We do get hit my ultraviolet rays whenever the Sun is up. But luckily the atmosphere screens out most of the UV rays. But when you go higher , for instance going up in the mountains, since there is less atmosphere, the intensity of the UV light is higher. UV light causes sunburn. That's why you get more easily sunburned at higher altitudes. Also, UV light can penetrate clouds -- so you can get sunburned even on a cloudy day. If the intensity of UV light is too great, it can cause damage to our cells and our DNA, causing mutations. Some scientist think that without our atmosphere protecting us from most UV rays, life might not have evolved on land.

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