Factors influencing UV at the surface

Ozone levels, clouds, the angle of the sun's rays, altitude, tiny particles in the atmosphere (which scientists refer to as aerosols), and the reflectivity of the surface (determined largely by the extent of snow cover, which is highly reflective), all influence the amount of UV reaching the surface.

Source: ACIA Impacts of a Warming Arctic: Arctic Climate Impact Assessment  (2004),
 Key Finding #9, p.99