Observation balloons are balloons that are employed as aerial platforms for intelligence gathering and artillery spotting. Their use began during the French Revolutionary Wars, reaching their zenith during World War I, and they continue in limited use today.
Historically, observation balloons were filled with hydrogen. The French colonel Charles Renard developed a mobile system with a trailer in 1880. Effectiveness was considerably improved with a new more aerodynamic design in 1914, by French engineer Albert Caquot. During World War I, from 1914 to 1918, both the Allies and Germany employed balloons, generally a few miles behind the front lines. The balloons were fabric envelopes filled with hydrogen gas, whose flammable nature led to the destruction of hundreds of balloons on both sides. Observers manning these observation balloons frequently had to use a parachute to evacuate their balloon when it came under attack. To avoid the potentially flammable consequences of hydrogen, observation balloons after World War I were often filled with non-flammable helium.
Typically, balloons were tethered to a steel cable attached to a winch that reeled the gasbag to its desired height (often above 3,000 feet) and retrieved it at the end of an observation session.
The first military use of observation balloons was by the French Aerostatic Corps during the French Revolutionary Wars, the very first time during the Battle of Fleurus (1794). The oldest preserved observation balloon, L'Intrépide, is on display in a Vienna museum. They were also used by both sides during the American Civil War (1861–65) and continued in use during the Franco-Prussian War (1870–71).
Balloons were first deployed by the British Army's Royal Engineers during the expeditions to Bechuanaland and Suakin in 1885. They were also deployed during the Second Boer War (1899–1902), where they were used in artillery observation at the Battle of Magersfontein and during the Siege of Ladysmith.
World War I was the highpoint for the military use of observation balloons. The British, despite their experience in late 1800s Africa, were behind developments, using spherical balloons. These were quickly replaced by versions of first Italian and then French designs, which were flyable and could operate in more extreme weather conditions. During World War I, artillery had developed to the point where it was capable of engaging targets beyond the visual range of a ground-based observer. Positioning artillery observers at altitude on balloons allowed them to see targets at greater range than they could on the ground. This allowed the artillery to take advantage of its increased range. The balloons were deployed on land and at sea for use in
- Observing enemy troops
- Locating submarines
- Artillery spotting
The idiom "The balloon's going up!" as an expression for impending battle is derived from the very fact that an observation balloon's ascent likely signaled a preparatory bombardment for an offensive.
Because of their importance as observation platforms, balloons were heavily defended by anti-aircraft guns and patrolling fighter aircraft. Attacking a balloon was a risky venture, but some pilots relished the challenge. The most successful were known as balloon busters, including such notables as Belgium's Willy Coppens, Germany's Friedrich Ritter von Röth, America's Frank Luke, and the Frenchmen Léon Bourjade, Michel Coiffard, and Maurice Boyau.
World War I observation crews, were the first to use parachutes on a wide scale, long before they were adopted by fixed wing aircraft. These were a primitive parachute type where the main part was in a bag suspended from the balloon with the pilot only wearing a simple body harness around his waist which lines from the harness attached to the main parachute in the bag. When the balloon crew jumped the main part of the parachute was pulled from the bag, with the shroud lines first, followed by the main canopy. This type of parachute was first adopted on a large scale by the Germans, and then later by the British and French for their observation balloon crews. 
Observation balloons also were used in substantial numbers during World War II, particularly by the U.S. Navy for anti-submarine work.  Observation balloons also played a role during the Cold War. For example, Project Mogul used high-altitude observation balloons to monitor Soviet nuclear tests. They were used by US and coalition military forces in Iraq and currently in Afghanistan. They are tethered to outposts near bases with cameras attached to the underside.
- Balloon (aircraft)
- Barrage balloon
- Espionage balloon
- Kite types
- List of American Balloon Squadrons
- Surveillance aircraft
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- May 1931, Popular Mechanics photo of WW1 British Royal Navy observation balloon gondola with external bag parachutes first used by Germany and then later by both the UK and France
- Time http://www.time.com/time/video/player/0,32068,56385884001_1947348,00.html
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- Pincus, Walter (20 August 2009). "High-Tech Balloon to Help Forces Keep Watch". The Washington Post.
|Wikimedia Commons has media related to Military observation balloons.|
- Memoirs & Diaries - In a Kite Balloon at firstworldwar.com
- Royal Engineers Museum Early British Military Ballooning (1863)