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Part 1 - Stepped Leader & First Return Stroke | Part 2 - Secondary Return Strokes
PHOTOS: Cloud-to-Ground Lightning Gallery
 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 bouyant 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.
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 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:
     
From video of a distant cloud-to-ground strike near Gothenburg, Nebraska: Frames 1 through 4 show the stepped leader descending, Frame 5 shows the intense first return stroke, Frame 6 shows the decaying first return stroke.
Below is a slow-motion movie of a cloud-to-ground strike showing its stepped leader racing toward the ground, followed by the first return stroke after ground connection, followed by two subsequent return strokes:
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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:
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. |
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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.
Part 2 - Subsequent Return Strokes >
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