between robe and skin.

Thus it was conclusively demonstrated that, at least for Bedouin robes, black is as cool as any other colour.

Thanks to Fuzz Crompton and Rebecca German for bringing this to my attention.

(Marc Abrahams is editor of the bimonthly magazine Annals of Improbable Research (www.improbable.com) and organiser of the Ig Nobel Prize)