why do we get sunburnt at a faster rate at different periods throughout the day?

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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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Any time other than that is not ok. Sitting in the sun at any point is damaging to your skin, and you should avoid it at all times if you can. It’s worst between 11-3 but other times are just less bad.

The intensity of the sun varies with the relative position of the sun to the earth.

If you can visualize your shadow on the ground the longer the shadow is the lower the angle of the sun and the less intense your sun exposure is.

Here is a link to an illustration that explains the concept by region of the earth

https://s3.amazonaws.com/ai2-vision-textbook-dataset/dataset_releases/rc2/train/question_images/earth_day_night_2572.png

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.)