E.g. I always heard that you should avoid sitting in the sun between 11 and 15 pm. But any time other than that is okay.
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The UV-A and UV-B light from the sun is absorbed by the earth’s atmosphere, and the amount of absorption is related to the path-length of the light through the atmosphere.
Since in the middle of the day the sun is highest in the sky, the path length through the atmosphere is shortest, and the UV levels highest. This is also why UV levels are low in winter (the sun is low in the sky, long path length). Similarly UV levels are generally greater the closer you are to the equator – because the sun there is almost overhead at midday.
The reason for “11-3pm” rather than symmetrically about midday (eg. “10-2pm”) is because of the widespread use of “daylight saving time”, where solar noon (sun highest in the sky) is actually around 1pm on the clock.
The general trend for UV levels with time of day in summer is shown on the main graph here: [https://phys.org/news/2021-12-uv-index-expert.html](https://phys.org/news/2021-12-uv-index-expert.html)
At lattitudes further from the equator the peak UV level is less (in northern Europe it rarely exceeds 6-7), but the general shape is similar.
There is no “hard” cutoff at 11am or 3pm – the change is gradual… but since the total UV dose is the area under the curve, you can often get approaching 75% of the total whole-day exposure in the central 4 hours of the day.
Not shown on the linked graph, but in an old paper which is no-longer readily available on the internet, the UV-B curve shown is actually slightly narrower than the UV-A curve, so the ratio of “burning” UV-B to “tanning” UV-A is worst in the middle of the day too.
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