An event as powerful as lightning needs something even more powerful to generate it- the thunderstorm.
It all begins when the atmosphere becomes unstable. In the summertime, the sun's energy heats the earth's surface, which in turn heats the atmosphere close to the ground. This warmer air close to the ground is buoyant relative to the air above it. If this instability becomes great enough, columns of warm air may begin to rapidly burst upward through the atmosphere in a process called convection. As the massive volumes of air rise, they condense in the cooler surrounding air. If there is sufficient moisture in the air, this process creates the enormous, towering cumulonimbus clouds (see diagram at above right), and a thunderstorm is born. An approaching cold front can also force warm, moist air at the surface upward, initiating thunderstorm development.
While there are several theories, the exact mechanism of lightning generation within a thunderstorm is not yet known. Strong updrafts in a thunderstorm carry water droplets into the subfreezing air high in the atmosphere. It is thought that electrification of a storm is related to the freezing of these small droplets of water as they are carried high into the cumulonimbus cloud. Some have also theorized that condensation of water vapor into water droplets during the convective process is the source of charge generation. Whatever the source, the large cloud eventually develops regions of positive and negative charge- usually positive charge high in the cloud and negative charge at the base. The negative charge at the cloud base induces a 'shadow' of positive charge on the ground, much like a magnet induces polarity on a metal paper clip.
The storm soon becomes supercharged with electrical energy as the convective activity continues. When the insulating air between the regions of opposite charge can no longer hold the two apart, a lightning flash begins to develop.
This animation depicts the stepped leader descending
to meet the upward leaders extending from the ground, and the first and subsequent return strokes. This is an extremely slow-motion animation- the actual process takes only a small
fraction of a second.
Air is not a good electrical conductor. But when it is subjected to a critical level of voltage, electrical breakdown of the air occurs. This 'broken-down' air can conduct electricity easily.
A cloud-to-ground lightning strike begins when the air 'breaks down' in a chain-reaction type fashion starting in the charge region in the lower section of the storm cloud. The air breaks down in narrow paths called leaders that split apart and branch out as the 'chain reaction' moves, in steps, toward the ground (watch first animation above). Picture the 'paths' as sticks being laid end-to-end, and every moment, adding another stick to make the path longer. These downward-moving paths of broken-down air are collectively called the stepped leader because of its incremental motion.
The stepped leader is dimly illuminated, but is not visible to the human eye because of its speed and closeness in time (a small fraction of a second) to the bright return stroke. However, playing video of cloud-to-ground lightning strikes in slow-motion can sometimes reveal part of the stepped leader just before it connects to the ground. The following is a frame-by-frame sequence from video of a cloud-to-ground lightning strike:
Fig. 1: From high-speed video of a cloud-to-ground strike near Arian, Texas: The stepped leader descends, followed by the bright return stroke in the last 3 frames.
When the stepped leader nears the ground (around 300 feet), one or more leaders are initiated from the ground (or objects on the ground), and move upward to meet the descending stepped leader (watch animation above). The photo below shows one of these small leaders reaching upward just to the right of the main lightning channel:
Upward leaders from the ground near cloud-to-ground lightning strokes. Click each image to view a bigger version.
The first of these three video frames shows a stepped leader in its final approach to the ground. The resulting return strokes follow. Click to view a slow-motion movie of this flash:
The video frames below also show a cloud-to-ground strike's stepped leader (frame 1) just prior to its ground connection (frame 2) followed by one of many return strokes (frame 3):
|Johnny Autery of Dixons Mills, Alabama caught the famous photograph of lightning striking a tree at close range that shows two upward leaders extending from the ground. View this amazing photo at his web site.|
By the time the stepped leader
gets that close to the ground, it has many branches
, so there is sort of a 'race', if you will, for which branch
will reach the ground first. Whichever downward-moving branch
touches an upward-moving leader
first, 'wins', completing a path of conductive 'broken down' air that connects the ground and the cloud- like a big,
long wire. When this connection is made, the opposing charges equalize
themselves rapidly by flowing upward
through this 'wire' at close to the speed of light.
