The gas balloon
A gas balloon is filled with either hydrogen or helium. These gases are much lighter than the air of the surrounding environment. If the weight of the lifting gas is subtracted from the weight of environmental air that is displaced, the result is the total lifting capacity of the balloon. Sand or water ballast is carried so that the balloon can be controlled. To climb, some of this ballast is jettisoned. To descend, gas is released from the gas valve on top of the balloon by briefly pulling on the dump line. To stop the descent, more ballast must be jettisoned.
The more ballast that can be carried the longer the duration of the flight. The balloons buoyancy will be affected by temperature and factors such as thermals or cloud shadows will cause it to climb or descend. The balloon may fly for hours in a stable air mass without climbing or descending. As evening approaches the lifting gas cools, loses volume and the balloon will start to descend. Each night about 80-100Kg (175-220 pounds) of ballast must be jettisoned. Each morning, the sun warms the lifting gas and the balloon will once again begin to climb. Each team in the Gordon Bennett race will carry about 700Kg (1’550 pounds) of ballast with them on their flight. Long-distance flights lasting many days are possible. The absolute record was set during the 19955 Gordon Bennett race by the team of Wilhelm Eimers and Bernd Landsmann starting from the Swiss town of Wil near Saint Gallen. They were in the air for more than 92 hours.