Posts Tagged +CG
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.