The Second Law of Thermodynamics is a forbidding principle - and in more ways than one: it forbids heat to flow from a colder body to a warmer one.
Of course, scientists think of the second law in esoteric terms and say that "the entropy of a closed system increases with time". By this they draw attention to the fact that when energy is transformed from one manifestation to another, such as from electrical energy into light, some of the energy is wasted; it is not lost, but it is converted into a form from which it cannot be usefully recovered. It is dissipated throughout the environment - usually in the form of heat.
Energy irretrievably gone from us in this way is reflected in the term entropy. Since energy is continually being converted from one form to another, and since there is some wastage at each conversion, the entropy of the universe is continually increasing, forming a vast and ever-increasing reservoir of used and useless energy - a huge imaginary cesspool into which all the frenzied activity of the world is finally and irresistibly drawn.
But it is easier to think in terms of the easier version Second Law of Thermodynamics - that heat always flows from a warmer body to a colder one. The concept dictates why the highest air temperature of the day occurs on average at about two or three in the afternoon, rather than at noon when the sun is directly overhead.
The surface of the Earth, responding to the radiant energy from the sun, gradually warms during the morning and is at its hottest, on average, very shortly after noon. But the air behaves differently. Air is transparent to shortwave radiation, and therefore virtually immune to solar heating; it takes its temperature mainly by contact with the ground beneath it.
Continually during the forenoon the surface of the Earth is hotter than the atmosphere in contact with it, and the temperature of the latter, therefore, gradually rises. At noon the ground is close to its maximum temperature and shortly afterwards begins to fall, but for an hour or two yet the surface of the Earth is still warmer than the air above it; heat continues to flow from ground to air for the time being - and for as long as it does so, the temperature of the air continues to rise.
It may be 2 or 3 o'clock before the ground temperature has fallen below that of the atmosphere - and only when this critical value has been passed does the evening fall in the temperature of the air begin.