The Best Science-Backed Way to Dress for the Cold

Credit - Photo-illustration by TIME

Forget catching big air on the ski slope or staying upright on an ice rink. If there’s any such thing as a winter superpower, it’s knowing how to dress to stay warm.

“When we dress appropriately, it allows us to enjoy the outside weather,” says Dr. Cheyenne Falat, assistant medical director of the adult emergency department at the University of Maryland Medical Center, who specializes in environmental emergency medicine. “But beyond just comfort and being able to enjoy activities outside, there are very serious health risks if your body fails to stay warm.”

The most sinister threat is hypothermia, which occurs when the body loses heat faster than it can produce it, leading to up to 1,500 deaths in the U.S. each year. Falat has also treated lots of patients who experience cold-weather injuries like frostbite, which can cause significant pain, skin blisters, or complete freezing of a body part. The worst cases—when your body’s tissue loses its blood supply—require amputation, she says. Plus, winter weather can lead to heart problems, trigger asthma attacks, and spike blood sugar levels, among other potential hazards.

Dressing appropriately can help insulate you from the elements and create a “microclimate” says John Castellani, a research physiologist with the U.S. Army Research Institute of Environmental Medicine. The goal is to feel slightly cool, but not cold—so when you start moving around, you won't get too sweaty and uncomfortable. “You want to feel the same way you would if you were in your house and had the temperature at 70°F,” he says. That will help you fend off potential health threats and make winter more palatable.

We asked experts to share the best science-backed ways to dress during the winter to keep your whole body warm.

What to look for in winter gear

The two most important properties to look for in cold-weather gear are thermal insulation and wind resistance, says Khubab Shaker, chairman of the department of materials at the National Textile University in Pakistan. “Clothing with high thermal insulation keeps you warm by trapping heat inside it,” says Shaker, who has researched the subject, “while wind resistance helps block wind and protect from its chilling effect.”

There’s a fancy formula that experts like Shaker use to calculate thermal insulation, but the average shopper can simply check labels and closely examine the product to predict how warm it’ll be. Does that shirt you might wear hiking have a thick fleece lining? Does your potential jacket have multiple layers and a down fill power somewhere between 500 and 900? All are indicators that the product has good insulation.

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It’s also important to make sure your clothing is moisture-wicking (meaning it draws sweat away from the skin and into the outer layer of your clothes, so it can evaporate) and breathable (allowing vapor to escape back into the environment, so you don’t start to feel sticky). That’s why wool rules: “It provides excellent insulation, resists odors naturally, and regulates temperature well,” Shaker says. It’s also fire resistant. The main drawbacks, he says, are itchiness and, sometimes, a heftier price than other materials. Merino wool isn’t as itchy, but it can be even pricier.

On the flipside, there are several materials he recommends avoiding. Plant fibers like cotton and linen are a poor choice for cold weather, Shaker says: “They have a tendency to absorb moisture and lose their insulating properties when they get wet.” Silk has the same limitations, while denim—despite its durability and the fact that it seems like it should be warm—is heavy, takes forever to dry, and provides inadequate insulation.

The best way to layer

Layering is essential on cold days—but you need the right technique. Fabrics like polyester, nylon, and wool work nicely for the base layer, which is closest to your skin, since they’re moisture-wicking and breathable. “When you sweat, it should take that sweat and pull it away from your body, so your body doesn't cool by evaporation,” Falat says.

The second layer is all about insulation, which is where you get most of your warmth. Falat recommends opting for wool or fleece. Cashmere also does the trick. “These are all materials that trap air,” she says. “Air is a very poor conductor of heat, so if you trap these micro-pockets of air in between your base layer and outer layer, that will prevent heat loss away from the body, and allow you to keep that trapped warm air around you.”

Your top layer, meanwhile—usually a jacket—ought to have a windproof or waterproof shell, typically made out of tightly woven synthetic fibers like polyester or nylon, as well as a layer of insulation. Its job is to create a barrier against the wind, reduce heat loss, and protect you from rain, snow, or whatever else the environment hurls at you. “The outer shell helps those inner layers stay dry and allows them to keep insulating you,” Falat says.

