Why is the UV Index (UVI) higher in the summer?

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I learned in school that seasonality is due to the earth’s tilt. Summer (in which hemisphere you live in) is hot because the sun shines on your part of the earth for more hours than it does in winter. Makes sense, but how does the UV Index work? I’ve been checking it recently and a hour of sunshine at this time of the year is much more damaging than other times. IDK if it’s in my mind but sunshine on my skin just feels… “burnier.” Presumably the amount of radiation the sun puts out doesn’t vary with seasons. It is also due to angle? Is it something with heat? Am I imagining this?

In: Planetary Science

3 Answers

Anonymous 0 Comments

>Summer (in which hemisphere you live in) is hot because the sun shines on your part of the earth for more hours than it does in winter. Makes sense, but how does the UV Index work?

So this understanding of summer is actually incorrect. For example the north pole gets 24/7 sunlight during the summer but is still colder than the equator which gets 12 hours of sunlight year round.

The actual reason for summer is, as you said, the tilt of the planet. During summer your hemisphere is tilted towards the sun. This means that more sunlight per square mile is hitting your hemisphere so it gets noticeable hotter than it does during the winter.

And since sunlight is more concentrated during summer the UV Ray’s in sunlight are also more concentrated.

Anonymous 0 Comments

>It is also due to angle?

You guessed it.

The higher the Sun is in the sky, the less sky (aka. atmosphere) the light needs to pass through to reach you. Since part of the atmosphere is made partly of ozone (which strongly absorbs UV), passing through less of it before hitting the surface means a lot more UV reaches you.

Edit to add: this is also why angle isn’t the only factor in calculating the UV index. Different parts of the atmosphere contain more or less ozone, and of course, things like clouds will flat out reflect a bunch of light (of all wavelengths).

>Is it something with heat? Am I imagining this?

The “burnier” feel is indeed mostly due to heat. The visible and infrared portions of sunlight carry far more energy than the UV portion, and the longer wavelengths in particular penetrate deeper into your skin.

>Presumably the amount of radiation the sun puts out doesn’t vary with seasons.

While true, the distance to the Sun does vary with season, but in the opposite direction of what you’d expect (living in the northern hemisphere).

We’re currently getting further and further away from the Sun, with the greatest distance (and as a result, least total radiation hitting Earth) being reached in early July.

Anonymous 0 Comments

The basic concept is that the sunlight comes more “directly” into us, rather than at an angle.

When you shine light in a angled plane the light spreads out, which means the light coming in for any position of the plane is less concentrated. Less concentrated light means less UV, less heat etc.

[https://upload.wikimedia.org/wikipedia/commons/thumb/e/e6/Oblique_rays_04_Pengo.svg/1920px-Oblique_rays_04_Pengo.svg.png](https://upload.wikimedia.org/wikipedia/commons/thumb/e/e6/Oblique_rays_04_Pengo.svg/1920px-Oblique_rays_04_Pengo.svg.png) I think this picture explains it best. Imagine at the summer the earth is tilted towards the sun rays meaning that northend hemisphere gets more direct sunlight