Lightning FAQ: Why do some lightning bolts have loops and knots in them?
Sometimes a lightning channel will have the illusion of 'looping back' on itself or having bright knots in it. The reason behind this phenomenon is quite simple.
Lightning is three-dimensional- it 'zigs' and 'zags' in all directions. Most of the time, you will be viewing a lightning channel generally from the side, and it will appear to travel in one general direction only. However, if that lightning bolt (or part of the lightning bolt) is coming straight at you (that is, you are looking at it from one end), it will appear to loop back on itself, sometimes even appearing go back upwards.
A good example of this phenomenon is this cloud-to-ground lightning discharge in Charleston, West Virginia:
To better visualize what is happening with a 'looped' lightning bolt like the one pictured above, take a metal coathanger and straighten it out.
Then, 'create' a model of a lightning channel by bending it in various
places in different directions, like this:
Now, take your 'coathanger model' of lightning and look at it from one
end, like this (be careful not to poke your eye!):
It appears that the coathanger loops around in some spots, but only
because of the way you are viewing it from the end. This is exactly
why lightning sometimes appears to loop - it's because you're looking at it
straight-on rather than from the side.
This diagram shows how part of a cloud-to-ground lightning flash can
appear to loop around in one spot:
Drawing (A) shows a side view of the observer (you)
viewing the lightning flash. Drawing (B) shows
how the same bolt would look to you from the vantage point in drawing
(A). The loop is simply an illusion created by
looking at the semi-horizontal section of the lightning bolt straight-on.
The bright knots in a lightning channel you sometimes see in lightning photographs are the result
of the same phenomenon- they are just smaller, tighter 'loops'. It
appears brighter because there is more than one section of lightning
channel 'overlapping' in the camera's frame.
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