US sailors launching a radiosonde at a military airport in 1943 during World War 2. The radiosonde consists of a battery-powered instrument package "(white box)" carried aloft with a helium-filled weather balloon. As it ascends it measures temperature, humidity, and air pressure and radios the information to a ground receiver. At an altitude of around 70,000 ft (23 km) the balloon pops, and the radiosonde floats back to earth suspended by the the red parachute visible on the support line. By WW2 the US Weather Bureau launched weather balloons daily from 80 sites around the US