Does our blue sky also scatter UV or just visible blue? I'm wondering if I should wear sunscreen or UV sunglasses when exposed to only blue sky, such as working in a room with large windows that let in a substantial area of blue sky.
Then, I wondered if there's UV from blue sky.
I wear sunscreen if driving long distance in California, but probably because sunshine reflects off the good and turns me pink if no sunscreen, even when not exposed to direct sun.
 A: The sky is blue because of a phenomena called Rayleigh scattering. Rayleigh scattering occurs whenever radiation of any kind encounters particles (in this case air molecules such as $\text{N}_2$, $\text{O}_2$, or $\text{CO}_2$) that are much smaller than the wavelength of the radiation. In addition, the shorter the wavelenght, the more light gets scattered by Rayleigh scattering. In fact, the total scattered light is proportional to $1/\lambda^4$, so blue light at 400nm will scatter 16 times as well then red light at 800nm!
For UV light that can get through the atmosphere without being absorbed (UV-A and UV-B), the wavelength is in the range of $280$ to $400\,\text{nm}$, while air molecules are $<1\,\text{nm}$, so air molecules scatter UV just like it does blue light. In fact UV-B light around 280nm will scatter more than 4x as easily as blue at 400nm (for the same reason blue scatters better than red). However, if you're behind a glass window/your windows of your car are rolled up, the glass acts as a sublock as ~90% of the light below 300nm (UV-B which causes sunburns) will be blocked, so wearing sunscreen is not as important as when you are outside in the sun.
