A Civil War observation balloon was an innovative reconnaissance tool used primarily by the Union Army to gather intelligence during the American Civil War (1861–1865). These balloons, made of lightweight silk or cotton coated with varnish, were filled with hydrogen gas to achieve lift. Tethered to the ground by ropes, they carried observers in a small basket to heights of up to 1,000 feet, providing a vantage point for monitoring enemy movements, directing artillery fire, and mapping terrain. Operated by the Union Army Balloon Corps, led by Thaddeus Lowe, observation balloons were an early example of aerial surveillance, though their use was limited by weather conditions and the availability of hydrogen.