UV levels can be more intense in the snow for two reasons:

The atmosphere is thinner at high altitudes and absorbs less UV radiation from the sun. UV radiation intensity increases by about 10–12% for every 1000 metres of altitude.
Snow is highly reflective. On a sunny day, clean fresh snow can reflect up to 90% of UV radiation. This means you can be exposed to almost a double dose of UV – directly from the sun and bouncing off snow-covered surfaces.
UV radiation from the sun – not heat – is the main cause of damage increasing your skin cancer risk.