UV is the abbreviation for Ultraviolet Radiation. These invisible rays produced by the sun are commonly classified in one of three categories: (UV- A, UV-B, or UV-C) and they are the major cause of sunburn, skin aging, and in some cases, skin cancer.
UV-A rays remain rather constant in intensity throughout the year and penetrate more deeply into the skin's layers than UV-B rays. UV-A rays are major contributors of delayed sunburn, wrinkling, and premature aging.
Please note the followings:

• Time of Day: The highest UV exposure during the sun's most intense period, in most climates, is between 11 am and 3 PM. 
• Season of the Year: The summer months are the only time when the sun and resultant UV rays are most intense and during this warmer period that you typically have more exposed skin due to lighter clothing. 
• Altitude: The amount of  UV rays are much more at higher altitude than at lower ones due to thinner, cleaner air. At altitude, the cooler air sometimes makes us forget we are getting high levels of UV exposure.