Tuesday Mar. 1, 2011
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You probably didn't sign up for this class thinking that you'd be spending so much time reviewing principles of electricity & magnetism, learning about small ions, conductivity, Maxwell currents and that sort of thing.   No you probably thought (hoped?) the class would be about lightning.  And much of the class will be, starting today.  We'll start with a fairly qualitative look at lightning phenomenology.  We'll concentrate primarily on cloud-to-ground lightning. 

Broadly speaking there are two types of lightning

Intracloud (IC) lightning remains in the cloud and is the most common type of discharge.  Approximately 1/3 of lightning discharges travel between charge in the cloud and the ground.  We'll use the term cloud-to-ground even though, as we will see, some relatively rare lightning discharges start at the ground and travel upward toward the cloud.

Most CG discharges, "negative cloud-to-ground", carry negative charge to ground.


Most CG discharges begin with an in-cloud, preliminary breakdown, process.  Also one of the lower positive charge centers is probably involved in most negative CG discharges.  Pay attention to the branching in this and the figures that follow.  The branching tells you the direction of the initial leader process in the discharge (we'll learn more about the leader processes shortly).

We've mentioned before that some CG discharges begin higher in the cloud and carry positive charge to the ground.


Positive CG discharges are more common
(i)  in severe thunderstorms where vertical wind shear is present (upper level winds are stronger than the winds at lower altitudes).  Normally the positive charge center is above the main negative charge and discharge between the two remain in the cloud.  Wind shear can cause the cloud to tilt and move some of the positive charge away from a position directly above the main negative charge center.  Discharges can then travel from the cloud to the ground.

(ii)  in storms at high latitudes.  The positive charge center is closer to the ground.

(iii)  in winter storms.  The positive charge center is closer to the ground.  Vertical wind shear may also be present

(iv)  at the end of summer storms.  The cloud may tilt and signficant amounts of positive charge may be found in the cloud anvil and away from the main body of the cloud.

Positive CG discharges usually just have a single return stroke though the return stroke peak current is often very large 100,000 A or more (negative CG discharge peak return stroke currents are usually 30,000 A or less).  The positively charged leader than initiates a positive CG discharge shows little or no stepping (we'll discuss the negatively charged "stepped leader" that initiates negative CG discharges shortly)

Tall buildings and towers are struck relatively often by lightning.  Often these are discharges initiated or triggered by the structure itself.


These discharges begin at the ground with the development of an upward moving, positively charged, leader (note the direction of the branching in these sketches).  The upward leader is followed by  downward negatively charged leaders and upward return strokes (i.e. the normal sequence of events in negative CG lightning). 

Researchers are now able to trigger lightning discharges using small rockets.  A spool of wire attached to the base of the rocket unwinds as the rockets travels upward.  If conditions are right, an upward leader is initiated when the rocket reaches an altitude of 100 m or so.  Direct measurements of lightning return stroke currents can be made.  Measurements of E and B fields can be made very close to the lightning channel (10 m or less).

Now we'll look at the sequence of processes that occur during negative cloud-to-ground discharges.


The discharge begins with some kind of in-cloud preliminary discharge (red in the figure above highlighted in yellow).  Then a downward moving, negatively-charged discharge begins moving toward the ground.  This is called the stepped leader discharge because the channnel advances in steps of about 50 m length every 50 microseconds or so.  The 50 m long channel extensions occur rapidly (less than 1 microsecond duration) and produce a bright flash of light.  Think of dropping a strobe light from aircraft altitude and watching it flash on and off as it falls to the ground.

It takes a few 10s of milliseconds for the step leader to travel a few kilometers from the cloud to near the ground.  This corresponds to an average speed of 1 - 2 x 105 m/sec.  Negative charge is carried from the main negative charge center and distritbuted along the length of the leader channel.   Currents flowing in the leader channel range from 100s to about 1000 Amps. 

The individual step, or extension of the leader channel, occurs in less than 1 microsecond.  The velocity of the step is 5 x 107 m/s of faster and the step current is 1 kA or more.

