What Is Lightning?

Many people know the story of Benjamin Franklin flying a kite in a rainstorm. At the tip of his kite, Franklin attached a pointed metal piece. Then at the end of the kite string, he fastened an iron key. Lightning struck the metal piece on the top of the kite, and Franklin received an electrical shock when he placed his knuckle near the metal key. Franklin was lucky to live through his discovery that lightning, indeed, was electricity.

Lightning forms high up in the atmosphere at altitudes of 15,000 to 25,000 feet. At this altitude, as the air rises and descends, some droplets of water stored in clouds freeze into ice. As the particles of ice move past each other, an electrical charge is generated. Particles with a positive charge travel towards the top of the cloud. Particles with a negative charge drop towards the middle and bottom of the cloud. As this cloud travels over the Earth's surface, it can attract positively charged particles on the Earth's surface.

The negatively charged particles stored up in the cloud may eventually travel to the ground in the form of lightning. As the charge travels towards the Earth, it creates a pathway, or channel. The charge traveling along this pathway is what we see as lightning. Sometimes a charge will travel back from the Earth to the cloud through the same channel that it traveled down towards the Earth. In fact, lightning does not just travel between clouds and the Earth. Lightning can travel from cloud to cloud and from cloud to air, as well.

Lightning can also start from the large positive charge near the top of a storm cloud. Positively charged lightning is very dangerous as it can take people by surprise. Negatively charged lightning tends to strike around the center of a storm. Positively charged lightning may strike up to ten miles away from the center of a storm. It may strike areas ahead of the storm's path or behind the storm's path.

Thunder is created by lightning. When an electrical charge large enough to cause lightning travels through the air, it generates a great deal of heat. The air around a bolt of lightning heats up to 50,000°F! When air gets that hot that quickly, it expands rapidly and causes thunder.

Each year, lightning strikes the Earth more than 20 million times. With that much lightning activity, it is important to know how to stay safe when there is lightning in the air. The National Weather Service recommends following the “30/30 rule.” Go inside if you see lightning and hear thunder. If you cannot count to 30 between seeing lightning and hearing thunder, remain inside. After you hear the last growl of thunder, wait for 30 minutes before going outside. Take thunderstorms seriously and find shelter.


A height measured from sea level.

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  1. What is lightning? Draw a diagram of a storm cloud. Show how lightning is generated inside a cloud and how the top of the cloud is different from the middle and bottom of the cloud. Diagram a bolt of lightning striking the Earth from a storm cloud. Write a few sentences that explain the different parts of your diagram.
    [anno: Diagrams may vary but should show a group of positively charged particles towards the top of a storm cloud and a group of negatively charged particles towards the middle and bottom of a storm cloud. Students may indicate the circulation of ice crystals inside a cloud as the source of an electrical charge. Diagrams should show a bolt of lightning traveling from the cloud to the ground. Students may label lightning as comprising negatively charged particles if the bolt originates from the middle or bottom of the cloud and is striking directly beneath the cloud. The lightning may be labeled as positively charged if it is originating from the top of the cloud and striking either behind or in front of the cloud. Students should provide a short description of what is shown in their diagrams.]
  2. What are two different kinds of lightning? How are they different?
    [anno: Two different kinds of lightning are positively charged lightning and negatively charged lightning. Positively charged lightning originates from the top of a storm cloud, and negatively charged lightning originates from the middle or bottom of a storm cloud. Positively charged lightning can strike for many miles beyond the center of a storm, whereas negatively charged lightning tends to strike near the center of a storm.]
  3. You might have noticed that some buildings have a lightning rod on the rooftop. A lightning rod is a metal pole that extends above the height of the roof. What purpose might a lightning rod serve? Write a sentence or two to explain your answer.
    [anno: Answers will vary but could include that people use lightning rods to protect buildings. If lightning is going to strike an area, it is better for the lightning to hit a metal pole than the building itself. Students may or may not know that a lightning rod is grounded and therefore diverts the electrical charge of a lightning bolt away from the building and into the ground.]