Why was this question
Why is the ground warm during winter and vice-versa?
deemed to be "about engineering" and closed, when the OP clearly stated he was not asking about how to employ the difference in temperature between surface and ground at a depth but instead why there is a difference in temperature?
"My question is not why we feel it warm, but why is it warm?"
From the meta post linked in the motivation to close it:
"Physics is a natural science. It seeks to study how and/or why nature works the way we observe. Engineering applies our understanding of nature to specific, real-world problems, creating new devices, products, or processes to solve those problems."
Is this not a question on why nature works the way we observe (why the ground layers are not at the same temperature of the surface) and not on how to employ this difference to solve the problem of heating/cooling?
It seems to me this question was closed because some answer focused more on the application of the physics than on the physics itself. But isn't the question about the underlying physics? The book I cited in my answer is "Partial Differential Equations in Physics", not "in engineering". I was entertaining the thought of expanding my answer with the reasoning employed by Sommerfeld, but not anymore.
As an aside: closing this question prevents further answers to the physics question to be added, for example on how much of the heat is due to the surface heating from the sun, and how much comes from the hot core inside. What will the average temperature at 10 or 100 meters be if we switch off the sun, or if we remove the hot core?
