When I checked the weather today, I saw that the UV Index was 11. "Ut oh", I thought, "I just wrote in my blog that the UVI was a scale from 1 to 10." Sorry about that - I was wrong.
The measure of the suns ultra violet rays (of which there are UVA and UVB) actually goes to 11+.
Regardless of the number, it is often suggested that we limit our time in the sun between the hours of 10a and 4p. We are encouraged to wear sunscreen that provides broad spectrum coverage along with protective sunglasses and hats any time we are exposed to the sun.
The Environmental Protection Agency of the USA has a webpage dedicated to helping us be SunWise and part of that section includes the UV Index information. I was delighted to see that the index is a global measure aligned with the World Health Organization's scientific evidence.
My summarizing the EPA information would be less helpful than just having you go to the page itself where the info is laid out quite simply. You can see it here.