Have you ever wanted to fly like a bird and soar through the sky? Do you think it would be fun to glide high above, seeing everything from the top down? Even though we’re unlikely to grow wings and fly on our own, we can still get a bird’s eye view of things by hopping in a hot air balloon.
You may have seen hot air balloons dotting the sky with bright colors and fun designs. But have you ever wondered how they stay up in the air?
Hot air balloons fly because of a very simple scientific principle: hot airrises. Hot air is lighter (less dense) than cold air, which means it rises.
If you live in a multi-story home, you have probably already experienced this principle on a warm summer day. The basement of your home will typically be quite cool, and the first floor may be quite comfortable. If you go upstairs, though, you will notice the air is much warmer. If you climb into the attic, you’ll find it holds the hottest air in the house!
The balloon part of a hot air balloon is called the “envelope.” It is typically made of nylon. Nylon is the perfect hot air balloon material, because it is lightweight, strong, and it will not melt as easily as other materials when heat is applied.
A burner is positioned below the balloon. It uses propane fuel to send the balloon up-up-and-away. When the pilot lights the burner, a jet flame heats the air and directs it into the envelope. As the air within the envelope heats, the balloon begins to expand and rise, taking the attached basket—and anyone inside—with it.
One cubic foot of air inside the envelope can lift about seven grams, which is only a tiny amount of weight. This is why hot air balloons have to be so massive. In order to lift the weight of a basket and passengers, it needs to capture a lot of hot air.
You’re probably wondering why there is a hole in the bottom of the envelope. Shouldn’t it be closed? Why doesn’t all the air just rush out?
Hot air is buoyant, which means it rises and thus won’t fall out of the bottom of the balloon. As long as the air stays heated, it will continue to rise, pushing the balloon upward and keeping it afloat. The more air inside the balloon, the greater its buoyancy. This means bigger balloons can go higher than small balloons.
When it’s time to come back down, the pilot releases a valve called a “parachute valve.” The parachute valve is a vent in the top of the envelopethat allows some of the hot air to escape. This also causes the remaining air to begin gradually cooling, and the balloon slowly sinks back toward Earth.