eli5: Why does the temperature start dropping before sunset?


I always thought that once the sun set the temperature started to drop, but today I noticed it started to cool down 2 hours before the sunset. Why is that?

In: 5

The sun is not at its strongest then That usually happens around the middle of the day and into the afternoon. In the morning, things warm up. In the evening, things cool down.

You know how the maximum temperature occurs between noon and three? It’s because the sun is at its maximum and it’s shining through a minimum amount of atmosphere. As the day progresses, the sun shines from an angle, through a lot more atmosphere and the effect of shade is more pronounced. Heat absorbed earlier in the day is radiated into space.

Take a piece of paper and hold it flat under a light directly above you. Observe the size of the shadow. Now tilt the piece of paper by about 45 degrees, do you see how the shadow shrinks in size? It is blocking less light when it is at an angle.

Imagine the paper is an area of Earth’s surface and the light is the sun. As the Earth spins bringing that patch of ground towards night the angle of the sun moves towards the horizon and the sunlight falling on the surface is more angled.

Just like with the paper the amount of sunlight falling on that area of Earth is reduced, which means less heat is being transferred from the sun.

See the [graphs here](https://www.sciencedirect.com/science/article/pii/S2405844018388352#fig2) to see how light and heat from the sun varies over 24 hours. Notice that the sun doesn’t simply turn on at dawn and off at dusk; its heat increases and decreases gradually due to a number of effects discussed in the article.

Mostly the reason is that the sun, when near the horizon, shines across the land and so relatively little hits the land to heat it up. It’s also more likely to hit clouds when near the horizon.