To see a hot air balloon in flight is truly something to see! A typical hot air balloon is 60 feet wide, 80 feet high, contains 77,000 cubic feet of air, and weighs about 800 pounds without passengers (skydrifters.com, sundanceballoons.com)! So how do these behemoths manage to float so gracefully through the air?
As it turns out, hot air balloons work based on the same principle that causes air currents and wind, thunderstorms, and tornadoes. They work because hot air rises, while cool air sinks! If you watch a hot air balloon being filled for flight, you’ll see a large gas burner forcing hot air into the opening at the bottom. This allows the balloon to fill with air that is hotter than the air surrounding the balloon. As the air inside the balloon rises in temperature, the balloon rises off the ground! Have a look at the YouTube video presented below to watch some hot air balloons being filled for flight (this great video was uploaded to YouTube by Geoffrey McKay--we did not make this video!!). You’ll notice at the beginning of the video that cool air (the same temperature as the surroundings) is being blown into the balloon with a fan—this fills the balloon with air, but does not cause it to lift. Only when the balloon is filled by the burners does it lift off the ground.
So why does hot air rise, while cool air sinks? First, let’s discuss what air really is. Air is a mixture of different gaseous elements that surround the planet because of gravity (for a more thorough review, see our previous blog, “Can Anything Really be Lighter Than Air?”). We call this layer Earth’s atmosphere. The density of this layer of gas—that is, the number of molecules per unit volume—depends on the temperature of the gas. As the gas heats up, the molecules move faster and faster, and therefore they spread out, making the gas less dense. As the gas cools down, the molecules slow down and thus get closer together, making the gas denser because there are now more molecules per unit volume.
This phenomenon is responsible for much of the weather on our planet: the air closer to the Earth heats up, as the temperature is warmer close to the ground. This air begins to rise, causing updrafts (you can see these updrafts clearly in the clouds during the summer, when the air near the Earth’s surface is very hot). These are called thermal columns, because the heated air rises in a column. As the air gets farther and farther from the Earth’s surface, it begins to cool. This cooler air then sinks back down to Earth’s surface, and the whole cycle begins again. This is called atmospheric convection.
Now that we understand why hot air rises, we can begin to understand why a hot air balloon is able to fly. The material the balloon is made of is able to keep all that hot air inside the balloon, separated from the cool surrounding air. This makes the air inside the balloon much less dense than that surrounding it; in effect, the hot air balloon is like a giant bubble of less dense gas! This is what makes it float up off the ground—it’s just like a bubble of less dense air rising through more dense water!
TRY FLYING YOUR OWN HOT AIR BALLOON!
Here’s what you’ll need:
1. One or more ignitable floating lanterns, such as Sky Lantern® (available at www.skylanterns.us, or at your local fireworks outlet)
2. Matches or a candle lighter
3. A fire extinguisher...just in case!
NOTE: Be sure to do this on a calm day! Wind will interfere, and will increase the risk of starting a fire. Also be sure that your area does not have burning restrictions!
Here’s what you’ll need:
1. One or more ignitable floating lanterns, such as Sky Lantern® (available at www.skylanterns.us, or at your local fireworks outlet)
2. Matches or a candle lighter
3. A fire extinguisher...just in case!
NOTE: Be sure to do this on a calm day! Wind will interfere, and will increase the risk of starting a fire. Also be sure that your area does not have burning restrictions!
Here’s what you need to do:
In general, just follow the instructions on the lantern’s packaging! You will light the fuel below the large paper lantern using your matches or lighter, being careful not to burn the lantern. Allow the lantern to fill with hot air, let it go, and watch it fly!
CHALLENGE:
Now that you’ve seen a paper lantern fly, try to make your own! Use light tissue paper, some strong glue, and some very thin wire for the base. Use old newspaper soaked in vegetable oil for the fuel. Try your lantern in an open area, far away from any homes!
If you make a paper flying lantern, take pictures or video, and share with us! Go to https://www.facebook.com/discoveryexpresskids, and post them on our page!!
In general, just follow the instructions on the lantern’s packaging! You will light the fuel below the large paper lantern using your matches or lighter, being careful not to burn the lantern. Allow the lantern to fill with hot air, let it go, and watch it fly!
CHALLENGE:
Now that you’ve seen a paper lantern fly, try to make your own! Use light tissue paper, some strong glue, and some very thin wire for the base. Use old newspaper soaked in vegetable oil for the fuel. Try your lantern in an open area, far away from any homes!
If you make a paper flying lantern, take pictures or video, and share with us! Go to https://www.facebook.com/discoveryexpresskids, and post them on our page!!
