Why are mountains so cold when they say that heat rises?
The mountains are cold because there is little oxygen or any other molecules in the air so high in altitude. Gravity pulls all things down including air molecules so the higher you go, the fewer molecules there are.
Heat rises because the molecules have higher energy and move faster and want to spread out so they move to a higher altitude where there are fewer molecules. When the hot molecules move up, the colder molecules move down.
But the heat that rises is not enough energy for the mountains to warm up. There are just not enough molecules up so high in the mountains. The air is too thin.