The inside of the fabric is either densely woven or sealed with a wind-proof membrane.
Cold is the absence of heat. When something is chilled, it means that
its heat disappears. Heat loss happens when warm air from one surface
circulates into the cooler areas surrounding it, a movement known as
convection. As the heat is released, an insulating layer of warm air
forms at the surface. Rapidly moving air (wind) disrupts the boundary
and allows for cool air to replace the warm. The ‘wind-chill factor’ is
the feeling of decreased temperature as a result of this flow of air.
The purpose of a wind-proof garment is to help insulation by stopping moving air from breaking the body’s own heat layer, and thus preventing the warm air from disappearing. Wind-proofing is achieved either by a densely-woven fabric structure or, as in water-resistant outerwear, by sealing the inside of the fabric with an impenetrable membrane.