Tethered weather balloons are filled with helium, a light weight, inert gas, which enables them to rise to altitudes of up to 30 km. At heights greater than that the gas expands to such an extent that there is a risk balloon will burst. Weather balloons carry numerous instruments aloft with them to measure temperature, atmospheric pressure, humidity and even the composition of gases in the atmosphere at different altitudes. A radio module transmits the data back to meteorologists on the ground, who then use the information to draw up a vertical profile of the atmosphere. The data is not only useful to weather forecasters, it also allows scientists to better understand the atmosphere, and particularly the effects that humans may be having on factors such as ozone concentrations and the build-up of so-called green house gases. Once all the data has been collected, the weather balloon can be pulled back to Earth using its anchor line.
A weather balloon is prepared for its ascent at a weather station on the Norwegian island of Spitsbergen.