Barrage balloon A barrage balloon is a large balloon tethered with metal cables , used to defend against low - level aircraft attack by damaging the aircraft on collision with the cables , or at least making the attacker ' s approach more difficult . Some versions carried small explosive charges that would be pulled up against the aircraft to ensure its destruction . Barrage balloons were only employed against low - flying aircraft ; the weight of the longer cable made them impractical for higher altitudes . World War II . In 1938 the British Balloon Command was established to protect cities and key targets such as industrial areas , ports and harbours . Balloons were intended to defend against dive bomber s flying at heights up to 5 , 000 feet ( 1 , 500 m ) , forcing them to fly higher and into the range of concentrated anti - aircraft fire — anti - aircraft guns could not traverse fast enough to attack aircraft flying at low altitude and high speed . By the middle of 1940 there were 1 , 400 balloons , a third of them over the London area . While dive - bombers were devastatingly effective against undefended targets , such as Guernica and Rotterdam , they were very vulnerable to attack by fighter aircraft , and their use by Germany against Britain with its effective Royal Air Force was rapidly discontinued . Balloons proved to be of little use against the German high - level bombers with which the dive - bombers were replaced , but continued to be manufactured nonetheless , until there were almost 3 , 000 in 1944 . They proved to be mildly effective against the V - 1 flying bomb , which usually flew at 2 , 000 feet ( 600 m ) or lower but had wire - cutters on its wings to counter balloons . 231 V - 1s are officially claimed to have been destroyed by balloons . Many bombers were equipped with devices to cut barrage balloon cables . Britain used large numbers of balloons , so Germany developed the most capable cable - cutters . Their systems consisted of small C - shaped devices attached to the leading edge of the wing . When a cable entered the device after sliding down the wing , it triggered a small explosive charge that drove a blade through the cable . British bombers were also equipped with cable - cutters although the Germans used few barrage balloons . The British added two refinements to their balloons , " Double Parachute Link " ( DPL ) and " Double Parachute / Ripping " ( DP / R ) . The former was triggered by the shock of an enemy bomber snagging the cable , causing that section of cable to be explosively released complete with parachutes at either end ; the combined weight and drag bringing down the aircraft . The latter was intended to render the balloon safe if it broke free accidentally . The heavy mooring cable would separate at the balloon and fall to the ground under a parachute ; at the same time a panel would be ripped away from the balloon causing it to deflate and fall independently to the ground . Disadvantages . Balloons were sometimes more trouble than they were worth . In 1942 Canadian and American forces began joint operations to protect the sensitive locks and shipping channel at Sault Ste . Marie along their common border among the Great Lakes against possible air attack . During severe storms in August and October 1942 some barrage balloons broke loose , and the trailing cables short - circuit ed power lines , causing serious disruption to mining and manufacturing . In particular , the metals production vital to the war effort was disrupted . Canadian military historical records indicate that " The October incident , the most serious , caused an estimated loss of 400 tons of steel and 10 tons of ferro - alloys . " Following these incidents , new procedures were put in place , which included stowing the balloons during the winter months , with regular deployment exercises and a standby team on alert to deploy the balloons in case of attack . The idea of free - floating balloons was also used in Operation Outward . Nuclear weapon tests . An unforeseen use for surplus barrage balloons was as tethered shot balloons for nuclear weapon tests , throughout most of the period when nuclear weapons were tested in the atmosphere . The weapon or " shot " was carried aloft to the required altitude slung underneath the barrage balloon , allowing test shots at much higher altitudes than tower shots , in controlled conditions . Several of the shots in the Operation Plumbob series were carried out using barrage balloons to carry the devices .