I believe that spidergoat post (post #2) is correct: There was similar thread about this, but I can't figure it out where. Something is hot or cold is depend on the heat (which represent the amount of energy), but not necessarily the temperature. Do you recall some heat formula, such as in definition of latent heat or sensible heat? Heat is a function of mass (and hence density), too. So, up in the mountain is colder because the air density is fewer. As what is explained by Spidergoat earlier.