In our last lecture we took a fairly detailed look at the
processes that occur during a negative cloud-to-ground discharge
leading up to the 1st return stroke.
Some cloud-to-ground discharges end at that point (you'll find some
statistics later in today's notes). I.e. they consist of (i) a
downward, negatively charged, stepped leader, (ii) a relatively short
upward connecting discharge, and (iii) an upward return stroke that
travels from the ground to the cloud at about 1/3 the speed of
Most cloud-to-ground discharges however contain multiple
strokes. A schematic illustration of a 4 stroke cloud-to-ground
is shown below.
The discharge begins at left in the figure with a branched stepped
leader that is followed
by a 1st return stroke. The
stepped leader takes a few 10s of milliseconds to travel from the main
negative charge center in the cloud to the ground. The ensuing 1st
return stroke travels back up the channel to the cloud is about 100
Then, with a few slight differences, the process repeats itself.
Additional discharges occur typically at intervals of 50 to 100
ms and are usually preceded by a dart leader that travels, without
stepping or branching, down the existing main channel. A short
discharge is initiated when the dart leader gets close to the ground
and then a subsequent return stroke travels back up the channel to the
cloud. We'll look at the dart leader and subsequent return stroke
processes in a little more detail
The 3rd return stroke in the illustration above is following by a
continuing current. This is a low amplitude (100s of Amperes)
current that continues to flow for 10s of milliseconds or more
which keeps the return stroke channel luminous. Short
rebrightenings of the channel sometimes occur during the continuing
current, these are called M-components.
The bright flashes of light produced by multiple return strokes
produces the flickering that you sometimes sees when observing cloud to
ground lightning. If you deliberately move a camera back and
forth while photographing lightning the separate strokes will be
displaced on the image. The technique is described in an article
"Panning for Lightning," by John Hendry Jr. that was published in
Weatherwise Magazine (vol. 45, No. 6, pps 18-19, Dec.
1992/Jan 1993 issue). Here is an example (source:
This appears to have been a 4 stroke flash. The rightmost channel
in the photograph is branched so that is the first return stroke.
Separation of the return strokes channels like this can sometimes be
caused by wind. That is referred to as "ribbon lightning."
As long as we're on the subject of lightning photograph, here is a good
Manual movement of the camera is sufficient to time resolve the
separate strokes in a lightning flash. Here's a short
description of the
Boys Camera that was used originally to
photograph the sequences of
processes in cloud-to-ground lightning with much faster time resolution
(source: Lightning, Martin A. Uman, Dover Publications, New York, 1984)
Now some more information about dart leaders and subsequent return
Because a dart leader is following
an existing channel it travels faster than a stepped leader.
Sometimes the dart leader will depart from the existing channel and
form a new channel to ground. Because it is traveling through air
that hasn't been ionized, the lower portion of the dart leader would
turn into a stepped
Occasionally there is sufficient time between strokes that the existing
channel can begin to cool and become less conductive. Under these
conditions the dart leader becomes a dart-stepped leader. The
steps of a dart-stepped leader are shorter and occur more frequently
than a stepped leader.
Some subsequent return stroke characteristics are summarized in
the table below
Statistics on the number of strokes per flash are shown in the
following two histograms (source: Lightning: Physics and Effects,
Cambridge Univ. Press, Cambridge, 2003).
A mean of 4.6 strokes per flash was observed in the 76 flashes in
Fig. (a) from Florida. In Fig. (b) the mean was 6.4 strokes per
flash in 83 flashes recorded in New Mexico.
|peak current derivative
|1 x 108 m/s
Many lightning parameters or characteristics are log-normally
distributed. This appears to be the case with the interval time
between return strokes distribution in the next figure (also from the Lightning: Physics and Effects
source mentioned above)
Shown are 516 interstroke intervals from 132 flashes recorded in
Florida and New Mexico. The shaded data are intervals preceding
strokes that initiated long continuing currents.
