Observation balloon

An observation balloon is a type of balloon that is employed as an aerial platform for gathering intelligence and spotting artillery. The use of observation balloons began during the French Revolutionary Wars, reaching their zenith during World War I, and they continue in limited use today. Synonyms include espionage balloon, reconnaissance balloon, spy balloon, and surveillance balloon. Historically, observation balloons were filled with hydrogen. The balloons were fabric envelopes filled with hydrogen gas, the flammable nature of which led to the destruction of hundreds of balloons. Observers manning these observation balloons frequently had to use a parachute to evacuate their balloon when it came under attack. Shortly after World War I, observation balloons were often filled with non-flammable helium to avoid the potentially explosive consequences of hydrogen. Typically, balloons were tethered to a steel cable attached to a winch that reeled the gas bag to its desired height (usually 1,000-1,500 metres) and retrieved it at the end of an observation session.