Winter presents an interesting apparel challenge for those who want to play outside. Wear too many layers and you’ll sweat; wear too little and you’ll freeze your fanny. Lately, technology has offered up a solution: a paraffin-wax-like substance known as phase change material encapsulated in microscopic balls of heat-resistant plastic similar to that used in dishwasher-safe dinnerware. Coated onto fabrics, the phase change material melts and freezes before you do and, in the process, stores and expels heat energy. Clothing can now be engineered to respond to your body temperature-and heat up or cool down to keep you feeling just right.

Phase change materials work because they are designed to maintain the midpoint of a narrow temperature range. One phase change material used in fleece jackets, for example, stays between about 27 °C and 38 °C when worn-or around 32 °C, which feels comfortable next to the skin. The specific range is determined by the lengths of the hydrocarbon molecules that make up the material; in different proportions they specify different ranges.

When a skier puts on the jacket, some of the phase change particles absorb body warmth and partially melt. During a strenuous run down the mountain, the skier’s body generates excess heat, which melts the remaining microcapsules. Because the heat is absorbed by the melting material rather than reflected back toward the body, the temperature inside the garment stays comfortable. On the chairlift ride back up the mountain, the skier cools down. But as the temperature between the jacket and the body drops, the microcapsules refreeze-in the process releasing their stored heat. The phase change material can run through these thermal cycles indefinitely, easily outlasting the life expectancy of the garment.

Two companies specialize in this material: Frisby Technologies in Winston-Salem, NC, and Outlast Technologies in Boulder, CO. By early 2003, just about every major brand of outdoor apparel will offer a line of clothing made with phase change materials, turning more fair-weather fans toward cold winter fun.