Even though the channel of 'broken-down' air is a better conductor than air, it is overloaded by the intense current flowing through it. This giant 'short circuit' causes the main lightning channel and all of the 'branches' to light up
brilliantly and heat up violently, like a filament in a light bulb. This flow of current is called the first return stroke, and is the visible 'lightning' flash that we see.
THUNDER: This currrent flow heats the channel of air to a temperature greater than the surface
of the sun in a split second. Heated air expands, explosively expanding when heated to such a high temperature with such speed. This explosive expansion generates supersonic shock waves moving outward from the channel in all directions. After travelling several feet, the shock waves slow to sound waves, which arrive to our ears as thunder.
lightning strikes often contain repeated discharges down the same path in rapid succession following the first return stroke
. These secondary return strokes
often make a lightning strike seem to 'pulse' or 'flicker' on and off.
If you could make the clouds and the earth invisible - allowing you to see an entire cloud-to-ground lightning strike from top to bottom, you would see a full dendritic structure very similar to a tree. Above, branches extending deep into the cloud tap into areas of electrical charge. In the earth below, a network of 'roots' penetrates below ground. The photo at right shows part of a distant cloud-to-ground lightning strike's in-cloud structure as some of the dendritic branches peek out of the cloud. This 'lightning tree' makes up the first return stroke of the cloud-to-ground discharge.
There are usually additional areas of electrical charge higher up or farther horizontally inside the cloud. Once the initial lightning channel is established, it is like a long wire connecting the cloud to the ground. 'Earth ground' is now essentially brought up inside the cloud by this 'wire' after the first return stroke (See Stage 3 below). Now you have a scenario in which these pockets of charge deep in the cloud now are 'within reach' of 'earth ground'. Previously, these pockets were too far away to overcome the insulating air to reach the ground. Now, the initial conductive lightning channel has brought 'ground' close enough to the charge pockets for them to discharge to it.
And that's exactly what happens. These charge pockets 'spark over' to the now-established lightning channel and use it to reach the ground - creating a second return stroke and pulse of light (See Stage 4 below). The branches of the 'lightning tree' grow deeper into the cloud, making 'earth ground' accessable to more areas of charge, and more return strokes follow (See Stage 5 below).
This chain reaction continues until all charge pockets within reach of the grounded 'lightning tree' are tapped and discharged. A complete lightning discharge event can contain anywhere from a single return stroke to more than twenty, depending on the electrical structure of the storm. The numerous pulsing return strokes occur in rapid succession, making the lightning bolt appear to flicker on and off.
The sequence below demonstrates the multiple return stroke process of a cloud-to-ground lightning strike. Two animations of the sequence appears below, one in labeled slow-motion and the second in full speed.
- Stage 1: Storm electification results in pockets of charge in the cloud. For the sake of illustration, polarity has been ignored here.
- Stage 2: Main Charge Pocket discharges to ground via stepped leader connection. Dendritic network of branches 'taps' charge in main charge pocket.
- Stage 3. Established lightning channel brings 'earth ground' close to Charge Pocket #3.
- Stage 4. Charge Pocket #3 discharges to the established channel. Earth ground is now brought deeper into the cloud.
- Stage 5. Charge Pockets #2 and #4 now are close enough to 'earth ground' via the conductive lightning channel that they too can discharge to ground.
A total of four return strokes occured with the 'sample lightning strike' above (First return stroke plus 3 secondary return strokes).
In this animation, the return strokes are identical in intensity. In actuality, however, different return strokes that make up a single lightning event will often vary in brightness and intensity. For instance, you might see: BRIGHT first return stroke, DIM second stroke, DIM third stroke, BRIGHT fourth stroke, DIM fifth stroke, MEDIUM sixth stroke, and so on. The brightness of each return stroke should indicate the size and strength of each charge area being tapped. In other words, larger and more intense 'pockets' discharging through the channel will result in a brighter, more intense return strokes. In some cases, secondary return strokes can be brighter than the first (branched) stroke!
The following animation shows what the above lightning discharge would look like in real time. Keep in mind that most of the upper structure of this discharge (the 'branched tree' sections) would be obscured from view in an actual thunderstorm. This demonstration here is using a fictional 'see-through storm' so the entire above-ground structure of the lightning is visible:
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