Air heated to 1500C provides about the same lift as cold helium, so this is no real advantage. Hot helium, however, can provide substantially increased lift. The rarefied helium approaches the lifting capability of a gasbag containing pure vacuum. Cold helium provides about 83% of the lift of a vacuum cell, while helium at 1500C provides about 97% of this lift. Cold hydrogen provides 90% of vacuum lift - at 1500C it would provide nearly 99% of vacuum lift, but could be extremely dangerous!