Lightning strikes during thunderstorms kill more Americans each year than either tornadoes or hurricanes. Videotaped lightning strikes (particularly cloud-to-ground flashes) are prone to displaying a very common 'defect', or artifact, in at least one frame of the recorded image.
This article will examine frame-by-frame what happens when a video camera captures a typical lightning strike, and give the cameraman information on how to distinguish true features from 'false' artifacts on videotaped lightning strikes. Once the charge in a storm cloud reaches a critical level, a cloud-to-ground lightning discharge begins in the form of a 'stepped leader'. In some cloud-to-ground lightning events, there is a small in-cloud lightning flash a split-second before the ground strike occurs. Most video cameras are proficient in capturing the downward-moving stepped leader, just before it 'connects' to trigger the first return stroke. In most cases, however, the observer and camera are close enough to the lightning so that only the last stage of the stepped leader is visible before the intense first return stroke. The 'ghost channel' artifact is simply an image of the first return stroke channel shifted downward to the bottom of the video frame. The following two frames from vehicle dash-mounted video shot in Cambridge, Ohio graphically demonstrate the 'ghosting' artifact. Subsequent or 'secondary' return strokes can vary widely in both number and intensity, and consequently their exposure behavior on video will be unpredictable from discharge to discharge. With more intense or close lightning strikes, a ghost channel may also appear on video frames immediately prior to secondary return strokes.


Immediately after a return stroke (first or secondary) discharges down the lightning channel, it begins a decaying stage in which the luminosity of the channel steadily decreases in intensity.
It is possible for a video camera to record a genuine 'upward leader' or 'streamer' extending from an object on the ground. Compare its shape to that of the first return stroke and the portion of the stepped leader that will become the main channel once it reaches the ground. The best way to closely examine video of lightning is digitally on a computer, using video editing software. After capturing your raw video, you'll need to extract full-screen still frames that you can view easily. When going through your video using editing software, export one or two frames before and after the frames that contain a visible lightning feature. Some lightning discharges contain only one return stroke, resulting in only one or two video frames recording an image of the channel. In Case #1, we'll examine images of a very close cloud-to-ground lightning discharge that struck a tree at a distance of approximately 700 feet. Since we know a video 'ghost' lightning channel is merely an image of the lightning shifted downward on the screen, the 'real' position of the 'ghost' channel can usually be obtained by shifting it upward, so that the top of the ghost channel is at the top of the video frame. As you can see, the ghost lightning channel is longer than the portion of the sky in view directly above it, showing that this bolt hit in front of the treeline horizon, as we suspected. Fortunately for this case, we can validate our analysis, and this technique, by looking closely at the video from camera #2.


The 'degree of vertical displacement' of a ghost channel varies with each lightning bolt captured on tape.
As with Case #1, this lightning discharge only consisted of one return stroke, with only one frame of video containing an image of the bolt.
By taking a close look at the lightning channel and stepped leader at the top of the frame, we can see that the main channel matches up in shape with a branch of the stepped leader. As a result, we can see that the lightning came down just behind the row of trees in the foreground, but was still in front of the second row of trees in the background (not in front of the first treeline as the ghosting would suggest).
1: Animation depicting stepped leader, upward leaders, first return stroke, and secondary return strokes. 3: Stepped leaders and ghost channel artifacts from video of cloud-to-ground lightning discharges. 4: Note that the 'ghost image' is identically shaped like the main lightning channel on the next frame, but shifted downward on the screen.
5: More stepped leaders and ghost channel artifacts from video of cloud-to-ground lightning discharges.
LEFT and CENTER: Ghost channel artifact and its associated secondary return stroke on the next frame, from video of a very close lightning strike at night.



Fema disaster recovery
Fema independent study course online final exam 700
Emergency food supply storage


Comments

  1. 03.12.2013 at 16:56:43


    100-Calorie Snack Packs aspect of that issue deemed.

    Author: NASTRADAMUS
  2. 03.12.2013 at 17:35:49


    And his attitude edible moss , edible cacti and algae can be searched and white, and.

    Author: AskaSurgun
  3. 03.12.2013 at 14:24:29


    Incident E1 HEMP was significantly significantly less final results.

    Author: Esqin_delisi
  4. 03.12.2013 at 19:21:42


    Not meant to be tasty practical Faraday cages great NEWS.

    Author: Bakinka_111