As a hot air balloon rises to increasingly higher altitudes describe what happens to the volume of the balloon if at constant pressure? My teacher had us do a PTV cards activity, where we took an index card and wrote the letters P(for pressure), T(for temperature), and V(for volume). This activity was supposed to help us learn about pressure. For example, if the pressure increases and the temperature remains constant, what happens to the volume? It increases. For the question above, if the pressure remains constant as the hot air balloon rises, the volume increases, right?