Images of stepped leader have been captured using streaking cameras (cameras with moving film) and, more recently, on video cameras with high frame rates.  The next figure tries to illustrate how a descending leader might appear on film.

Because the film is moving, each of the images is displaced slightly on the film.  Only the bottom extension of the channel produces much light (the remainder of the channel is weakly illuminated).  This is highlighted in yellow above.  The descending channel breaks up into brances and then the ends of the separate branches step and move toward the ground.

We really won't cover the attempts that have been made to understand or simulate stepped leader channel development.  It would be very difficult to produce a stepped leader discharge in a laboratory because even a single step is 50 m long.  Here's my conception of what leads up to a step of the stepped leader.


Basically the leader consists of a low resistance arc core that is surrounded by a space charge envelope (see Question #2 on the 1st homework assignment).  High fields at the bottom end of the propagating channel create discharges that grow out into the air ahead of the leader.  Initially these are high resistance glow discharges.  As the filaments of this discharge increase in number and lengthen more and more current flows from the bottom end of the arc core.  At some point, a portion of the glow discharge channel will change to a conducting arc.  It is this glow discharge to arc transition to produce a brief pulse of current and light.  This is the step that effectively lengthens the leader channel. 

This might be a good time to have a look at one of the slow motion videos of lightning shown in class.  The first video is a negative cloud to ground discharge with just a single return stroke.  The discharge was captured with a video camera that captures 7200 images per second (the video is then slowed down considerably during play back).

When the stepped leader approaches to within a few hundred meters of the ground, the electric field at the ground intensifies to the point that several positively charged, upward propagating discharges are initiated.  One of these will intercept the stepped leader and determine where the lightning will strike the ground.




The striking distance referred to in the figure is typically 10 to 20 m above flat ground and 20 to 100 m above taller objects. 

There are probably less than a dozen good photographs of upward connecting discharges.  The example I showed in class was taken by Johnny Autry in Alabama.  The photograph is copywrited but I've sketched it below and have pointed out some of the main features.

You can see the actual photograph on the photographers homepage.  There were at least 3 upward discharges initiated by the approach of the stepped leader (1, 2, and 3 in the sketch).  Streamer 1 connected to the bottom of the stepped leader.  It isn't clear where the exact junction point was.  The downward branching at Point 4 indicates that was part of the descending stepped leader.  A very faint upward discharge can be seen at Point 3.

Most of the remaining examples can be seen on pps 141 & 142 in Rakov and Uman's book "Lightning Physics and Effects."  As chance would have it this portion of the book can be viewed online here (you may have to scroll up or down to pps 141 & 142).

Junction between the upward connecting discharge and the stepped leader electrically connects charge in the cloud to ground and a large ampltitude pulse of current travels rapidly back up the channel.  This is the first return stroke discharge.



The tip of the return stroke propagates up the channel at a speed of about 1 x 108 m/s (1/3 the speed of light) taking about 100 microseconds to travel from the ground to the cloud.  The 1st return stroke has a peak current of typically 30 kA.  Current rises to peak value in a few microseconds and the peak dI/dt is about 100 kA/us.  The return stroke heats the air to a peak temperature of about 30,000 K (5 times hotter than the surface of the sun).  The overpressure creates a shock wave that quickly decays into a sound wave that we hear as thunder. 

I often use the following analogy to help understand the return stroke process.

Now imagine what you would see if the hand at the bottom of the tube is removed.




And now back to the stepped leader channel and all the negative space charge surrounding it.

The intense action is really at the tip of the upward propagating return stroke.  The large potential difference (cloud potential in close proximity to ground potential) ionizes the air, creates large currents, heats the air, and produces a bright flash of light.  The potential difference wave can progagate rapidly up the channel (and out into the branches).  Once the potential wave passes a point on the channel the space charge envelope collapses and electrons flow more slowly down the conducting channel to ground.


This is about where we stopped on Tuesday.  I showed a couple more slow motion videos of lightning.  I'll leave the links below but will also embed them in Thursday notes.

negative cloud-to-ground flash ( a different version was shown in class with just the stepped leader and the 1st return stroke)
(7200 images per second)

upward discharge
(1000 frames per second?)