>Aren't the black cloak things worn by Arab Nomads also a heavy wool? And aren't they worn day and night?

They're probably cotton or hemp. Although the theory behind the voluminous cut of the garment is that air can circulate within the folds of the fabric, so it doesn't much matter what it's made of. I don't know about the day & night principle, but it's always puzzled me that black would be the color of choice, being that it absorbs heat...