As the weather grows cold this winter, you may be one of the many Americans pulling their winter jackets out of the closet. Not only can this extra layer keep you warm on a chilly day, 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 through the three classical modes of heat transfer – conduction, convection and radiation – all while remaining breathable so sweat can escape.
In a fireplace, heat transfer occurs by all three methods: conduction, convection and radiation. Radiation is responsible for most of the heat transferred into the room. Heat transfer also occurs through conduction into the room’s floor, but at a much slower rate. Heat transfer by convection also occurs through cold air entering the room around windows and hot air leaving the room by rising up the chimney.
Douglas College Physics 1207, CC BY
The physics has been around for centuries, yet modern material innovations represent a leap forward that let those principles shine.
Old science with a new glow
Physicists like us who study heat transfer sometimes see thermal science as “settled.” Isaac Newton first described convective cooling, the heat loss driven by fluid motion that sweeps thermal energy away from a surface, in the early 18th century. Joseph Fourier’s 1822 analytical theory of heat then put conduction – the transfer of thermal energy through direct physical contact – on mathematical footing.
Late-19th-century work by Josef Stefan and Ludwig Boltzmann, followed by the work of Max Planck at the dawn of the 20th century, made thermal radiation – the transfer of heat through electromagnetic waves – a pillar of modern physics.
All these principles inform modern materials design. Yet what feels new today are not the equations but the textiles. Over the last two decades, engineers have developed extremely thin synthetic fibers that trap heat more efficiently and treatments that make natural down repel water instead of soaking it up. They’ve designed breathable membranes full of tiny pores that let sweat escape, thin reflective layers that bounce your body heat back toward you, coatings that store and release heat as the temperature changes, and ultralight materials.
Together, these innovations give designers far more control over warmth, breathability and comfort than ever before. That’s why jackets now feel warmer, lighter and drier than anything Newton or Fourier could have imagined.
Trap still air, slow the leak
Conduction is the direct flow of heat from your warm body into your colder surroundings. In winter, all that heat escaping your body makes you feel cold. Insulation fights conduction by trapping air in a web of tiny pockets, slowing the heat’s escape. It keeps the air still and lengthens the path heat must take to get out.
