Recent works shed light on how a relatively uncommon type of lightning came into being. These results confirm a hypothesis expressed for several decades, but which had never before been the subject of a precise theoretical evaluation.
The electric discharges that punctuate the life of cumulonimbus clouds unfold in forms as diverse as they are varied. Sometimes, they emanate from prominent structures (radio antenna, tower, wind turbine or even roughness of natural relief) and develop in the direction of the base of the storm cloud. Also, their outline draws a very characteristic physiognomy, a little as if the sky and the earth had exchanged their places.
When lightning gives birth to other lightning
These ascending flashes, as the researchers call them, are much less well understood than those more usual from the cloud to the ground, in particular with regard to the mechanism of initiation. As such, in a study recently published in the Journal of Geophysical Research: Atmospheres, a group of scientists looked into the issue.
Their work focused on the formation of ascending lightning bolts that occur shortly after the onset of a first lightning strike. More precisely, it shows that the appearance of this initial discharge greatly increases the ambient electric field. Above all, it is able to increase its intensity tenfold near elevated structures. The precise repercussion on the electric field varies according to the proximity and the intensity of the discharge.
Overcome air breakdown voltage
Although the phenomenon lasts only a fraction of a second, it is sufficient to lead to the air breakdown threshold being exceeded, the moment when the latter loses its insulating capacity and becomes a conductor. In short, it is able to cause the initiation of an ascending flash from an asperity. This initiation can just as easily be triggered by the tracers of the initial lightning as by its back arc. In addition, the model used by the researchers shows that the trigger may arise structures only a few tens of meters high.
” This study confirms theoretical experimental observations and provides new information on the initiation mechanisms of ascending lightning from elevated structures. », Indicates the paper in its summary. It remains to be seen why certain ascending discharges occur without any flash having preceded. If there is one certainty about lightning, it is because it is far from having revealed all its mysteries to us!