It is well known that thunderstorms drive electrical current upwards from clouds into the ionosphere - the electrically charged layer of the upper atmosphere that starts at altitudes of about 90 km - and that charge drifts downwards through the atmosphere during fine weather. But scientists have long suspected that other effects may help to maintain the potential difference of about 300 000 volts between the Earth’s surface and the ionosphere.

During a thunderstorm in the South China Sea in July 2002, Su and co-workers used low-light-level cameras to photograph the clouds every 17 milliseconds. The five jets they observed - dubbed carrot-jets or tree-jets according to their shapes - were visible for some tens of milliseconds. But crucially, the team also detected simultaneous bursts of radio waves in four of the five cases, which indicates that the jets had transferred significant amounts of charge. The thunderclouds were at an altitude of 16 km.

Such electromagnetic bursts have only previously been linked with powerful lightning strikes, which are known to transfer large quantities of charge. But Su and colleagues do not believe that lightning triggered the radio waves they detected, since the local lightning detection network registered no strikes at the times of the jets.

Similar atmospheric events known as blue jets have previously been observed leaping up to 70 km from thunderclouds - high enough to reach the ionosphere. But since no electromagnetic emissions have been associated with blue jets, scientists do not believe that they are significant carriers of charge in the so-called global electric circuit.