Most winter jackets have three key parts: lining, insulation, and shell. Here’s what to look for in each of these layers.

Lining

In the not-so-old days (circa 1990s), a jacket’s lining served mainly  to transport moisture away from your body. It still does, but now it helps keep you warmer too. Up to 87 percent of your body heat is lost through radiation in the infrared spectrum. That bit of science is important because the latest jacket linings can reflect infrared heat back toward your body, keeping the heat from being lost. Inspired by NASA’s space blankets, these linings are now more supple and less noisy than early versions of this material.

Insulation

Insulation traps warm air from your body in tiny air pockets. Down is king among insulators: efficient, cozy, and packable. But not all down jackets are created equally. Down fill is rated on a scale of 300 to 900, which refers to the cubic centimeters one ounce of down takes up in a standardized cylinder. The higher the quality - fewer feathers, more real down - the less it compresses. High-rated down has more air pockets and thus higher heat retention. (Plus it feels softer!)

Don’t worry about getting wet, which used to be down’s downfall. Today, high-tech down jackets have outer fabrics that shed water. And they are seam-sealed, meaning needle holes no longer act like tiny vents that allow moisture to get in.

If you’re allergic to down or if you need to be outdoors in very wet, cold weather, synthetic insulations - such as Primaloft® or Climashield® - are good down substitutes, they are just a little bulkier if you need to stash your jacket in a pack. They get heavier than down when soaked but still retain their warmth.

Shell

The shell material is a jacket’s main defense against the elements. Nylon fabrics laminated with GORE-TEX have long been the gold standard, although many manufacturers now have their own brands of waterproof, breathable shell constructions.

The latest shells are “PFC free,” which means the brand does not use perfluorinated acid (PFC) - a chemical that bioaccumulates in the environment and in humans - in the shell’s water- proof coating. According to the Centers for Disease Control and Prevention, high levels of PFCs in the bloodstream have been linked to serious health issues, including cancer and immune-system disorders.

Stretch

Another innovation in outerwear is the fact that the entire system - lining, insulation, and shell - stretches. In the old days, the shape of the garment was fixed, and you moved within it. Now, it moves with you. Weather still stays out, heat stays in, perspiration escapes, and the fit is better.

Convinced you need a new parka? Be careful you don’t get one that’s too warm! In general, outerwear has become much lighter weight-wise for the amount of warmth provided. If you get one designed for your activity level - sitting all day in a duck blind vs hiking in the back country looking for elk - you’ll get it right.