The specific volume of air is basically the reciprocal of Air Density. As the temperature of the air increases, its density will decrease as its molecules vibrate more and take up more space (as per Boyles law). Thus the specific volume will increase with increasing temperature. At one standard atmosphere of pressure and 20Ã‚Â°C / 50% RH, air has a volume of 0.8402mÃ‚Â³/kg. At 30Ã‚Â°C / 50% RH, the volume will have increased to 0.8774mÃ‚Â³/kg. This is the basis upon which hot air ballons work, filling the enclosed volume with less dense air than its surroundings.
The specific volume of air is also affected by humidity levels and overall atmospheric pressure. The more moisture vapour present in the air, the greater the specific volume. With increased atmospheric pressure , the greater the density of the air - so the lower its specific volume. The Figure below shows lines of constant air volume plotted on the Psychrometric Chart.