The Science of Warmth

The turn of the last century was the heroic age of Antarctic exploration. Amundsen, Scott, Shackleton, and their teams traversed unknown icy lands and waters, battling frigid elements. Frostbite and hypothermia were constant fears. Looking at photos of these men, you see determination but also utter cold and exhaustion on their chapped faces.

Antarctic expedition members Edward A. Wilson, Robert F. Scott, Edgar Evans, Lawrence Oates, and Henry Robertson Bowers looked very cold at the South Pole. Even though their clothing might have seemed like warm choices, it made these men colder in the long run.

Today, thousands of scientists, researchers, tradespeople, and even chefs work in the same conditions, many still sleeping in tents outdoors. These folks rarely get even mild frostnip, and they have the energy for a party or two on the weekends. The difference? They are warm. Their coats, hats, long underwear, gloves, socks, boots, gaiters, tents, and sleeping bags are made of fabric that is better able to withstand the chill than those of their predecessors one hundred years ago. Fabric gear protects our fragile, hairless bodies from frosty weather in a variety of ways. So, what makes fabric “warm”?

WICKING

Have you ever jumped into a lake on a toasty summer’s day and then shivered as you emerged? Water conducts heat better than air, magnifying the impact of both hot and cold. The thermal conductivity of water is more than 20 times higher than the thermal conductivity of air. On a microscopic level, the molecules in water are like a room full of toddlers: they are always moving. Every once in a while, two of these toddler molecules run into one another. These tiny collisions contribute to the evaporation process as water molecules move from liquid to gaseous states, and the remaining liquid water cools. If water stays in contact with your skin, you will grow colder and colder as the remaining water evaporates. Fabric can protect against this heat transfer through a process called wicking. Moisture can be wicked through a garment, evaporating or passing to the next layer. If wicking doesn’t occur, the moisture can cause rapid conductive heat loss. A material good at wicking will also dry rapidly because water doesn’t saturate the fabric.

LOFT

Looking for something that will really trap the heat close to your body? You’re in luck! It’s already all around you! Air is one of the finest insulators you can find. The greater the space between molecules in a substance, the harder it is for heat to transfer through that substance. As a gas, air has molecules that are spread out, making heat transfer more difficult. Because of this, air is commonly used as insulation; think double-paned windows. But as with double-paned windows, something has to trap the warm air and hold it in place for that insulation to be effective. In fabric, this is known as loft. Yarn or fabric with a lot of loft will contain elements that hold tiny pockets of air to help the user maintain warmth.

WEAVE

Weave is arguably the simplest way to control warmth in fabric. A tight weave means that threads are placed closely together in the material. The closer the threads, the less air and water can permeate, keeping the elements outside and the comfort inside. Together, the warmth these properties provide in the fabric is called thermal insulation. When fabric is used for clothing, the combination of permeability (weave), moisture transfer (wicking), and insulation (loft) is described as thermophysiological comfort. There is one final—and significant—factor that impacts the warmth of a fabric: use. A fabric that would keep you warm on a sunny day might be useless in the rain. A material could create a delightfully warm blanket but terrible pants. Some clothing is better when the wearer is active, others when the wearer is sedentary. The warmth of a material used for clothing also varies depending on how tightly it is worn.

—K. Rose James