While clouds do reduce some of the sun's UV rays, they don't block all of them, as the Skin Cancer Foundation explains. UVA rays can penetrate clouds, and they can also reach below the water's surface. ... Reflective surfaces like snow and ice also intensify UVB rays and their effects on the skin.

Do clouds increase UV?

Clouds can block up to 70-90% of these UV-B rays during times of extreme overcast. ... When compared with completely clear skies, studies have shown that partially cloudy skies have raised the UV-B rays by 25% and increased DNA damage up to 40%!

Does cloud cover affect UV index?

Cloud Cover: Thick, heavy cloud cover can block most UV radiation, however, thin clouds can let most of the UV rays through. Fluffy, fair-weather clouds are deceiving because they reflect the rays and can increase the amount of radiation reaching Earth.