Archive for October, 2022
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.