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?