Definition

A non-power-driven, unmanned, lighter-than-air aircraft in free flight.

Source: ICAO Annex 2 - Rules of the Air

Description

Unmanned free balloons are mostly used for scientific purposes. They usually operate at very high levels (i.e. above 60 000 ft / 18 000 m). While at these levels they are not a considerable hazard to other aviation operations, the ascent and descent phases need specific attention. 

Unmanned free balloons use light gas (hydrogen or helium) as hot air is impractical for reaching their high operation altitudes. Based on the volume of the gas, balloons are divided into "zero-pressure" and "superpressure".

Zero-pressure unmanned free balloons change their shape as they ascend (see image below). The reason is that as the balloon rises, ambient pressure gradually decreases and the gas inside extends until it equates that pressure.