London, March 24 : Scientists have claimed that through a new computer model, they have finally worked out how lightning forms, how it escapes the storm cloud that fosters it - and whether it will form a "bolt from the blue".
According to a report in the New Scientist, the theory is the first to explain all of the different types of lightning that exist - from regular cloud-to-ground lightning, to gigantic "jet lightning" that escapes up through the top of clouds, to "bolts from the blue", which can strike the ground under blue skies, several kilometers away from a thunderstorm. Previous theories had difficulty explaining why lightning would ever escape a cloud," said Jeremy Riousset of Penn State University, US.
Scientists knew that lightning forms between layers of positive and negative charges within a cloud. But, this could not explain how lightning ever made it out of the cloud.
"According to those theories, all lightning should remain trapped between the charged layers, inside clouds," said Riousset. "We had suspicions about why it would go towards the ground, but we had no idea why it would go upwards."
Riousset's model shows that when the layers of positive and negative charges inside a cloud are redistributed, this can create the conditions necessary for lightning to escape by amplifying the difference in potential between the layers.
The direction in which it escapes depends on how the charges were initially distributed inside the cloud, their magnitude, and the weird fact that charges move upwards more readily than they move towards the ground, according to the report.
The computer model estimated that intra-cloud lightning moves charges around inside the cloud, and cloud-to-ground lightning removes charges by channelling them down to the ground. Both of these disturb the charges further, creating the conditions for lightning to escape upwards.
A lightning bolt can even be re-directed from its original path, as happens with bolts-from-the-blue.
These massive upwards jets get re-routed by charges lining the sides of the cloud. The lightning ends up exiting the cloud sideways and can hit the ground more than 40 kilometres away from a storm.
"Lightning mapping shows that bolts-from-the-blue are surprisingly common," said Riousset.
According to Riousset, the computer model he and his colleagues have developed can be used to predict what type of lightning a storm cloud is likely to generate. This means they can say if the lightening will escape upwards, hit the ground beneath the cloud, or strike Earth miles away from the storm.