Teamdroid writes –
Everyone knows about hot air balloons. A great big sack filled with hot air that floats as if by magic in the sky. I’m sure most people think that the air needs to be heated by great honking propane torches to obtain positive buoyancy but that just isn’t the case. With a thin enough envelope (ballooning lingo for the thing that holds the hot air) you can use the mighty power of the sun to do this job on the cheap and clean. This site is loaded with useful info (some may be in French however) and data for your own personal exploration of the atmosphere. Did I say personal? Yep, you bet. Some people have actualy build solar balloons large enough to lift a person. There are even instructions on how you can build and use a 1.9 meter diameter mini solar balloon.
Solar hot air balloon – Link.