Reno Reporter

Residents of Nevada near Rancho San Rafael Park are used to seeing hot-air balloons as the summer draws to a close.

The yearly Great Reno Balloon Race draws University of Nevada, Reno (UNR) staff, professors and students to partake in the festivities; a recent news release from the school said.

UNR Department of Physics Professor Pat Arnott, who studies atmospheric sciences, explained the mechanics of what causes hot-air balloons to take to the sky.

“Hot air rises because of gravity and the dependence of air density on temperature – hot air is less dense than cold air at the same pressure,” he said in the release. “Hot air responds to the buoyant force and floats upwards, Archimedes' principle in action. The net effect is that the higher density, colder air urges the less dense warm air upwards.”

Arnott added that thinking of a cylinder of cold air with the flat sides parallel to the ground is a great way to visualize the hot-air balloon.

“Since pressure in the atmosphere above a certain location decreases with altitude, the bottom of our cylinder of cold air is pushed upwards more than the top of the cylinder is pushed downwards,” he said. “We might think that this would make the cold air cylinder go up because of the unbalanced pressure on the cylinder.

“But in static equilibrium the force of gravity pulling the cold air downwards balances the net force due to pressure exerted on the flat ends of our cylinder. The force of gravity is equal to the density of the cold air multiplied by the volume of the cylinder multiplied by the acceleration due to gravity.

“Now let’s replace our cylinder of cold air with hot air. The same pressure imbalance happens as before, though now the force of gravity pulling the hot air down is less than it was with the cold air, so the hot air rises up.”

Arnott also noted that the ideal time to fly hot-air balloons is when it's cold outside and at sunrise as the glow from the fire provides a great visual for spectators.