Torn between down and synthetic insulation? There are pros and cons to each: Down (made out of the soft undercoat of feathers from waterfowl, like geese or ducks) is highly compressible and lightweight; however, it tends to be on the pricier side and requires careful cleaning, or the material will degrade with use. Synthetic insulation, meanwhile, is great for damp conditions since it dries quickly, and it’s less expensive and easier to care for. Though research suggests it provides a warmth-to-weight ratio comparable to down, it can be heavier and bulkier, which is why some people don’t prefer it. You’ll do fine opting for whichever you find most comfortable, experts agree.

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Falat gravitates toward natural materials like goose down, but that's based on personal preference rather than science. “If someone is planning on doing manual labor outside or outdoor activities, and they’re going to repeatedly get dirty, then maybe synthetic material is a better choice,” she says. Her ski jacket, for example, has synthetic insulation—and it keeps her as warm as she could hope. “But I do have a few down jackets I use more for walking in town and just enjoying myself outside, and doing non-physical activities,” she says.

If you’re spending an active day outdoors—hiking or skiing or shoveling mountains of snow—keep in mind that the goal is to be able to easily add and shed layers. Otherwise, if sweat dries on your body, it will remove heat, which could make your body temperature plummet before you even realize what’s happening. “The last thing you want to do is truly get very sweaty in the afternoon, and then have those colder, sweatier layers still on as it starts to get colder again,” Falat says. “We often see people suffering from mild to moderate hypothermia because they do some activities and sweat during the day, and then those layers cool them.”

The more layers you wear, the more versatility you have, which heightens your ability to adapt. If the sun is shining while Falat is skiing, for example, and it’s not windy or wet out, she often tosses her outer shell into a backpack, and then puts it back on when it gets colder. Or, if it's relatively warm but windy and rainy, she’ll shed her mid layer—ensuring she doesn’t get too hot—before reuniting with it when the temperature drops.

Ideally, you’ll have a friend with you whenever you’re spending a lot of time outside in winter weather, Falat adds. That way, you can keep an eye on how the other person is responding to the temperature. Some people with hypothermia experience "paradoxical undressing,” which refers to a false sense of warmth that leads them to remove layers of clothes—worsening their condition. “I'm all about shedding that mid layer if you need to, if it's 35° and sunny outside,” she says. “But maybe have someone remind you, ‘Hey, it's 20° degrees, and it's cloudy and overcast and rain is coming. Maybe it's not the time to start undressing.’”

How to accessorize well

Your head is one of the fastest ways you lose body heat. To prevent that, opt for a hat made from wool, fleece, or acrylic—all of which offer excellent insulation—and make sure it has a snug fit. Or, if it’s especially cold, consider a balaclava that covers much of your cheeks, nose, chin, and neck. If you're planning to be active, your forehead and scalp might start sweating, in which case a headband that covers your ears can work well, Castellani says. “That gives you a place to get rid of some of the heat,” he notes. “Then you won't be sweating in your hat.”

The best way to warm up cold hands is to be physically active, Castellani says. In general, though, he recommends opting for mittens over gloves: “Gloves protect your hands and reduce heat loss, but it’s with your fingers separated,” he says. “There's a lot of area of skin that radiates out to the environment.” Separated fingers get rid of heat faster than the alternative, he adds; if you're wearing mittens, your fingers will be pressed together, and that facilitates warmth. A bonus tip: Castellani likes balling his hands into a fist inside his mittens. “I’m basically reducing the area of heat loss within that protection,” he says.

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Don’t forget about your feet, either. Falat recommends socks made out of a wool blend (natural wool blended with stretchy synthetic fibers, like polyester and nylon), which has moisture-wicking properties in addition to providing warmth. Your feet won’t sweat in your shoes, but they’ll stay nice and warm, she says. She always carries an extra pair with her, so if her feet get wet, she can sub in the backups.

And make sure your shoes are waterproof and wind resistant. “It's not the time to be wearing your Crocs or cotton tennis shoes,” she says. Pull them out of your closet again in the spring—a reward for making it through the winter thanks to the warmest possible wardrobe.

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