Archive for category Lightning Science
On September 17th, I got a chance to see the world’s largest Tesla coil in operation at the Tesla Science Center at Wardenclyffe which is located in Shoreham, New York. As part of a belated birthday celebration of Nikola Tesla’s 166th birthday, which was on July 10th, Greg Leyh (@LightningOD on Twitter) operated his 40 ft tall Tesla coil in a spectacular and educational demonstration. The Tesla coil is a 1/3rd scale prototype for his endeavor to build two 121 ft tall Tesla coils. At his website, Lightning On Demand, you can read about the science behind this project and the objectives he hopes to achieve.
During the demonstration, Greg first had selected members of the audience hold onto fluorescent bulbs. He slowly raised the surrounding electric field by activating the coil and soon the bulbs lit up in their hands. Next he operated his own “Original Tesla Roadster” which used the invisible electric field to power a motor onboard the small “go kart sized” three wheeled buggy.
Turning things up a notch, Greg then demonstrated “Saint Elmo’s Fire” which is a cold corona discharge that occurs on pointed objects when the electric field reaches a certain breakdown threshold. These faint blue/purple arcs of light are cold streamers formed when that air ionizes without enough energy to cause significant thermal energy from kinetic collisions. The light comes from emissions during molecular and atom recombination or primarily nitrogen and oxygen after ionization or excitation to higher energy states.
He then demonstrated how these static discharges can ignite gasoline but not diesel fuel, followed by an impressive ignition of gun powder and hydrogen filled balloons.
Turning things up once again, Greg increased the voltage output of the Tesla coil and brilliant arcs finally began leaping from the Tesla coil itself as the air broke down under the electric stress produced by the tuned oscillators and coils. He zapped a long piece of wood which burst into flames as the power increased. His assistants then hoisted a human wood cutout holding an umbrella that had a covering of metal mesh spread out across its top. The Tesla coil struck the metal mesh which acted as a Faraday cage protecting the wood cutout below. After the metal mesh was removed, the arc did not hesitate to propagate down to the wood cutout burning a scar with ease.
I first met Greg in 2008 when he asked me if I could film his then smaller Tesla coils using my high-speed camera. I jumped at the opportunity and filmed them in action in San Francisco at recording speeds up to 66,000 images per second. The high-speed recordings, timelapes and integrated image stacks are below.
I learned then that Greg likes to run things until they break, and he did just that with his biggest coil in San Francisco. I suspected he would do the same at the Tesla Science Center, and sure enough he kept increasing the power to see what happens. Arcs shot up to the sky and down to the ground to the delight of all.
My research on upward lightning has involved human-made tall objects such as communication towers, tall buildings and wind turbines. However, upward lightning from natural rock formations is also possible as the conditions that allow for upward lightning require an object that enhances the electric field locally due to its shape. Any object that protrudes above it surroundings can enhance the electric field and this is the basic concept that allows for air terminals in lightning protection systems to serve as more favorable attachment points for descending lightning leaders.
However, in the case of upward lightning, in which a leader initiates and travels upward toward the overlying thunderstorm charge region, studies have shown that tall objects have to be of suitable size and shape to initiate the leader. These studies indicate that objects require an “equivalent height” of at least 100 m (~300 ft) or in the case of wind turbines, need to be rotating, in order to experience upward lightning. Equivalent height takes into account the object’s shape and nearby ground topography as a narrow object (e.g., communications tower) can enhance the electric field more so than a broader object of the same height such as a much wider building or a sloping mountain. However, a small narrow tower on top of a sloping mountain can have a much greater equivalent height than the height of just the small tower considered separately.
In addition, upward lightning can be triggered by nearby lightning activity in which a component of a triggering flash causes a rapid electric field change over the tall object resulting in the initiation of an upward leader. Alternatively, upward lightning can be self-initiated in which an upward leader initiates from the tall object without a preceding triggering flash. In this case, the strength of the electric field and possibly the removal of corona space charge by the wind allows for the initiation of the upward leader. If there are multiple tall objects, self-initiation upward lightning tends to occur from only one of the tall objects, whereas lightning-triggered upward lightning flashes can see multiple tall objects initiate upward leaders during the same flash.
I have not investigated upward lightning from natural rock formations as it seems to occur much less frequently based on the amount of images and videos captured of such events. Communication towers and tall buildings can be prolific upward lightning producers so we focus our research on those.
Paul M. Smith (@PaulMSmithPhoto on Twitter), who is an incredibly skilled transient luminous event photographer, shared an image posted on Instagram that showed apparent upward lightning from some rock formations in Canyonlands National Park in Utah. The image was captured at Mesa Arch, a beautiful arch that provides a wonderful framed sunrise canvas opportunity. The photographer was Chris Markes (@chrismarkes on Instagram). Because I had previously seen so few upward flashes from rock formations and the fact that these formations were in a canyon, which would be less favorable for electric field enhancement, I was skeptical of the validity of the image at first. However, Paul reached out to Chris and was able to obtain an approximate time of the event. With that information, I asked Chris Vagasky (@COweatherman on Twitter) who works for Vaisala, Inc. (@VaisalaGroup on Twitter) which operates the National Lightning Detection Network if he could see if there were any lightning events recorded. The data that he found strongly suggests that what Chris Markes recorded was in fact a lightning-triggered upward lightning flash that initiated upward positive leaders from at least three rock formations.
