I was reading the altitude records for hot air balloons on Wikipedia, and noted that the max hot air balloon altitude was about 60,000ft. It didn't really say if there was a reason why. I know that closed envelope balloons frequently climb much higher.
I was wondering... If you have a pressurized cabin (for keeping the operator conscious), and you mix oxygen into the fuel to heat up the air, is there really any reason that a balloon cannot ascend higher? Does the thinning of the air prevent it from holding heat as well? Do other atmospheric events come to play in not allowing a hot air balloon to ascend higher? I don't understand hot air balloons that well and could be missing a major component =)