Lightning-Triggered Upward Lightning in the Canyonlands?

Upward positive leader channels from Airport Tower (middle channel) and Monster Tower (right). Notice the upward branching indicating that the leader initiated from the rock and propagated upward.

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.

Location of a +CG return stroke (southern-most measured point) relative to Airport Tower rock formation.

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.

Mesa Arch (left) relative to Airport Tower (right)

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.

, , ,

%d bloggers like this: