Why are lightning rods effective in keeping tall trees and homes safe from lightning?
During thunderstorms, many people stand under trees in an effort to stay dry. However, this can have dire consequences. In the spring of 1991, a lacrosse game at a Washington, D.C., high school was postponed after lightning was observed in the sky. Over a dozen spectators ran for shelter under a tall tree to protect themselves from the rain. A few seconds later, lightning struck the tree, injuring twenty-two people and killing a fifteen-year-old student.
Trees are tall points where positive streamers can originate and attract the stepped leader, starting a lightning bolt. By standing under a tree, holding an umbrella, swinging a golf club, or batting with an aluminum bat, people are making themselves part of a lightning rod.
Lightning rods are pointed metal rods that are installed above a tree or rooftop to protect the object. The rod, connected to the ground by a metal wire, both encourages and discourages a lightning strike. The rod discourages the lightning strike by “leaking” positive charges out of its pointed top to satisfy the need for positive charge in the clouds. If the rod cannot leak out enough charge to satisfy demand the stepped leader from the cloud is instead attracted to the rod, and a flash of lightning occurs. Therefore, the rod attempts to discourage lightning, but if it cannot satisfy the negative charge, it attracts the lightning to the rod instead of the tree or house.
If the lightning rod doesn’t have a good connection to the ground through the wire it can increase the danger to the building. Often these heavy grounding wires come loose from the lightning rods, and if the rod is then hit by lightning, the charges will flow along the surface of the building to the ground and could cause a fire. The rods can become disconnected from lack of routine maintenance; it is wise to check these connections on a regular basis.