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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Tom Skilling, famous American meteorologist, says that the sun’s position and distance from earth means that from 11am to 2pm in summer is peak tanning times. It takes twice as long to get the same tan at 3pm as it does at noon.
The same goes for the calendar. September tans take longer than a June tan, when the sun is closer.
Trigonometry.
No, seriously. The more directly a source of radiation (like light, or UV rays) hits a surface, the more of that intensity is absorbed by the surface. If you lie down in a rainstorm, you’ll catch more raindrops on your front than if you’re just leaning backwards. Sunrays work the same way. How much more? Well, if you catch all of them lying flat, you’ll catch sin(x) of them leaning, where sin is a way of measuring things that have to do with circles and triangles, and x is how far rotated you are from lying flat.
But you aren’t lying flat at certain times of day, so why do you get more sunburned? The earth is lying flat! When the sun is directly overhead, it’s like the earth (and you) are flat as compared to the direction of the sunbeams. When the sun is low in the sky, it’s more angled, and you’re catching less of them.
Two reasons.
One, the Sun is brighter from your perspective at noon than it is in the morning or evening. That’s because the Sun is at a larger angle from the ground, and it’s the same reason midday is hotter than the morning.
And two, that change in angle also means sunlight travels through more of the air. Since air absorbs UV to some extent, longer paths through the air result in more UV being absorbed, and thus less of it hitting your skin. (This is the same reason why you get sunburned much more easily at high altitude.)
Two reasons.
One, the Sun is brighter from your perspective at noon than it is in the morning or evening. That’s because the Sun is at a larger angle from the ground, and it’s the same reason midday is hotter than the morning.
And two, that change in angle also means sunlight travels through more of the air. Since air absorbs UV to some extent, longer paths through the air result in more UV being absorbed, and thus less of it hitting your skin. (This is the same reason why you get sunburned much more easily at high altitude.)
Two reasons.
One, the Sun is brighter from your perspective at noon than it is in the morning or evening. That’s because the Sun is at a larger angle from the ground, and it’s the same reason midday is hotter than the morning.
And two, that change in angle also means sunlight travels through more of the air. Since air absorbs UV to some extent, longer paths through the air result in more UV being absorbed, and thus less of it hitting your skin. (This is the same reason why you get sunburned much more easily at high altitude.)
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.
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.
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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