Originally posted by Fishu
and I thought the heat increases lift, but thats based on the sail planes.
Guess it might affect to the lift, if the plane is unable to create enough power due to heat readings 
Balloons rize because the air inside is less dense than the air outside. Archemedies principle says heavier air displaces the lighter air, and the air rises.
Sailplanes get lift from upward moving air currents, whether from waves blowing over mountain ranges or thermal upwellings. The density of the air is not paramount, it is the vertical wind on which the glider sails.
The less dense the air, the less horsepower the engine can produce. Also with less density, the wings cannot produce as much lift with the same air velocity.