Up to now we really haven't considered the possibility that dart
leaders and subsequent return strokes might depart from the channel
created by the stepped leader and 1st return strokes. That does
often turn out to be the case. This means that the separate
strokes in a flash do not always strike the same point on the
ground. The illustration and statistics below come from "Statistics
W.C. Valine and E.P. Krider, J. Geophys.
Res., 107(D20), 4441, doi:10.1029/2001JD001360, 2002)
In a new channel flash one (or more) of the subsequent strokes
follows a different channel to ground. A connection between the
new and original channel is assumed to occur somewhere in the
cloud. In an altered channel flash the new portion of the channel
can be seen departing from the original channel somewhere between the
cloud base and ground.
386 flashes in 7 Arizona air mass thunderstorms were photographed
(video photography) for this study. 135 flashes (35%) contained
just a single stroke, 251 flashes (65%) were multistroke flashes.
Note that on
average a flash has 1.45 strike points. We will make use of this
number later in the course when we discuss lightning location
techniques and estimates of lightning flash densities, an important
parameter in lightning protection and lightning risk assessement.
A distribution of the number of strokes per flash observed in the
Arizona study. The mean, 2.8 strokes/flash, is a little lower
than the mean values mentioned earlier for the Florida and New Mexico
Next we look at how a multi-stroke
cloud-to-ground discharge might
appear on slow and fast electric field antenna records.
The sequence of events shown on the sketch of a high-speed
photograph record of a discharge at the top of
the figure should be familiar by now.
Point 1a on the slow E field record shows the field change produced by
the stepped leader (the E field at the ground points upward toward the
negative charge being lowered by the leader, so it looks like the
atmospheric electricity convention for E field polarity is being used
on this figure). The dart leader field changes are shown at
1b. They are smaller in amplitude (the dart leader doesn't lower
as much charge as the stepped leader) and have shorter durations.
The abrupt return stroke changes are shown at Points 2 on the slow E
and the fast E records. Note one the slow E field record that the
leader and return stroke field changes are of opposite polarity.
The return stroke removes negative charge from the leader channel,
charge that came from the main negative charge center in the
thundercloud. The overall effect is the same as if a quantity of
positive charge had been added to the cloud and used to neutralize the
Points K on the fast E field record are "K changes." During the
interval between return strokes, in-cloud leader processes seek out
additional pockets of charge in the cloud. Occasionally, when the
leader finds a concentration of charge, a rapid recoil streamer travels
back along the
in-cloud leader channel (in some ways like a return stroke propagates
back along the leader in a cloud-to-ground strike). The K changes
are probably produced by these recoil streamers.
The preliminary discharge process that initiates cloud-to-ground
discharges is not well understood. Sequences of bipolar waveforms
like shown at Point 3 though are thought to be associated with the
Point 4 shows E field pulses thought to be produced by the last few
steps of the stepped leader as it approaches the ground.
The fast E field waveforms produced by 1st and subsequent strokes are
distinctly different. The 1st return stroke field variations
following the peak are considerably noisier than is observed with
subsequent strokes. Some of the "noise" on the 1st return stroke
record is probably produced by branches. Some of the structure on
both the 1st and subsequent stroke waveforms is probably due to channel
Some of the modern lightning detection and location instrumentation is
able to distinquish between return stroke waveforms and waveforms
produced by leaders or discharges that occur during the preliminary
The next figure is essentially the same except that some continuing
current has been added following the second stroke.
A continuing current is a low level current (10s to 100s of
Amperes) that continues to flow after a return stroke for 10s to a few
100s of milliseconds. The continuing current can transport
signficant quantities of charge to the ground (100 ms x 250 Amps = 25 C
). Continuing currents can burn through metal sheets or metal
skins on aircraft bodies. A CG lightning discharge with
continuing current is sometimes referred to as "hot lightning" because
these discharges are more likely to cause a forest fire than a
discharge without continuing current. A field change associated
with the continuing current is visible on the slow E field
record. M components are a rebrightening of the lightning channel
that occurs during a continuing current. The K changes seen on
the fast E field trace may be associated with the M components.
The next figure illustrates a triggered lightning discharge.
The figure shows a typical slow E field record and a record of the
current that would be measured at the ground instead of a fast E field