THE SCIENCE OF LIGHTNING STRIKES

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.