Cool black and warm white

The American engineer Harold Hay designed houses in the 1960s and 70s that were cooled, not using mechanical ventilation or air conditioning, but by natural means. The houses were sited in parts of the United States with clear skies at night like California and Arizona. They had bags or tanks of water on the roof that were warmed during the day, and radiated their heat back to the blackness of the sky during the night. This created the cooling. Hay called them Sky-Therm houses. In desert regions the effect has been used for centuries to make ice at night.

In 1976 Hay was interviewed by Mother Earth News. The magazine asked him about his theory of ‘cool black and warm white’. “It’s become almost holy writ”, Hay said, “that black always absorbs the sun’s rays and warms up, while white always reflects solar energy and stays cool.” For these reasons, solar water heaters are painted black, and houses in hot countries have traditionally been painted white. In recent years scientists have advocated painting both roofs and road surfaces in light colours to reflect sun back to the sky and reduce urban temperatures.

So why, asks Hay, are polar bears white, and why do so many tropical animals like the great apes have black fur? On thermal grounds one might expect the opposite. (But there is also the matter of camouflage.) Hay’s explanation has to do with *thermal mass *and the way that different materials store heat. Put a black piece of metal and a thick black coat out in the summer sun for an hour, Hay says. Then pick them up. The metal will burn your hand, but the coat will not. The surface of the coat is just as hot as the surface of the metal. But the fibres of the cloth below the surface provide insulation, so the mass of material does not warm, and the surface heat is reradiated to the atmosphere.

Hay’s theory explains why the Bedouin and Tuareg peoples of the Sahara have tents woven from the black wool of their black goats. The sides of the tent do not come right down to the ground. When the sun strikes the cloth, it gets hot, but only on the surface. The air next to the cloth gets hot. The warm air rises and creates a strong updraft. This continually pulls a current of air towards the tent and in under its sides. The heat of the sun is used to power a form of natural ventilation.

Could a similar mechanism apply to clothes? The 19th century English colonial administrator wore a white suit (and a solar topee) to try to keep cool. But some natives of hot countries wear loose black clothing. In 1980 Amiram Shkelnik and colleagues published a paper in *Nature *asking ‘Why do Bedouins wear black robes in hot deserts?’ They carried out experiments and found disappointingly that “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.”

So a voluminous black robe does not it seems create a pleasant current of air over the skin, inside the garment, like a black tent. And in any case, in the summer, most Bedouins wear the loose-fitting white cotton dishdash, like the white *jellabiya *of other Arab peoples.

‘Plowboy Interview: Harold R. Hay Talks About Solar Energy’, Mother Earth News, September/ October 1976

Amiram Shkolnik, C Richard Taylor, Virginia Finch and Arieh Borut, ‘Why do Bedouins wear black robes in hot deserts?’ *Nature *Vol.283, 1980, pp.373-375