Superheating in Gas Balloons

It is the heat from the sun that warms the earth’s surface, also the air, as well as the gas in a balloon.  It might be expected that the temperature of the gas in a balloon floating in the air would be the same as the temperature of the atmosphere itself, in the same way that the temperature of a body immersed in a liquid soon equalizes and the temperatures of the body and the liquid become the same.  This is not the case with a balloon. A balloon could contain gas that was exactly the same temperature as the air if it was inflated in hangar and allowed to equalize. It would take only a short time after the sun’s rays are encountered the temperature of the gas within the envelope will increase.  This can be as much as 30 degree F in extreme cases a much greater increase in temperature is possible depending on the fabric of the envelope.

This increase in gas temperature is due to the light rays being changed to heat as they pass through the envelope.  It is the same effect that takes place in a greenhouse or tent.  On a sunny day the temperature in the greenhouse or tent that is not ventilated is always higher than the temperature of the surrounding air.  The pilot will notice changes in lift as soon as the sun disappears behind the clouds, or after sunset, which is caused primarily be the reduction in superheating and not by changes in atmospheric temperature.

The subject of superheat is one of the easiest parts of aerostatics, and at the same time, one of the most important.  The effects of an increase or decrease in superheating is directly noticed and is always of sufficient magnitude to make it important during the flight.  Whenever the temperature of the gas inside the envelope becomes greater without a corresponding change in the surrounding air, the balloon will pick up additional lift due to the increased volume and decreased density of the expanded gas.  The lower the density of the gas the greater the lift the balloon has. On clear days when the sun is shining brightly, the temperature of the gas inside an envelope is always higher that that of the surrounding air and the lift is consequently greater than that computed by using the air temperature only.  This difference between the temperature of the gas and that of the outside air is called superheat.  Superheat may be either positive or negative.  It is always positive during hours of sunshine and reaches the negative condition at night.

If sunshine strikes the outer cover of the envelope some of its heat energy will be absorbed by the gas and the temperature of the contained gas will be increased.  As soon as this happens, the gas will begin to expand and when the envelope is not full of gas (below pressure height) the volume of gas will increase to fill the available space and displace any air which will be pushed out of the appendix of the balloon.  (Pressure Height is when the balloon envelope is completely emptied of air and is completely full of gas). 

Superheating has no effect on the weight of the outside air displaced by the balloon.  The lift of the balloon will be increased by the amount exactly equal to the weight of the air (or gas being valved) forced out of the envelope due to the expanding gas.