A very large balloon flown above towns, cities, factories, gun emplacements and other important sites, to prevent attacks by low-flying aircraft. The balloons were tethered by cables, so that they could be winched up or down. Often they had netting hanging down. They were flown in groups
A barrage balloon is a large balloon tethered with metal cables, used to defend against bombardment by aircraft by damaging the aircraft on collision with the cables. Some versions carried small explosive charges that would be pulled up against the aircraft to ensure its destruction. Barrage balloons were only regularly employed against low-flying aircraft, the weight of a longer cable making them impractical for higher altitudes.