Windwear clothing excels during high-output aerobic activities like trail running, hiking, or cycling. It allows excess heat and moisture to escape in circumstances where rainwear or waterproof-breathables would soak you in perspiration. MEC windwear materials are tightly woven to prevent wind penetration. The fabrics let perspiration pass out readily, or in tech-speak, they have a high rate of Moisture Vapour Transmission (MVT).
In cold, dry conditions, windwear is an inexpensive and comfortable alternative to waterproof breathables (such as Gore-Tex® or Entrant®). The trade-off is that windwear garments are not waterproof, although they can shed some drizzle or snow. Their water repellency varies, but all will perform best if kept clean, and dried in a hot dryer.