You can get a sunburn on a cloudy day, but you can't get a sunburn even on a sunny day if you are behind glass. why?

To understand why you can still get a sunburn on a cloudy day but not when you are behind glass on a sunny day, it's important to know how sunburns occur.

Sunburns are caused by the ultraviolet (UV) radiation emitted by the sun. There are two types of UV radiation that reach the Earth's surface: UVA and UVB rays. UVB rays are primarily responsible for sunburns.

On a cloudy day, although the sun is not visible, it does not mean that UV radiation does not reach the Earth's surface. Clouds can block or scatter sunlight, leading to a reduction in visible light, but some of the UV radiation can still penetrate through the cloud cover and reach your skin. Therefore, you can still get a sunburn on a cloudy day, even though it might not feel as hot or as sunny.

When you are behind glass on a sunny day, typically used for windows or in cars, the glass acts as a barrier that blocks most of the UVB radiation. However, it is important to note that UVA rays can still pass through glass, which can contribute to long-term skin damage and aging. This is why even though you might not get a sunburn behind glass, it's still essential to protect your skin from UV radiation by wearing sunscreen or seeking shade.

In summary, on a cloudy day, UV radiation can still penetrate the cloud cover and cause sunburns. When you are behind glass on a sunny day, the glass primarily blocks UVB radiation, reducing the risk of sunburn.