Question

why temperature of the atmosphere remains almost constant from year to year

Answer 1

Essentially, there is no yearly gain or loss of total energy, and the average temperature of the earth and the atmosphere remains fairly constant from one year to the next.

That balance leaves us with an average earth temperature of about 59 degrees F. This equilibrium does not imply that the earth's average temperature doesn't change, but that the changes are small from year to year, usually less than one degree. These changes become significant only when a set trend is measured over many years.