Why are the sun’s UV rays more harmful in the afternoon than in the morning or evening?

Question

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

Answer 1

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.