A sealed balloon made from a material of high modulus of elasticity. The balloon has an excess of lifting gas, producing an overpressure at the altitude of neutral buoyancy. As the balloon gas heats and cools, the excess pressure stresses the balloon film. If the balloon material has a high enough modulus, strain is minimal. With system mass constant and balloon volume almost constant, the balloon floats at a near-constant density level night and day in the absence of convection or wetting.