I always hear that we should avoid the sun or make sure to put on sunscreen in the afternoon when the sun is the most intense. Why is the sun’s rays more harmful when it is directly overhead vs in the morning soon after sunrise? Either way, the sun is shining directly on me so not sure what causes the difference in intensity.
In: Planetary Science
Remember that there’s a layer of atmosphere between you and the sun. When the sun appears lower in the sky, it has a lot more air to go through to get to you. When directly overhead, the sun’s rays only have to travel through the air above you- the shortest journey or the thinnest filter.
In the morning and evening the light travels through many more miles of air. Air absorbs some UV, and more air absorbs more of it. Not all of it, you can still get a sunburn in the early morning.
You’re forgetting there’s the air between you and the Sun. Sure it’s mostly transparent, but there are many kilometers of it. And when the sunlight goes through it at an angle the range it has to travel through air increases more.
Using DST, on average the suns rays will be most direct at 1pm, depending on where you live. It’s still going to be pretty direct at 12pm and 2pm but the further you get from solar noon, the more atmosphere the suns rays have to go through because it comes in at more of an angle. Gasses in the atmosphere can absorb a portion of the UV radiation.
Distance from the Sun to you + possibly percentage of the Sun’s UV rays that can reach you.
For example, if you’re out at dawn when the Sun partially appears, more of the UV rays get blocked/attenuated by various things other than air and the rest have to travel longer to you compared to midday.
Geometry. During the middle of the day, the sunlight comes mostly straight down, passing through a lot less air, than it does in the early morning and evening when the sun is lower in the sky. This is also a large part of the reason for the seasons. In winter, the sun is higher in the sky, and it’s light passes through less air, resulting in stronger sunlight. In the winter, the sun is lower in the sky, even at midday, and the light passes through all that air and is weaker when it finally hits the ground.
Imagine you point a flashlight directly at a globe with the flashlight even with the equator. The globe is surrounded by a layer of atmosphere made of gasses and other particles like dust in the air, which blocks/scatters some of the light.
The light that hits the equator is traveling a shorter distance to reach the surface because the round globe bulges most at the middle, and that light is taking a direct route through the atmosphere to reach the surface. This is the path of last resistance.
If you look further up/down the globe, the surface curves away from the flashlight. The light must to travel further to reach the surface and it must pass through more atmosphere because of its angle. More light is blocked/scattered this way, which is what reduces the intensity of the UV rays that reach us.
This is a simplified and slightly altered version of reality, but the concept for UV index is the same.
Take a very powerful flashlight. Aim it directly towards your father’s eyes. Then aim it to your mother’s feet.
Guess who’s now slapping your face.
You get more UV in the hours around noon when the sun’s rays travel through less atmosphere. This is why most recommendations for avoiding exposure give ranges like 9 am to 3 pm.
At sunrise and sunset, the sunlight has the most atmosphere to travel through to get to your skin.
More air in the way means more harmful UV rays get absorbed by the air, so less of them reach your skin.
In the afternoon, the sun is close to its ‘highest’ point above your head, where it then has the lowest amount of atmosphere to travel through.
This means more harmful UV rays reach your skin, causing more damage than in the morning or evening.
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