The lifting power of an aerostat equals the difference between the weight of air displaced by the aerostat and the weight of gas it contains. In other words, the lifting power is proportional to the difference between the densities of air and of the gas which fills the aerostat. As the density of a gas is in inverse proportion to absolute temperature, so the difference in densities is also in inverse proportion to absolute temperature, i.e. the lifting power is greater, the lower the temperature.