10/23/2017
Hot-Air Balloons and the Workings of the Atmosphere
What makes the spectacle in Albuquerque and elsewhere possible?
By Helen Czerski
Oct. 19, 2017
Every morning for nine days in October, hundreds of hot-air balloons join a gigantic atmospheric waltz in the clear blue skies over New Mexico for the Albuquerque International Balloon Fiesta. I’ve loved flying in a balloon, and I’m so envious of the thousands of spectators at the fiesta. It’s a beautiful celebration of the earliest form of human flight. There’s something about balloons that planes, helicopters and rockets have never replaced.
The startling variety of balloons in a festival like New Mexico’s hides the simplicity that’s keeping them aloft. A hot-air balloon is fascinating not because it’s in the sky, but because it becomes part of it.
It’s simple to integrate a basket of humans into the atmosphere. The nylon fabric of each balloon is just the container for a pocket of air that’s emptier than the atmosphere around it. As a burner heats the air inside, the molecules speed up, which means they push harder on their surroundings and spread out. Some of the original air gets squeezed out, so the space in a balloon has perhaps 20% fewer air molecules in each cubic foot than the air outside.
These missing molecules matter—the hot air in a medium-size balloon weighs about 6,400 pounds, instead of the 8,000 pounds of air it would have taken the fill the balloon while cold. The difference is made up by the cargo of people, the fuel and the structure of the balloon itself. The weight of everything you see suspended from a balloon exactly matches the weight of the missing air from inside the balloon. So it just blends in with the atmosphere. If you’ve ever wondered what it would be like to ride with the wind, this is it.
Balloons go where the air goes, and they make the layers of the sky visible. Close to the ground, rough terrain, trees and buildings slow the wind down, but the wind speed increases rapidly as you rise. The bumpier the ground, the higher you need to go to be free of its influence—perhaps 700 feet over a flat plain but 1,500 feet over a city. Balloonists can control their speed by choosing their height, but they have a surprising amount of control over direction, too. In the Northern Hemisphere, the spin of the planet means that winds turn to the right as you go higher—due to what’s known in physics as the Coriolis effect.
This and the weather determine the major wind direction, and that will carry the balloon along. But in the slower wind speeds nearer the ground, the Coriolis effect is less influential. That means that the pilot does have a bit of control over direction when the balloon is within 1,500 feet of the ground, because the wind near the surface has a distinctive pattern.
As the balloon descends from higher up, it will start to follow the lower winds that aren’t turned to the right by the Coriolis effect. By controlling the balloon altitude, the pilot takes advantage of these different wind directions. These wind spirals are present all the time, but you can’t see them directly. And Albuquerque is a particularly good place for a balloon festival, because the winds high above the spiral often run in the opposite direction to the low-level winds. So by traveling in different layers of the atmosphere on the outward and return legs, it’s possible to launch and land in the same spot.
There are hazards. Thermals form when dark patches of ground heat up quickly in the sun and the warmed air above them rises in an invisible fountain. Balloonists avoid thermals because they make the ride very bumpy. That’s why balloons fly in the early morning and evening, when solar heating is minimal.
The vast ocean of air above us is constantly on the move, invisible but dynamic. In the end, that’s what my love of ballooning is about: It’s the closest we ever come to seeing the atmosphere directly, being able to visualize the flow, to see the sea of air we live in and appreciate the dance of the sky. A human in a hot air balloon is just joining the party.
