Full of Hot Air!

Colorful hot-air balloons are certainly beautiful, as they float through the sky. They stay up as long as the air inside them remains hot. If you're watching the balloon from a distance, they may seem very quiet, and much of the time, the balloons do just float along silently. But every now and then, the balloonist has to turn on the burner to heat the air again, and that's quite noisy if you're in or near the balloon!

Hot-air balloons rise because hot air is less dense than cooler air. Density is the weight of something divided by its volume. A hot-air balloon typically holds about 100,000 cubic feet (2800 cubic meters) of air. Cooler air (68^oF, 20^oF) weighs about 7500 pounds (3400 kilograms); hot air (250^oF, 120^oC) weighs about 5600 pounds (2500 kilograms). That's a difference of 1900 pounds (900 kilograms)! When the air in the balloon is heated, the whole thing becomes lighter, and it rises into the air.

You may notice that you usually don't see hot-air balloons during the day. They're most often spotted in the early morning or just before sunset. That's because it's safest to take off when the air is at its calmest. Also, the air tends to be cooler at these times, so the difference between the cool air and the heated air is greatest.

Another thing about hot-air balloons - you can't really steer them! Balloon flights aren't used to get from one place to another, because where the balloon goes depends on which way the winds blow. Balloons are followed by chase vehicles, cars or trucks that meet the balloon when it lands, driven by people who help the balloonists pack the thing up and take it back to the starting point.

Have you ever been to a balloon festival? As beautiful as one or two hot-air balloons are, it's really amazing to see over one hundred of them take off at once. Some famous balloon festivals are the Quick Chek Festival of Ballooning in Readington NJ (where Dr. B took the photo above) and the Albuquerque International Balloon Fiesta in NM. You can look for a balloon festival near your home here.