New materials, old physics – the science behind how your winter jacket keeps you warm

As the weather gets colder this winter, you may be one of the many Americans pulling your winter jackets out of the closet. Not only can this extra layer keep you warm in cold weather, but modern winter jackets are also a testament to centuries-old physics and cutting-edge materials science.

Winter jackets keep you warm by managing heat via the three classic modes of heat transfer – conduction, convection and radiation – while remaining breathable so sweat can escape.

Physics has been around for centuries, but modern hardware innovations represent a step forward that allows these principles to shine.

An ancient science with a new shine

Physicists like us who study heat transfer sometimes consider thermal science “settled.” Isaac Newton first described convective cooling, the loss of heat caused by the movement of a fluid that carries thermal energy away from a surface, in the early 18th century. Joseph Fourier’s 1822 analytical theory of heat then placed conduction – the transfer of thermal energy by direct physical contact – on a mathematical basis.

The work of Josef Stefan and Ludwig Boltzmann at the end of the 19th century, followed by that of Max Planck at the dawn of the 20th century, made thermal radiation – the transfer of heat by electromagnetic waves – a pillar of modern physics.

All of these principles inform modern materials design. However, what seems new today is not the equations but the textiles. Over the past two decades, engineers have developed extremely fine synthetic fibers that trap heat more effectively and treatments that allow natural down to repel water instead of absorbing it. They designed breathable membranes filled with tiny pores that allow sweat to escape, thin reflective layers that reflect your body heat back to you, coatings that store and release heat when the temperature changes, and ultralight materials.

Together, these innovations give designers far more control than ever before over warmth, breathability and comfort. That’s why jackets are now warmer, lighter and drier than anything Newton or Fourier could have imagined.

Traps still air, slows the escape

Conduction is the direct flow of heat from your warm body to your colder surroundings. In winter, all that heat escaping from your body makes you feel cold. Insulation combats conduction by trapping air in a network of small pockets, slowing the escape of heat. It keeps the air still and lengthens the path that heat must take to escape.

High-loft down makes up the large clusters of fluffy feathers that create volume inside a down jacket. Combined with modern synthetic fibers, down traps warm air and slows its escape. New types of fabrics infused with ultra-light, highly porous materials called aerogels pack even more insulation in surprisingly thin layers.

Tame the wind, protect the boundary layer

A good winter jacket should also resist wind, which can remove the thin boundary layer of warm air that naturally forms around you. A jacket with a good outer shell blocks the pumping action of the wind with tightly woven fabric that retains heat. Some jackets also have an outer layer of lamination that keeps water and cold air out, and a woven pattern that seals any heat passages that might escape around the cuffs, hems, flaps and collars.

The outer membrane layer of many jacket shells is both waterproof and breathable. It keeps rain and snow out and also allows your sweat to escape as water vapor. This feature is essential because insulation, like down, stops working if it gets wet. It loses its fluff and can’t trap air, meaning you get cold quickly.

These shells also block the wind, which protects the warm air bubble created by your body. By stopping wind and water, the shell creates a quiet, dry space for the insulation to do its job and keep you warm.

New tricks for reflecting infrared heat

Even in still air, your body radiates heat by emitting invisible waves of thermal energy. Modern jackets solve this problem by using new types of fabrics and technologies that allow the interior surface of the jacket to reflect your body heat back to you. This type of surface has a subtle spatial blanket effect that adds noticeable warmth without adding bulk.

However, how jacket manufacturers apply this reflective material is important. Coating the entire material with metal film would reflect a lot of heat, but it wouldn’t allow sweat to escape and you could overheat.

Some liners use a micro-dot pattern: reflective dots reflect heat while the spaces between them keep the material breathable and allow sweat to escape.

Another approach moves this technology to the outside of the garment. Some models add a pattern of reflective material to the outer shell to prevent heat from spreading in cold air.

When these outer spots are dark in color, they can also absorb a touch of heat from the sun. This effect is similar to window coverings that trap heat inside while taking advantage of sunlight to add more warmth.

Heat only matters if you stay dry. Sweat that cannot escape wets a jacket’s insulation layer and accelerates heat loss. That’s why the best winter systems combine moisture-wicking interior fabrics with ventilation options and membranes whose pores let water vapor escape while keeping liquid water out.

What’s coming

Describing where heat spreads through textiles remains a challenge because, unlike light or electricity, heat spreads through almost everything. But new types of materials and unique surfaces with ultra-fine patterns are allowing scientists to better control how heat travels through textiles.

Heat management in clothing is part of a broader heat management challenge in engineering, which spans microchips, data centers, spacecraft and life support systems. There is still no universal winter jacket for all conditions; Most clothing is passive, meaning it does not adapt to its environment. We dress for the day we think we will face.

But some engineering researchers are working on environmentally friendly textiles. Imagine fabrics that open microscopic openings as humidity increases, then close them in the bitter, dry air. Image liners that reflect more heat in blazing sun and less in darkness. Or a loft that inflates when you’re outside in cold weather and relaxes when you come inside. It’s like a sci-fi costume made practical: clothing that senses, decides, and subtly reconfigures itself without you ever touching a zipper.

Today’s jackets don’t need a new law of thermodynamics to work: they combine basic physics with the use of precisely engineered materials and thermal fabrics specially designed to retain heat. This marriage is why today’s winter clothing feels like a leap forward.

This article is republished from The Conversation, an independent, nonprofit news organization that brings you trusted facts and analysis to help you make sense of our complex world. It was written by: Longji Cui, University of Colorado Boulder and Wan Xiong, University of Colorado Boulder

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The authors do not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.