THE SCIENCE OF LIGHT­NING STRIKES

EQUUS - - Eq Tack& Gear - Source: Na­tional Weather Ser­vice (www.light­ningsafety.noaa.gov)

Thun­der­storms de­velop when ther­mal up­drafts carry large num­bers of pos­i­tively charged ice crys­tals into the up­per lay­ers of a storm cloud. This leaves a stronger neg­a­tive charge be­hind in the lower and mid­dle por­tions of the cloud. An­other, thin­ner layer of pos­i­tive charges re­mains along the lower edge of the cloud.

When th­ese dif­fer­ences grow too great, and the in­su­lat­ing qual­i­ties of the air can no longer con­tain them, the neg­a­tive charges in the cen­ter of the cloud cre­ate a chan­nel to flow to­ward the pos­i­tive charges. In­tra-cloud flashes oc­cur when the dis­charges re­main con­tained within the cloud. How­ever, when neg­a­tive charges from the cen­ter of the cloud flow down­ward to­ward the bot­tom sur­face, the thin layer of pos­i­tive charges there may not be pow­er­ful enough to con­tain the surge from above. In that case, the surge will con­tinue down­ward, to­ward the ground in search of a con­nec­tion that will equal­ize the neg­a­tive charge.

The ex­treme neg­a­tive charge mov­ing down­ward causes pos­i­tively charged “stream­ers” to rise from mul­ti­ple ob­jects on the ground. When the de­scend­ing neg­a­tive charge, called a “stepped leader,” makes con­tact with a streamer, the con­nec­tion is com­plete, and a huge amount of elec­tri­cal en­ergy is trans­mit­ted into the ground. This is a cloud-to-ground strike. The pos­i­tive charges at the top of the cloud can also send a strike di­rectly to the ground, es­pe­cially if strong winds at high al­ti­tudes are push­ing the cloud side­ways or into an anvil shape. Th­ese strikes can oc­cur eight to 10 miles away from the main storm cloud, far from any rain that is fall­ing.

In­jury and dam­ages oc­cur when peo­ple, an­i­mals, trees and build­ings get caught in or too close to a cloud-to-ground strike. Light­ning can strike you or your horse in five dif­fer­ent ways. Stay­ing safe dur­ing a

light­ning storm means avoid­ing all of the po­ten­tial paths the elec­tric­ity can take:

• A di­rect strike is when a per­son or an­i­mal is the main chan­nel by which the bolt of light­ning reaches the ground. The elec­tric­ity will split: Part will travel over the sur­face of the skin, cre­at­ing a dis­tinct burn line, and the rest will travel straight through the car­dio­vas­cu­lar and neu­ro­log­i­cal sys­tems within the body—most likely stop­ping the heart. The risk of a di­rect strike is greater when the vic­tim is stand­ing ex­posed in an open area.

• A side flash, or side splash, oc­curs when light­ning strikes a taller ob­ject, and as it trav­els down­ward, it jumps side­ways into the vic­tim to com­plete its cir­cuit with the ground. The most com­mon sce­nario for this phe­nom­e­non is when a per­son or an­i­mal takes refuge un­der a tree, which is struck. The side flash is more likely when the vic­tim is stand­ing within a foot or two of the trunk.

• Ground cur­rent causes more light­ning fa­tal­i­ties than all other types com­bined, and it is es­pe­cially deadly to large an­i­mals. When light­ning strikes any ob­ject, once the cur­rent reaches the ground it can ra­di­ate out­ward along the

sur­face. If some­one is stand­ing on the ground, elec­tric­ity will travel up the leg clos­est to the strike, pass through the body, and exit via the leg far­thest from the strike. The far­ther apart the two legs are, the more elec­tric­ity will be drawn through the body. A per­son stand­ing with feet to­gether will ex­pe­ri­ence less volt­age than a per­son stand­ing with feet spaced widely—but both vic­tims are likely to sur­vive, since in peo­ple, this charge will travel through only the lower ex­trem­i­ties. Be­cause horses and other large an­i­mals stand over four legs with a long torso, they will to draw in twice as much elec­tric­ity as a per­son would, and all this elec­tric­ity is likely to travel right through the heart. Ground cur­rent can kill an en­tire herd that takes refuge un­der a tree that is struck.

• Con­duc­tion is an­other com­mon killer of live­stock. If on its path to the ground the light­ning en­coun­ters a con­duc­tive metal struc­ture, the elec­tric­ity is likely to travel lat­er­ally along that metal path­way un­til it can find an­other route to the ground. One com­mon sce­nario is when a strike trav­els along fenc­ing made of wire on wooden posts. If an­i­mals are touch­ing the fence, the cur­rent will go through them to com­plete the cir­cuit; even if they are sim­ply stand­ing near the struc­ture, the cur­rent may side-flash to reach the ground.

• Stream­ers, the pos­i­tively charged bolts that arise from ob­jects on the ground to meet the neg­a­tively charged strikes from the clouds, can also kill or in­jure a per­son or an­i­mal, even if the con­nec­tion to the main light­ning bolt is not made.

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