Newswise — If your mother ever advised you to dress in layers when it's cold outside, she was right -- but not for the reason you may think.
Elizabeth McCullough, a professor of textiles at Kansas State University, said the versatility of adding or subtracting garments to stay comfortable is just a benefit of layering. The real reason it's a good idea when the thermometer dips is because those layers of fabric help trap your body's heat and keep it from escaping.
"When you add a garment, you're actually adding a layer of air that's trapped between garment layers as well as inside the component fabrics," McCullough said. "It's the still air that's doing the insulating."
Thick fabrics containing down or synthetic fiberfill are warmer than other types of fabrics because they're able to trap more air, she said.
McCullough is co-director of K-State's Institute for Environmental Research, where she tests garments for companies like The North Face and L.L. Bean. She said the most effective way to keep warm is to cover as much of the body's surface as possible and to distribute the layers evenly over the body.
"People think they're doing the right thing by covering up the torso," McCullough said. "But your body works as a system. If one body part is uncovered, you will lose heat from your skin's hot surface to the colder environment. The bigger the difference between hot and cold, the faster heat flows out."
In places where it's extremely cold -- like Maine, Wisconsin or Montana -- McCullough said it's important not to forget to insulate the legs as well as the head, hands and feet.
"You can't just put on a jacket and expect to be comfortable in very cold conditions," she said.
Mom was right if she told you to wear a hat, too. McCullough said that hats are important because your head is physiologically different from other parts of your body.
"Blood vessels in your head don't constrict," she said. "The first place vessels constrict is in the hands and feet so that your body won't lose heat as quickly. But the blood vessels in the head don't constrict, because your body's priority is keeping the brain functioning."
McCullough said the main reason a hat will make you feel warmer is that it's the last place many of us think to cover up.
"Often people have most of their other body parts covered," she said. "If the head is the only area left uncovered, you can make a tremendous difference to your clothing insulation by simply adding a hat."