Twice a day, about 900 weather stations around the world launch balloons in a coordinated international programme to measure conditions in the upper atmosphere.
These weather balloons may reach an altitude of 115,000ft (35,000 metres) and can drift more than 186 miles (300km) from their launch point in a journey lasting up to two hours. Similar balloons have been taking soundings, as they are known, since the 1930s.
Each helium-filled balloon is about a metre and a half in diameter on the ground but gets larger as it rises and the outside pressure decreases. A radiosonde, a package of meteorological instruments, is suspended 20 metres below the balloon.
The radiosonde sends back readings of temperature, pressure and humidity at different altitudes. Meteorologists may also track the location of the balloon to determine wind speed and direction from its movement. When it reaches maximum altitude the balloon, now about 6 metres across, bursts and the radiosonde parachutes harmlessly back to earth.
While people on the ground occasionally mistake weather balloons for UFOs, pilots and air traffic controllers are usually quite familiar with them. And, especially if the instruments are recovered, distinguishing an innocent short-range weather balloon from an intercontinental stratospheric spy with a solar-powered payload the size of a passenger jet is generally not too difficult.
