Archive for April, 2020
This is one of the best positive cloud-to-ground flashes that I have filmed. When you watch the video remember that lightning leaders grow as bidirectionally with a positive and negative end. We see the positive leaders of this flash below cloud base and the negative end of the leader network is higher up in the clouds and therefore not visible. There are two sets of positive leaders to focus on. The farther leaders are on the left descending to ground and the right positive leaders closer to the camera spread out horizontally along cloud base. Once the far positive leaders reach ground a return stroke occurs. Once the return stroke traverses the leader network, the connected channel grows as an upward propagating negative leader higher up in the storm. The closer leaders also have a negative end that is growing unseen in the upper part of the storm but these leaders do not connect with ground and continue to spread out horizontally. Frequently, some of the positive leader branches become cutoff and develop fast moving bidirectional recoil leaders that attempt to reionize the decayed positive leader branches. The negative end of the recoil leaders travel toward the negative end of the flash by racing toward the place where the positive leaders emerged below cloud base. This continues for quite some time. You may consider this to be a hybrid flash with a ground flash component (farther) and an intracloud flash component (nearer) both raising negative charge upward toward a positive charge region. This flash was filmed at 5,600 images per second.
This complex negative ground flash captured at 7,200 images per second shows negative leaders, negative return strokes with different termination points as well as multiple return strokes in the same channel. It also shows how negative leaders can redevelop from a decayed negative channel branch point and extend the negative leader branch further. The final return stroke is caused by a recoil leader that initiates in the cloud at the positive end of the flash (not visible) with the negative end of the recoil leader traveling along the previous return stroke channel and causing a final negative return stroke.
Early in the morning of 26 June 2018 in southwest Kansas, something wonderful happened. A lightning flash occurred that caused additional lightning to rise up from 14 wind turbines filling the sky with blinding channels of light. Hank Schyma (an accomplished storm chaser, photographer/videographer and all around interesting guy also known as Pecos Hank) was there to witness this amazing spectacle and captured it on video. A huge mesoscale convective system had developed earlier in the evening, and he had positioned himself on the trailing side in hopes of capturing massive horizontally extensive lightning flashes that tend to develop in the trailing stratiform region. He was not disappointed. He witnessed numerous spectacular flashes and a number of these involved upward leaders developing from a wind turbine complex nearby. He reached out to me and other scientists to share his observations, and we were floored by what he captured.
I have been studying upward lightning flashes since 2004 primarily in Rapid City, South Dakota where there are 10 towers positioned along a ridgeline that runs through the middle of town.
In 2013, we participated in a project to observe upward lightning from a wind turbine farm in north central Kansas. We managed to capture a few events with one involving 4 wind turbines.
Our research, analysis and findings show that most upward flashes in the summer convective season are triggered by preceding nearby positive ground flashes and/or cloud flashes in which horizontally extensive negative leader activity passes nearby tall objects. The rapid electric field change from the negative leader activity or positive cloud-to-ground return stroke combined with the shape of the tall object, which enhances the electric field locally, results in the initiation or triggering of upward positive leaders from the objects.
I had always wondered just how many wind turbines could initiate upward leaders when triggered by a nearby flash. Hank’s capture showed that up to 14 wind turbines could initiate upward leaders in a single flash. As far as I know, this is the most that has been observed to date. This flash was truly a Perfect Upward Flash and followed the textbook on how preceding flashes can trigger upward leaders.
Hank’s video shows incloud brightening that propagates toward the camera and over the wind turbines. This is negative leader activity that frequently travels through layers of horizontal positive charge that build up in the trailing stratiform region of mesoscale convective complexes. Lightning develops as a bidirectional leader which ionizes the neutral air due to the strong electric field caused by charge regions within a thunderstorm. The bidirectional leader has a negative end that has a surplus of electrons and the a positive end with a deficit of electrons.
Often when the negative leaders travel a large distance, they tend to become cutoff from the other end of the leader. Due to the still present strong electric field, the cutoff segment, which is still conductive, can polarize and develop a new positive end resulting in new positive leader propagation and corresponding renewed negative leader growth. Frequently, the new positive leader end will travel to ground and connect causing a positive cloud-to-ground return stroke, and that is exactly what happened as recorded by Hank’s camera. Positive leaders propagate to ground on the right side of the video and connect to ground causing a return stroke. This return stroke, which involves an incredibly fast propagating region of rapid electron acceleration, heating and intense light emission, travels up the channel at about 1/3rd the speed of light and through to the negative end of the leader network that was overlying the wind turbines. The resulting electric field change causes positive leaders to initiate and grow from the highest of the wind turbine blades. These upward positive leaders travel upward driven by the newly modified electric field created by the return stroke.
To have so many upward positive leaders develop shows that the area covered by the triggering leader network and magnitude of the electric field change from the return stroke was very large influencing all the wind turbines nearly at once. It truly was a Perfect Upward Flash and something to behold.
I would like to thank Hank for sharing this video with me so I could share its explanation with all of you. He recently created an excellent video on How Lightning Works which you can see on his YouTube channel. It is definitely worth seeing and explains our latest scientific understanding of lightning using his amazing video and imagery.