The body constantly gives off warmth and moisture, even while resting. Ambient moisture and temperature levels are typically lower compared to the inside of the garment. Here, you have a partial pressure gradient between warmer, moister air inside and less moist, colder air outside. As a result, in its search for equilibrium, moisture moves through clothing to the exterior when you are active. This principle applies to every kind of apparel. Breathability defines the ability of a given fabric to allow moisture to escape (diffuse) to the outside, thereby allowing complete comfort.