Solar hot air balloons are a great addition to any outreach event. You can purchase one online for as little as a few dollars or make your own using thin trash bags or sheets of polyethylene.
The heated air inside the balloon expands, making it less dense than the cooler ambient air. This causes the balloon to float. While conducting science experiments, kids can learn the basics on how the balloons generate lift, solar radiation, and solar energy.
Most balloons I’ve seen are shaped like long, slender tubes (sealed on both ends), tear drops (your typical hot air balloon) or tetrahedrons (also referred to as tetroons). But don’t let that deter you from making unique designs of your own!
The best time to launch is in the morning on a calm day while the ambient air is cool.
Materials needed for a tube-shaped balloon
5-7 thin black trash bags (0.3-0.4 mil)
Lightweight, strong tape
Lightweight string tied to a rubber band on one end, a piece of cardboard on the other
Cut away the ends of all but one of the bags.
Starting with the bag that has the uncut end, overlap two bags by placing the end of one bag inside the other by an inch or two.
Tape the seam. I used black duct tape that I split in half lengthwise. Cellophane tape would have been better because of its light weight. I’ve read where other people use masking tape.
Continue steps 2 and 3 for the remainder of the tube.
Prepare the tie-down string by tying one end to a rubber band, the other to a piece of cardboard
Fill the bag with air using a hair dryer or fan.
Seal the open end with the rubber band.
Wait for the Sun to warm the air inside of the balloon, then watch it float!