. Weather balloon. Meteorologist launching a weather balloon. The balloon, which is made of rubber and filled with hydrogen, carries aloft a radiosonde (in hand at left). This is a group of instruments for measurement and radio transmission of meteorological data, including temperature, pressure and humidity. As the balloon gains altitude, and the local pressure drops, the balloon expands. Eventually, at around 35,000 metres, the balloon bursts and the radiosonde is returned to Earth by a small parachute. Photographed at Salisbury Plain, Wiltshire, UK. MODEL RELEASED