Archive for May, 2017
Insight gained from the analysis of high-speed camera observations and correlated electric field measurements has allowed for lightning leader polarity classification in some standard-speed video and still image recordings. To date, recoil leaders appear to be solely associated with positive leader development and therefore provide a unique signature that can be identified in standard-speed video recordings (60 ips). A majority of recoil leaders that form on positive leader branches tend to fade/decay without connecting to a main luminous channel, and their bipolar/bidirectional development can only be seen at recording rates greater than 5,000 ips. Even though their duration is typically less than 500 µs, their intense brightness will record well on standard-speed video camera sensors. During a single standard-speed video image exposure of 17 ms, numerous recoil leaders may form. If any of the recoil leaders that form during the long exposure do not connect with a main luminous channel their integrated luminosity traces will appear detached from a main channel. In essence, they appear as floating leader segments. Furthermore, the positive end of the recoil leaders, upon arrival at the positive leader tip, tend to illuminate a short forked segment. This forked segment also records clearly on standard-speed exposures and occasionally digital still images.
The video segment below shows the development of an upward positive leader recorded at 7,207 ips with a high-speed camera as well as with a standard-speed video camera (60 ips). The high-speed recording resulted in 135 µs exposures (139 µs image intervals) and 17 ms exposures for the standard-speed recording. A total of 122 high-speed images were recorded during each standard-speed video exposure. The standard-speed video image is, therefore, an integration of the activity recorded by the high-speed camera during the 17 ms exposure. Annotations on the standard-speed video show the features that identify the leader as positive due to the recoil leader production.
The following is an integrated high-speed video segment that corresponds in time to a single standard-speed video image from the previously shown upward flash. The detached recoil leaders are clearly visible in both images.
Here are more standard-speed video images showing recoil leader development during upward positive leader propagation.
The decreased sensitivity of digital still camera sensors compared to video sensors and the longer exposure times used at night (i.e., 20 s) results in recoil leaders recording as faint leader segments. Below is a video showing positive leader development captured at 1,000 ips. Three different positive leaders of differing intensity show the spectrum of behavior modes exhibited by positive leaders. The weak positive leader (top) was weakly luminous, highly branched and produced numerous recoil leaders. The middle positive leader was brighter and only branched a few times near the end of the recording and produced fewer recoil leaders. The bright positive leader branch at the bottom did not branch and did not produce any recoil leaders.
The image of this event below shows how the spectrum of positive leader development appears when captured by a digital still camera. The image was captured using ISO 100, f/6.3 and a 20 s long exposure. Although the recoil leaders where intensely bright in the high-speed video, their short duration and the decreased ISO sensitivity of the digital still camera results in them appearing faint in the upper portion of the image. The non-branched lower leader channel remained brightly luminous during its entire development and this recorded as a brightly luminous leader on the still image.
Below are additional examples of positive leader development associated with +CG flashes as captured by digital still camera. The recoil leader producing positive leader branches are the primary indicator of leader positive polarity.
Negative leaders do not exhibit similar recoil leader behavior as shown in a related post.
On May 13th, my daughter and I went out to chase storms that were forming over the Black Hills. A nice cluster of storms moved over Sturgis, South Dakota (home of the Sturgis Motorcycle Rally), and we filmed some close flashes as the storms passed over us. We then followed the cluster toward Bear Butte which is an isolated uplifted hill on the east side of the Black Hills, northeast of Sturgis.
Our primary target decayed and so we focused on new storms that had formed over the Black Hills and were moving directly toward us. They put down some nice CGs, and as they reached us, I repositioned to have Bear Butte in my field of view. A few minutes later we were treated to two spectacular CG lightning flashes directly in front of us and close. They were very bright and very loud. I suspected they were +CGs given their long duration continuing current and exceptional brightness. The Black Hills area and Northern High Plains for that matter exhibits an atypically high percentage of +CG flashes, and trying to understand and explain this anomoly was part of a study I was involved in during the UPLIGHTS research campaign.
For the first flash, I had my infrared triggered cameras set to f/8 and ISO100 in aperture priority mode. Although this setting is ideal for the average CG flash between 5-15 km, the LCD image review showed significant saturation. I reset the aperture to f/11 and the second flash was still somewhat saturated.
Below is the image for the first flash. You will notice there is two CG channels, one in front of Bear Butte and one beyond.
National Lightning Detection Data provided by Vaisala, Inc. indicated the closer CG was in fact positive (electrons traveled upward along the channel) with an impressive 159.6 kA estimated peak current. It struck 2.5 km away. NLDN data indicated the second channel was also a positive CG 12.6 km away and had an estimated peak current of 58.4 kA.
The second flash which is shown below only had one CG termination point.
NLDN data indicated it was a +CG, 2.2 km away with a peak current of 143.1 kA.
Positive CG flashes tend to exhibit higher peak current compared to negative CGs on average and usually do not have multiple return strokes. If my memory serves, I believe the latest published scientific literature has the average peak current for -CGs around 30 kA and 50 kA for +CGs. So these flashes were exceptionally strong. Unlike what we were taught in school, they DO NOT always originate from the top of a thunderstorm or anvil area and DO NOT always strike away from the main storm and rain area. It all depends on where the charge regions form, and in the Northern High Plains, we see a lot of storms with inverted charge regions, which leads to more +CGs. In the near future, I will be adding an education section on my blog which explains this in more detail.
Below is video of the two flashes captured on a Panasonic HPX-170 at 1280x720p60 which uses a global shutter (no annoying rolling shutter artifacts). In the slow playback you will see an artifact on the frame preceding the return stroke. This is saturating brightness bleed over from the subsequent return stroke that occurs in the following frame. After the CCD records a frame, the voltage values from each photosite (which corresponds to each pixel in the image) are shifted to an adjacent storage photosite that is covered. The voltage is then read out from the covered storage photosites while the next exposure is taking place in the non-covered photosites. If the non-covered photosites experience a saturating brightness, some of the voltage can bleed over into the adjacent storage photosites during their readout adding a voltage increase to their recorded values. Since the covered photosites are readout row by row with the data shifting up the CCD array to higher covered photosites after each row is read, the artifact will usually show up lower in the image as the “image data” from the previous frame has moved up when the saturating brightness occurs. These artifacts are often misidentified as attempted leaders that occur close to the camera, when in fact they are only “ghost images” of the bright return stroke channel that occurs in the subsequent frame but shows up on the previous frame (forward in time…que Twilight Zone music.)
You will also notice the integrated recoil leader activity associated with descending positive leaders in the distant second CG during the first flash. This integrated recoil leader activity is a clear identifying characteristic of positive leaders, and I explain this in the previous post.
Below are some additional images from flashes we captured before the storm moved over Bear Butte.