When I was in Meteorology doing Shuttle and ELV support we would release a bunch of these Jimspheres, http://www.ehartwell.com/afj/Jimsphere
They would often get caught in the Jet stream and I was told that it was possible for them to stay up in the atmosphere for weeks since they were made of aluminized mylar and because of their design would eventually float at around 55k feet.