Temperature ratings define the specific range each winter jacket is most comfortable being worn in. At the highest level, the two warmth categories are simply "WARMER" and "WARMEST" as defined here:
The low end of the range is at or above zero degrees Fahrenheit
The low end of the range is below zero degrees Fahrenheit.
All of our temperature rating testing is done at the renowned Kansas State University, the first lab to develop the modeling for metabolic rates. It starts with manikins of kids or adults, always dressed in the same cold-weather clothing. Then they're placed in a controlled environment where we measure the energy required to maintain a constant body temperature, using the metabolic rates of kids or adults walking slow and fast.
The ratings are an easy way to help compare which jacket, parka or coat will fit your, or your family's, winter needs best. After looking at the jacket's shell construction (waterproof or water resistant, for example), and insulation (down or synthetic), temperature ratings give you point to compare, helping you make the right decision.