Movies have been lying to us all for years!
The saying "heat rises" is an acknowledgement of the fact that warmer air has a lower density than the surrounding atmosphere, and therefore, being of lower density, it will rise. This rising is limited however - think about it - if there were no limit to hot air rising, then the entire atmosphere would float out into space. The reason it is "cold" in the upper atmosphere is actually because of a lower air density.
