Why doesn't the evening/morning sun feel much hotter on your face? 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?
 A: 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):

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:
 
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:

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

indicates the instantaneous amount of skin damaging UV radiation.

A: 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.
