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I understand that places on the Earth's surface get hotter in summer, and in the middle of the day rather than morning or evening, because the surface of the Earth is presented 'face-on' to the Sun at those times, rather than at a slant. Simple trigonometry (or a simple drawing) shows that the same amount of radiation is spread over a smaller or larger area depending on the angle. (I learnt this in grade school, please correct me if I am wrong.)

Analogously, I find that, in the heat of the middle of the day with the sun overhead, I get sunburnt on the top of my head (I am bald), the top of my nose, or my shoulders, far more than any part of my body which is a 'vertical surface', such as my face.

However, in the evening or the morning, when the sun is low, I don't feel experience lots of heat and sunburn on body parts which are facing the sun directly. Why isn't the effect of the low sun on individual vertical surfaces just as strong as the effect of noonday sun on horizontal surfaces?

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    $\begingroup$ more atmosphere $\endgroup$ – chharvey Apr 17 '15 at 3:00
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In general, the sun's light (particularly the UV that causes sunburn) has to pass through a lot more atmosphere (or a greater amount of air mass) in the morning and evening to get to a vertical surface than it does when it is at zenith to a horizontal surface.

An example is shown in the generalised image below (all graphs are obviously generalised):

enter image description here

The caption from the source (The Daily Cycle of Sunlight: Part 1) is (the relevant statements are in bold):

When the sun is close to the horizon, its rays pass through a thicker slice of the atmosphere, which warms the color of sunlight (orange arrow). During the middle portion of the day, the sun’s rays pass through a thinner slice of the at-mosphere (white arrow). We define noon sunlight as being neutral white—neither warm nor cool.

As such, the solar radiation is absorbed and scattered more in the atmosphere in the morning or evening. The net effect can be shown on the following diagram:

enter image description here

Source: It's Coldest After Dawn

In terms of sunburn, the erthemal dose rate (related to UV radiation) follows the same pattern, as seen below:

enter image description here

Image source: NOAA, where the define erythemal dose rate as

indicates the instantaneous amount of skin damaging UV radiation.

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  • $\begingroup$ As a note: the higher frequencies are scattered first, which is why the sky looks blue and sunsets are red and yellow. UV scatters even faster than blue, so less of it is present in those reddish sunrises and sunsets. $\endgroup$ – Cort Ammon Apr 14 '15 at 18:02
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    $\begingroup$ Thanks for this answer! This is making me think that filtering of radiation by the atmosphere could be as important as the angle in making the difference between winter and summer and noon and evening/morning sunlight. Is this the case? $\endgroup$ – jwg Apr 15 '15 at 7:17
  • $\begingroup$ Yes, this is a fair analogy to make. $\endgroup$ – user77400 Apr 15 '15 at 7:19
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    $\begingroup$ Your answer is good, but there is a problem. "Solar radiation per square meter graph" is misleading, since it corresponds to parallel to earth surface square meters, and the OP asks why even if he is facing the sunset, his body directly perpendicular to solar rays, gets still less radiation than at noon. $\endgroup$ – Santropedro Nov 18 '16 at 23:34
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I dont think the Sun light has to travel more atmosphere and hence the mornings and evenings are less warm than noon. The sun light is falling in gross all over half of the earth which is facing the sun at the time of rotation in its axis. So if that is true all the area which is exposed to the Sun's rays has to be equally heated up and not only one longitude at a time. coz the dimension of earth is meager considering the radius of the exposure of the sun's radiation. A simple physical drawing from Sun's POV will show earth is a tiny object while Sun radiates all over around its area. So there must be some other reason for morning and evening to be colder and noon is warmer.

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    $\begingroup$ Thanks for your answer, but some of your reasoning isn't clear to me. Do you have a justification for morning and evening to be colder, or are you not sure? $\endgroup$ – jwg Mar 27 '17 at 7:29

protected by Community Sep 6 '17 at 7:32

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