April 17, 2002
Q: What causes lightning?
A: The electrical charges that cause lightning originate high in a cumulonimbus cloud, in a region of snow crystals, snow and ice pellets (graupel). In the rising and sinking motions within the storm there are a lot of collisions between these precipitation particles. The collisions cause the snow crystals to become positively charged, while the snow and ice pellets become negatively charged. Upward motion within the storm causes the positively charged snow crystals to be carried upward to the top of the storm, while the heavier snow and ice pellets fall to the middle and lower part of the storm. Enormous charge differences develop within the cloud and can cause cloud-to-cloud lightning. The negative charge in the lower part of the storm causes positive charges to build up on the ground directly beneath the cloud and in the area immediately surrounding the cloud. As the thunderstorm cloud moves, these induced positive charges on the ground follow the cloud like a shadow. As the differences in charges between the cloud and ground increase, the likelihood of cloud-to-ground lightning increases. Initially, an invisible, negatively-charged channel of air forms near the cloud base and surges downward toward the ground. As this "step leader" approaches the ground, streamers of positive charge reach upward from trees, buildings, and other objects on the ground. Once a connection is made and the channel is complete, a surge of electrical current moves from the ground to the cloud causing the visible "return stroke" that we call lightning. Sometimes there are several return strokes of electricity within the established channel and you will see these as flickering lightning. It is important to note, however, that not all lightning forms in the negatively charged area low in the thunderstorm cloud. Some lightning originates in the cirrus anvil at the top of the thunderstorm. This area carries a large positive charge. Lightning from this area is called "positive lightning."
An Approaching Thunderstorm—When should I seek safe shelter?
Lightning can strike as much as 10 miles away from the rain area in a thunderstorm; that's about the distance that you are able to hear the thunder from the storm. If you can hear thunder, chances are that you are within striking distance of that storm. Also, remember that each thunderstorm has a first stroke of lightning, which is just as dangerous as any other stroke. Be alert to threatening skies and increasing winds, and take shelter before hearing thunder. After a storm passes, wait 30 minutes or more before leaving proper shelter!
What are some of the safest places from lightning? The least safe?
For a shelter to provide protection from lightning, it must contain a mechanism for conducting the electrical current from the point of contact to the ground. A house or other substantial building offers the best protection from lightning. Hard-topped vehicles are also safe, because the metal shell distributes the lightning's charge around the car and into the ground. On the outside, lightning can travel along the outer shell of the building or may follow metal gutters and downspouts to the ground. Inside a structure, lightning can follow conductors such as the electrical wiring, plumbing, and telephone lines to the ground. Unless specifically designed to be lightning safe, small structures do little, if anything, to protect occupants from lightning. Many small open shelters on athletic fields, golf courses, parks, roadside picnic areas, schoolyards and elsewhere are designed to protect people from rain and sun, but not lightning. It is important to stay safe from lightning when inside a structure as well. Once inside the home lightning can travel through the electrical, phone, plumbing, and radio/television reception systems. Phone use is the leading cause of indoor lightning injuries in the United States. Lightning can travel long distances in both phone and electrical wires, particularly in rural areas. Stay away from windows and doors as these can provide the path for a direct strike to enter a home. Avoid contact with concrete walls, which may contain metal reinforcing bars. Avoid washers and dryers, since they not only have contacts with the plumbing and electrical systems—but also contain an electrical path to the outside through the dryer vent.
Summary of Lightning Safety Tips for Inside the Home:
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Avoid contact with corded phones
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Avoid contact with electrical equipment or cords. If you plan to unplug any electronic equipment, do so well before the storm arrives
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Avoid contact with plumbing. Do not wash your hands, take a shower, wash dishes, or do laundry
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Stay away from windows and doors, and stay off porches
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Do not lie on concrete floors and do not lean against concrete walls
Outdoors is the most dangerous place to be during a thunderstorm. The most dangerous places outside are near water or trees, on high places such as house roofs, or in other open areas.
What should I do if I am caught outdoors during a thunderstorm?
If you are caught outside during a thunderstorm and there are no safe structures available, there are things you can do to minimize the lightning danger. You should avoid being the tallest object in the area. If only isolated trees are nearby, crouch on the balls of your feet in the open, keeping twice as far away from a tree as it is tall. This "lightning crouch" is safer than lying flat on the ground, because lying flat increases your chances of being hit by a ground current caused by lightning hitting the ground nearby. If you are caught in a boat, crouch down in the center of the boat away from metal hardware. Don't stand in puddles of water, even if wearing rubber boots.
If someone is struck, what should I Do?
In the unfortunate event that a person is struck by lightning, medical care may be needed immediately to save the person's life. Since the human body does not store electricity, it is perfectly safe to touch a lightning victim to give them CPR. Cardiac arrest and irregularities, burns, and nerve damage are common in cases where people have been struck by lightning. However, with proper treatment—including CPR if necessary—most victims survive a lightning strike, although sometimes with long-term effects on their lives and the lives of family members.
What are some of the medical effects of a lightning strike?
Lightning kills an average of 73 people per year in the United States. This is more than the average number of deaths for hurricanes and tornadoes. People injured by lightning may suffer from lasting affects, particularly to their nervous system. Personality changes, short-term memory difficulties, headaches, dizziness, insomnia, fatigue and other ailments have been noted by lightning-strike survivors.
