Lightning bolts are synonymous with unbelievable speed. To understand how they form, scientists turned to super high-speed video and captured details in the microseconds before a lightning bolt appeared.
The details appear in a study published on February 1 in the journal Geophysical Research Letters. The high-speed video captured frames just 2.63 millionths of a second apart, Nicoletta Lanese reports for Live Science. The video shows the movement of electric charge stretching down from the sky and up from the ground. And for two frames, a thin thread of light connects the two. The observation is the first of its kind, and settles a controversy about lightning formation, Maria Temming reports for Science News.
During thunderstorms, unbalanced electric currents build up in the clouds and the ground, according to NASA. The air between them is really good at insulating the electric charge and keeping the negative charge in the louds separate from the positive charge in the ground. But as the saying goes, opposites attract. When the positive and negative charges become strong enough, they begin to stretch across the atmosphere toward each other.
The negative charge in the clouds reaches down in the shape of a stepped ladder, taking several zig-zagging steps down from the sky, per the National Weather Service. The “stepped ladder” charge, also called a leader, is visible in the researchers’ video. At the same time, a positively charged leader reaches up from the ground or through a path like a lightning rod.
Thin fingers of electricity called streamers branch out at the end of each of the leaders. Scientists knew that when the streamers join, the connection point between the leaders makes a sudden change to a channel of hot plasma, and the positive charge shoots into the sky in a flash of lightning. The new study illuminates the structure of that streamer connection.
“The target of the lightning strike is not determined at the beginning when it initiates from the cloud,” says atmospheric physicist Rubin Jiang of the Chinese Academy of Sciences Laboratory for Middle Atmosphere and Global Environment Observation in a statement. “The attachment process is the process that eventually determines the object that’s struck by the lightning flash.”
In 2017, Jiang and the research team pointed a high-speed camera at a 1,066-foot-tall meteorology tower in Beijing during a thunderstorm. The footage of a lightning bolt shows that when the positive and negative leaders came within 75 feet of each other, their streamers made a brief, thin connection. The thread of light appears in two frames, one slightly brighter than the other, before a lightning bolt lights up the sky.
Because the researchers captured the thin connection in two frames instead of one, they conclude that the lightning bolt formed after just one streamer connected the positive and negative currents. A streamer from each charge reaches across the gap like the fingertip connection in Michelangelo’s Creation of Adam.
A competing theory suggested that many streamers from each leader would have to connect, like a clasping hand, in order to spark a lightning bolt. But that would have looked wider and brighter on camera, the researchers say.
The photos mark the first time that the lightning bolt’s formation has been caught in more than one frame, making it the most definitive observation of lightning’s breakthrough phase.