Lightning Explained, Part 3
The mechanism of space charge shielding goes beyond simple rods but can also explain much of the seemingly mysterious behavior of lightning. For example, the reason tall towers are sometimes hit below the top may simply be that discharge activity from the tower top during the slowly varying electric field, shields the tower top in the same way as was observed with the sharp rods. And so when the descending leader arrives, a lower part of the tower is more prone to producing the upward connecting leader as compared to the tower top. But it will only occur if the slowly varying field reaches critical values for sufficient durations relative to the timing of the arrival of the descending leader. It is predictable but sporadic.
The Issue of space charge shielding is also shedding new light on rocket-triggered lightning experiments and the unique characteristics of that situation.
Can anything be done to prevent a lightning strike?
To the best of our understanding, there is absolutely nothing that can be done to influence any aspect of the descending stepped leader. However it is possible to influence a grounded structure's propensity to produce upward leaders and from where on the structure such leaders would emanate.
The taller the structure and the more charge associated with the particular descending stepped lightning leader, the more the length of the upward leader from the structure can grow. So for a low structure with a large footprint, the probability of being struck will not be greatly influenced by the building's ability to launch leaders. However, for tall slender towers, their ability to launch long leaders is the reason they are struck so frequently.
Influencing the possibility of producing upward leaders from a structure will thus have a much greater effect on taller more slender structures as compared to low massive ones.
For very tall towers, in the 100m range and more, the possibility of another kind of lightning strike comes into play. These structures can generate their own lightning. An upward flash is a lightning strike where the upward connecting leader is created without there having been any descending leader to which to connect. It is true that such strikes can be initiated by nearby flashes but the distinction is that these upward flashes do not connect with any descending lightning leader but rather, move toward localized charge centers in the clouds. Return strokes of comparable magnitudes to downward lightning result.
It is with these structures that the most can be done to influence the probability of a direct strike. The production of charges at the tops of these very tall towers can shield them enough to raise the required space potential for upward leader initiation to the point of significantly suppressing their occurrence.
-- continued on page 3
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