Young Brunette Girl With Long Hair Outside

Warmth
While humans have developed clothing and other means of keeping warm, the hair found on the head serves as primary sources of heat insulation and cooling (when sweat evaporates from soaked hair) as well as protection from ultra-violet radiation exposure. The function of hair in other locations is debated. Hats and coats are still required while doing outdoor activities in cold weather to prevent frost bite and hypothermia, but the hair on the human body does help to keep the internal temperature regulated. When the body is too cold, the erector pili muscles found attached to hair follicles stand up, causing the hair in these follicles to do the same. These hairs then form a heat-trapping layer above the epidermis. This process is formally called piloerection, derived from the Latin words 'pilus' ('hair') and 'erectio' ('stiffening'), but is more commonly known as 'having goose bumps' in humans. This is more effective in other mammals whose fur fluffs up to create air pockets between hairs that insulate the body from the cold. The opposite actions occur when the body is too warm; The erector muscles make the hair lay flat on the skin which allows heat to leave.