Why do mountains often get colder with height?

As you go higher, the air becomes thinner and contains less heat-trapping pressure. When air rises up a mountain, it expands in the lower pressure and cools adiabatically, which is why mountain tops are usually colder than nearby lowlands. The average environmental lapse rate in the lower atmosphere is about 6.5°C per 1,000 m, although local conditions can make it different. That temperature drop is one reason mountain climates can support snow, glaciers, and alpine ecosystems even at tropical latitudes.