Arctic Hat evaporative cooling inner liner retains cool water and vaporizes to keep your head up to 20 degrees cooler than the outside temperature. Simply wet the inner lining, then start feeling cool instantly and stay cool for hours. Arctic Hat also has a built-in radiant barrier to deflect and protect you from dangerous UV rays arctic hat provides 99.89% protection from the sun's harmful UV rays. Arctic hat is adjustable, so one size fits most.