Sunglasses should be worn all year round since UV radiation is present in all types of weather. This is because UV rays reflect off surfaces such as water, snow, sand, and buildings. Hence, both radiation from the sun and reflected UV radiation are harmful to the eye. UV exposure during winter can cause photokeratitis (snow blindness). The UV index is high when the sky is clear and high clouds are present. The World Health Organization states that 90% of UV rays can go through light cloud coverage.