Why does putting on a coat in cold weather make us warmer? asks a reader.
Go out lightly dressed on a frigid day, and thermal energy will quickly drain away from you into the cold December air. But unlike a run-down mechanical bunny, you don’t need new batteries. A warm coat pulled from the back of the closet will do just fine.
A warm parka
Although you might assume the culprit is heat radiating from exposed skin (such as your red, cold nose), the body actually loses thermal energy in five different ways.
First, we are indeed champion radiators: Exposed or under- insulated parts of our bodies give off infrared radiation. (Think of those infrared warming lights fast food restaurants; they keep the fries hot.) We also absorb infrared radiation from the environment. But when the temperature drops — when the air is cooler than the body’s temperature — we lose more heat than we gain. On a cold winter day, the loss is speedy, and mounts up: Infrared radiation accounts for more than half of all the heat we lose.
How do we lose the rest? First, there’s conduction, direct contact between a warmer object (us) and a cooler object (say, a cold plastic subway seat). Heat slowly transfers from you to the seat. If you’ve ever sat down on a toasty, just-vacated seat in winter, you’re being warmed by the previous sitter’s lost body heat.
(Water is even better at conducting heat than plastic or wood. Which is why falling into a pond in winter can cause a disastrous loss of body heat.)
So we are radiators and conductors. (No wonder cats like to snooze on our chests.) But we also lose heat through convection. Air isn’t a great conductor of heat, which is why a body-warmed layer of air hangs around our skin. But when the wind blows, it carries away this toasty layer. On a windy winter day, we become chilled very quickly. The higher the wind speed, the faster heat is carried away, so the wind-chill index is actually a handy clothing guide.
We also lose heat through evaporation. In the summer, evaporation of our skin’s watery sweat helps keep us from overheating. In the winter, sweating just makes us colder. Finally, we lose thermal energy just by breathing, exhaling warm water vapor into the wintry air.
Winter coats help contain some of this five-pronged heat loss. Coats, hoodies, scarves, gloves and other clothes don’t have built-in heaters like electric blankets. But they do help keep your body’s thermal energy from simply warming the air, seat, and metal pole in a chilly subway car.
How? Coats made of real or synthetic leather create a physical barrier to wind. Fluffy down-type jackets keep a layer of warmed air virtually trapped near your body, minimizing loss by convection. Wool, furry, or sherpa-style coats hold air in pockets created by their fibers. (Such coats are warmest if the fuzzy bits are on the inside.) Sweaters and shirts underneath a coat enhance the insulating effect, adding layer upon snug layer of trapped toasty air, courtesy of the body’s inner furnace.
