Explanatory note regarding the abbreviations below: UPF means ultraviolet protection factor, SZA means solar zenith angle, UVR means ultraviolet radiation,
“The longer hair provides a higher UPF at the smaller SZA, but the difference between the protection provided by the longer hair compared with the shorter hair reduces with increasing SZA. Protection from UVR is provided by the facial hair; however, it is not very high, particularly at the higher SZA.”
Also see: Solar Ultraviolet Protection Provided by Human Head Hair (Photochemistry and Photobiology, Volume 85, Number 1, January/February 2009 , pp. 250-254)
BONUS (unrelated): Feline Reactions to Bearded Men