envelope (in ballooning context)

In ballooning, the envelope refers to the large fabric bag that holds the heated air or gas, which creates lift to carry the balloon. It functions like a flexible container that expands or contracts based on temperature and pressure changes. When the air inside heats up, the envelope expands slightly, making the balloon lighter than the surrounding air and allowing it to ascend. Conversely, cooling the air causes contraction and descent. The envelope's material must be lightweight yet strong to withstand heat and altitude conditions, enabling safe and controlled flight.