The variation of the temperature at the different times during the day is the result of two primary causes: the difference of the thickness of the atmosphere the sunlight must traverse to reach the surface; and the change of the concentration of sunlight over the same surface area of the Earth.
Flat-Earthers claim that the change in Sun’s distance caused such a difference in temperature and that it can only be explained in a flat Earth. They are wrong.
Earth’s atmosphere has practically the same thickness. During the early and late in the day, sunlight arrives at an angle, and thus, has to penetrate a thicker atmosphere before it can reach the surface. The thicker the atmosphere it traverses, the more energy is absorbed by the atmosphere, and the less energy reaches the surface. As a result, during early and late in the day, we receive less heat energy from the Sun than at midday.
Also, because the sunlight is coming at an angle, the same amount of sunlight is spread over a larger area during early and late in the day; but is concentrated in a smaller area during midday.
It is hotter later than earlier in the day because heat is accumulated and cannot be released all at once. In the day, the Earth receives more energy from the Sun than it releases. At night, we don’t get energy from the Sun, but the solar energy we get during the day is retained and will be slowly released throughout the night.
Flat Earthers claim that the difference in the temperature at the different times of the day cannot be explained in a spherical Earth. They are wrong. Such the difference in temperature can already be explained comprehensively and consistently.