Balloonists prefer winds to be twelve miles per hour or less. Gas balloons fly as a result of a lifting gas in their envelopes . In the United States pilots fly with helium or hydrogen as the lifting gas. Both gases are lighter than air. Gas balloons get their initial lift from the helium or hydrogen pumped into their envelopes.
Pilots have two ways to control the altitude of their balloon.
To descend, pilots can either vent helium or hydrogen through the top of their envelope or wait for the cooler evening temperatures to contract the gas in the envelope. Gas balloon pilots also prefer winds to be below twelve miles per hour for launching and landing.