There's little question that on a cold day, if your head is warm, your whole body thermostat gets a boost. We can't find any proof that keeping a cool head cools your whole body, but a comfortable hairstyle in hot weather has got to be some help.

There are two routes to a cool summer hairstyle . . . the obvious short, easy-care haircut, and long hair tamed and turned up.

Several seasons back Yves Saint Laurent showed his collection in Paris with many of his models wearing a single braid starting high on the back of the head and dropping down the back. It didn't take long for the offstage models to follow, and within the last year the single French braid look has caught on with a lot of stylish young women.

It may look as simple as pigtails, but it's not. It takes some fancy handiwork. Rather than simply dividing the hair in three parts for the usual braid technique, sections of hair must constantly be worked into the braid.

The single braid not only gives a neat look down the back, but it can be turned up in various ways or bundled into the nape of the neck. CAPTION: Illustrations 1 2,3, and 4, Start by pulling hair straight back or starting with two small braids from the sides of the forehead. Then at the back of the head start working in other clumps of hair into the braid pattern. Martha Vaughan for The Washington Post