The density of the air, the higher altitute, the smaller density: http://en.wikipedia.org/wiki/Atmosphere_of_Earth#mediaviewer/File:Atmosphere_model.png http://

The density of the air, the higher altitute, the smaller density: http://en.wikipedia.org/wiki/Atmosphere_of_Earth#mediaviewer/File:Atmosphere_model.png http://en.wikipedia.org/wiki/Atmosphere_of_Earth#mediaviewer... And the balloon has to have a smaller density than the surrounding air to be floating.