"The 65,000-foot mark is the sweet spot where the airship would get as high as possible while still having enough air to propel against, because it needs propulsion to stay in the same spot. It's also a good altitude in terms of average wind speed."
Source: Jason Rhodes, an astrophysicist at NASA's Jet Propulsion Laboratory in Pasadena, California, who is leading the effort on a possible NASA challenge for developing stratospheric airships that can break records in terms of duration of flight at high altitudes. The agency has issued a request for information for this contemplated "20-20-20 Airship Challenge."