I have read some threads on this topic, but I am still confused. Why does temperature drop as you go up a mountain? I have read that since pressure depends on the weight of the air on top of you and as you increase your altitude the amount of air over you decreases, therefore pressure decreases, gas expands and expansion makes its temperature drop. The rising hotter air gets continually cooled due to expansion (or would it start rising at all?). Gas loses internal energy when it has to expand against a force, i.e. do work. But isn't exactly the same amount of work the gas does received as internal energy of the lump of gas next to it that the work is being done upon?