I’ve heard that on a cloudy day that you can get a sunburn. I’ve also heard that the clouds actually focus the UV rays and increase the chances of getting a sunburn. I’ve heard these very two things mentioned about swimming in water also. But I don’t recall ever getting a sunburn on a cloudy day. And the parts of my body that spend the least time in the water get burned the most (shoulders and neck).
Well it depends on the intensity of the UV rays if you get a burn or not.
The reason you can be burned on an overcast day is due to the scattering of light by particles in the atmosphere. It’s the same reason that the sky seems blue. Blue light by its nature scatters more than the other visible colours; it has a short wavelength. UV rays have even smaller wave lengths and so can be scattered about the sky instead of simply being absorbed within the clouds.
The water may reflect additional UV onto you leading to a higher chance of a burn/tan.
I figured that the parts of your body above the surface would get it bad because of the surface reflecting the UV. Which is the same reason why people get burned more easily on boats or when they are on the slopes. But that tells us that the water is reflecting the UV away from the parts of your body underneath the surface of the water.
So anyways… you are saying that you can get a sunburn on a cloudy day. But the clouds do protect you from direct UV rays? They just don’t protect you from the scattered UV rays? So all in all, you are safer from getting a sunburn on a cloudy day?
Well EM waves with short wavelengths attenuate quickly in water (a reason why the US Navy uses Extremely Low Frequency to signal subs). So most gets absorbed by the water, some gets reflected.
I’m saying that clouds do not block UV rays as they do visible light. The potential to get a burn remains despite overcast conditions.
I know this thread is kind of old. But since UV light is scattered and the shade from the clouds doesn’t protect you, then what about the shade from a tree?