3.2 DEVELOPMENT OF A LIGHTNING FLASH.
As the charge builds up in a cloud, the electric field in the vicinity of the charge center builds up to the point
where the air starts to ionize. A column of ionized air, called a pilot streamer, begins to extend toward earth
at a velocity of about 160 kilometers per hour (100 miles per hour) (3-3). After the pilot streamer has moved
perhaps about 30 to 45 meters (100 feet to 150 feet), a more intense discharge called a stepped leader takes
place. This discharge lowers additional negative charge into the region around the pilot streamer and allows the
pilot streamer to advance for another 30 to 45 meters (100 to 150 feet) after which the cycle repeats. The
stepped leader progresses towards the earth in a series of steps with a time interval between steps on the order
of 50 microseconds (3-4).
In a cloud-to-ground flash, the pilot streamer does not move in a direct line towards the earth but instead
follows the path through the air that ionizes most readily. Although the general direction is toward the earth,
the specific angle of departure from the tip of the previous streamer that the succeeding pilot streamer takes is
rather unpredictable. Therefore, each 30 to 45 meter (100 to 150 foot) segment of the discharge will likely
approach the earth at a different angle. This changing angle of approach gives the overall flash its
characteristic zig-zag appearance.
Being a highly ionized column, the stepped leader is at essentially the same potential as the charged area from
which it originates. Thus, as the stepped leader approaches the earth, the voltage gradient between the earth
and the tip of the leader increases. The increasing voltage further encourages the air between the two to break
The final stepped leader bridges the gap between the downward progressing column and the earth or an
extension of the earth such as a tree, building, or metal structure that is equipotential with the earth. While
the stepped leader is approaching the earth, a positive charge equivalent to the negative charge in the cloud is
accumulating in the general region underneath the approaching leader. Once the stepped leader contacts earth
(or one of its extensions), the built-up positive charge in the earth flows rapidly upward through the ionized
column established by the stepped leader to neutralize the strong negative charge of the cloud. This return
current constitutes what is generally referred to as the lightning stroke. If additional pockets of charge exist in
the cloud, these pockets may discharge through the ionized path established by the initial stroke. Continuous
dart leaders proceed from a remaining charge pocket toward the earth down this path. Once the dart leader
reaches the earth, another return stroke of positive charge propagates up the channel to neutralize the
secondary charge in the cloud. This cycle may be repeated several times as succeeding charge centers in the
cloud are neutralized.
3.3 INFLUENCE OF STRUCTURE HEIGHT.
Flashes to earth are normally initiated by a pilot streamer from the cloud. As the charged leader approaches
the ground, the voltage gradient at the surface increases. Ultimately the voltage becomes high enough for an
upward-moving leader to be induced. Over flat, open terrain, the length of the upward leader does not exceed a
few meters before it unites with the downward leader to start the return stroke. However, structures or other
extensions from the earth's surface experience intensified electric field concentrations at their tips.
Consequently the upward leaders are generated while the downward leader is some distance away; the upward