Q: If heat rises, why is it so much colder at high altitudes?

A: Heat doesn’t really rise. When air comes into contact with warm ground, it acquires some of that heat and grows less dense, so it rises (as lighter things tend to do). As the air rises, it mixes with cooler air, cooling it to a point where it’s too dense to rise any more. Because the cool air on a mountaintop (or high in the sky) doesn’t make contact with the warmer ground, it remains cold.