The data showed nine lightning events beginning at 005509 UT on 3 Oct 2022 (1855 MDT on 2 Oct 2022) and lasting just under 0.7 seconds. The first was a very strong 108.6 kA estimated peak current positive cloud ground return stroke 16 km south of the Airport Tower rock formation. This was followed by a -5.2 kA intracloud event close to the location of the return stroke. 326 ms after the +CG return stroke the first of seven small peak current negative intracloud events were recorded by the NLDN. All of these were less than -10 kA estimated peak current and all grouped within 400 m of Airport Tower as measured using Google Maps.
In the image captured by Chris, Airport Tower is the formation in which the middle lightning channel is attached and is 4.9 km from Mesa Arch. Monster Tower, to the right of Airport Rock in the image with the rightmost lightning channel attached is 2.3 km from Mesa Arch.
Based on the image and the NLDN data, I believe the nearby +CG flash caused the initiation of upward positive leaders from the rock formations, and in the case of the upward leader from Airport Tower, some of the upward positive leader branches decayed and subsequently formed recoil leaders in which the negative ends traveled back down the channel resulting in subsequent return strokes (if the main channel had also decayed) or impulsive m-components (if the main channel was still active). In the latter case, the negative end of the recoil leader would connect to the main channel at the branch point causing and impulsive current increase in the luminous main channel and branch. Below is an example of this process recorded by high-speed camera.
Our research has shown that the recoil leader generation and attachment process on decayed upward positive leader branches tend to register as low peak current negative intracloud events by the NLDN.
Regarding the topography and its potential to enhance the electric field locally, Airport Tower is 223 m taller than its lowest measured contour that encircles it (prominence height) and Monster Tower has a prominence height of 120 m. This suggests that they both have effective heights greater than 100 m as the canyon floor extends 8 km east and nearly 5 km to the south.
I would like to thank Chris Markes for allowing me to repost his image on my blog, Paul Smith for showing me Chris’ post as well as Chris Vagasky for sharing the lightning data.
Lightning Safety Awareness Week runs from Sunday, June 19th through Saturday, June 25th. There are many sites to learn about lightning safety and myths related to lightning. If you wish to learn more about the physics and behavior of lightning you can read through my education section.
This year I was able to capture upward lightning flashes from a newly installed wind turbine complex northeast of Newell, South Dakota as well as lightning associated with the monsoon season in the Colorado Plateau. There were also some spectacular lightning displays in the my home area in the Northern High Plains. Below are some of the images captured.
Standard- and high-speed video highlights from the 2021 storm season are now available on my YouTube channel.
Captured a close negative ground flash while driving near Guernsey, Wyoming on 19 May 2020. You can hear the thunder on the dash cam in about a second after the flash. Recorded with the Phantom M321S at 1,500 images per second. In the frame prior to the return stroke, there appears to be a dim connection to the downward leader and ground. This is a camera artifact due to the saturating bright return stroke recorded in the following image (frame). The brightness “bleeds” over into the previous frame making it appear there is a connection or upward connecting leader present when in fact it is not.
Just as we have documented positive leaders developing from negative leader channels, we have also observed and analyzed negative leaders develop from positive leader channels. However, the physical process is noticeably different as it involves the development of recoil leaders in decayed positive leaders. Negative cloud-to-ground return strokes can occur during the horizontal propagation of positive leaders when the positive leaders decay and become cutoff from their original negative ends. A recoil leaders that develops along the positive leader channel paths can have its negative ends “veer off” the previously ionized channel and travel to ground via negative breakdown through virgin air causing a negative return stroke. The growing positive leader that follows the return stroke frequently decays with additional recoil leaders forming. The negative end of subsequent recoil leaders travel down the newly established channel path to ground, since it is more conductive due to its more recent ionization, causing additional negative return strokes resulting in repeated extension and growth of the horizontal positive leader end.
It is sometimes possible to recognize this type of flash solely from digital still imagery due to the geometry and shape captured during a single exposure. Below is a digital still image of the flash shown in the video above. The negative leader development that traveled to ground from the decayed positive leader channel displays recognizable negative leader patterns (erratic direction change and branching) and the brightness of the return stroke illuminates the channel back to the positive leader end which is in the left portion of the image. Notice the left curve where the negative leader return stroke channel joins the positive leader channel. If the downward negative leader was simply a branch of the initial horizontal propagating negative leader there would have been a right curve in the bright channel segment that traveled back in the direction from which the leader initially propagated (to the right).
We know from observation and analysis of horizontally extensive lightning flashes that often negative leaders travel horizontally through a layered positive charge region that spans large areas. We frequently observe that positive cloud-to-ground return strokes occur along the path the negative leaders travel but in trail of the negative leader tips. Current thinking is that the negative leaders become cutoff from their original positive ends and then develop new positive leader ends that propagate downward to the ground and cause a +CG return stroke that then further extend the negative leaders. Although we have frequently documented the positive leaders growing toward ground after negative leaders propagate in cloud, due to the clouds, we rarely are able to see the positive leader development initially take place from the previously formed negative leader channel. This video contains three cases where we see the negative leader channel from which a new positive leader develops, propagates to ground and causes a +CG return stroke that travels toward the end of the negative leaders, thus furthering their propagation. What is interesting and has yet to be understood is how the positive leader seems to develop from a still luminous negative leader channel segment. The luminosity in the negative leader channel suggests it is still actively carrying current and not completely cutoff. Therefore, we need to determine through further research the mechanism by which a positive leader is able to form and develop from this luminous channel. This behavior was first documented and described in a paper by Saba et al., 2009 using high-speed camera imagery.
Saba, M. M. F., L. Z. S. Campos, E. P. Krider, and O. Pinto Jr. (2009), High-speed video observations of positive ground flashes produced by intracloud lightning, Geophys. Res. Lett., 36, L12811, doi:10.1029/2009GL038791.
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.