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NOVA scienceNOW: Lightning

Viewing Ideas


Before Watching

  1. Tell the class that before people understood lightning to be a physical phenomenon, nearly every culture constructed myths to explain lightning and thunder. The Greeks believed lightning was thrown by Zeus. Vikings thought Thor's hammer struck an anvil while the god was riding in a chariot across clouds. Buddha was thought to carry a thunderbolt with arrows at the tip. Some Native Americans believed flashing feathers of a bird caused lightning, and the bird's flapping wings made thunder. Ask students to write down their own understanding of what lightning is and how it is caused.

  2. On the board, make a chart headed "Lightning," with two columns underneath, one titled "What You Know" and the other titled "What You Want to Know." Conduct a class discussion and complete the chart. Then, give teams of students an answer to one of the following questions—one per team (cut and paste onto a sheet of paper, copy onto index cards, or print out this page and cut it apart). Have them read the answer and generate the matching question. Have teams share the questions they wrote and read aloud each answer.

    • What is lightning?
      It is a discharge of electricity accompanied by a bright flash of light.

    • What causes lightning?
      While how it occurs is not fully understood, researchers believe that ice particles in a cloud break apart. The larger particles gain a negative charge, and smaller particles gain a positive charge. Gravity separates the particles. Updrafts carry the smaller, positively charged particles to the upper part of the cloud. Gravity pulls the larger, negatively charged particles to the bottom of the cloud. Thus, an electrical potential both within the cloud and between the cloud and the ground is created. Rays from dying stars may provide the energy necessary for triggering the lightning.

    • What is thunder?
      The rumbling noise caused by lightning. The lightning heats the air it passes through—lightning bolts are hotter than the sun's surface. The sudden thermal expansion causes the air to expand suddenly. The shock wave produces the thunder.

    • Why do we hear thunder after lightning strikes?
      Thunder moves away from the lightning bolt as a sound wave. Sound travels at 340.29 meters per second, while light travels at 299,792,458 meters per second. Thus, while light reaches our eyes almost instantly, sound travels to our ears much more slowly.

    • How do you approximate a lightning strike's distance?
      While light travels a mile nearly instantaneously, sound takes approximately five seconds to travel a mile. After you see lightning, count the number of seconds until you hear the thunder. Divide this number by five, which will give you the approximate distance in miles you are from the lightning strike. Divide by three to find the distance in kilometers.

    • About how many lightning strikes are there in the world each year?
      There are approximately 1.2 billion flashes. Many are cloud-to-cloud flashes, so not all of them reach the ground.

    • About how many lightning strikes are there in the United States each year?
      There are about 20 million lightning strikes in the United States each year.

    • Which part of the United States has the greatest amount of lightning? The world?
      Central Florida, between Tampa and Orlando, is the lightning capital of the United States with an average of 90 thunderstorms each year. Central Africa is the world's lightning capital with an average of 280 thunderstorms each year.

    • How do scientists study lightning?
      Benjamin Franklin studied lightning during the 1750s. He designed experiments to show that clouds are electrically charged and that lightning is electrical in nature. During the late 1800s, cameras and light spectroscopy were available for lightning research. Time-lapse photography helps experimenters follow the lightning discharge from the clouds to the ground. Researchers also use sensors mounted on rockets and weather balloons.

    Hear a lightning expert on NOVA scienceNOW describe what scientists think may trigger the four million lightning flashes that occur on Earth each day.

    View this NOVA scienceNOW slide show to learn about nine types of lightning, including red sprites, elves, and volcanic lightning. (Flash plug-in required; printable version available.)

  3. Lightning is a dramatic discharge of static electricity. Perform the following activity to show how to build static electric charges. Students will also observe electrical forces, caused by static electricity, that attract and repel. (The activity will work best in dry weather.)

    1. Gather the following materials:

      • several plastic forks
      • 1 piece of wool or rabbit fur, 10 centimeters square
      • 10-20 pieces of plastic foam, crumbled into bits
      • 10-20 paper punchouts or small pieces of paper, about 1 centimeter square each
    2. Stroke a plastic fork with the wool or fur. Rub in one direction for best results. Electrons have a negative charge. Plastic is a material that readily loses some of its electrons. Wool and fur, on the other hand, accept extra electrons. Rubbing the plastic leaves it with an overall positive charge and the wool or fur with an overall negative charge.

    3. Place the fork next to a small piece of paper. Try to make the paper stand on edge and try to move the paper back and forth. Try to pick up several pieces of paper at once.

    4. Recharge the fork by rubbing it again. Holding the fork with the tines facing up, drop a small bit of plastic foam onto the fork from different heights. Both plastics are negatively charged, and the foam will be repelled.

    5. Record your observations.

After Watching

  1. Have student pairs complete the word search to help them become familiar with the following terms: cosmic rays, dying star, electric field, electricity, electromagnetic spectrum, emit, energy, frequency, gamma ray, light, lightning, radiation, rays, static, visible light, wavelength, and X-rays. The following is the answer key to the student sheet.

                E L B I S I V       M
      E L E C T R I C I T Y       U  
          N         O   A       R   C
          E         S   T     T     I
          R         M   I   C R   E T
      F   G         I   C E   A   L E
    X R A Y S       C   P     D   E N
      E             R S   L   I   C G
      Q             A     I   A   T A
      U   R   G   D Y I N G S T A R M
      E   A   A     S     H   I   I O
      N   Y   M     T     T   O   C R
      C   S   M     H     N   N   F T
      Y       A     G     I       I C
              R     I     N       E E
            W A V E L E N G T H   L L
    E M I T   Y                   D E

  2. Remind students that energy from the sun and stars is called radiation, and it reaches Earth in waves. The waves that transfer the energy from the sun and stars are called electromagnetic waves, or electromagnetic radiation. The radiation is grouped according to the wavelength and frequency of the waves. Waves that have long wavelengths and low frequencies are lower in energy than waves that have short wavelengths and higher frequencies. All electromagnetic radiation travels at the speed of light, and it travels in a vacuum. It does not need a medium in which to travel.

    The scientists in the program report that the amount of energy needed to spark lightning is not present in clouds. Current theory states that the initiation energy comes from the X-rays generated by cosmic rays emitted by dying stars. To help students understand how energetic X-rays are, review the electromagnetic spectrum. Have teams research one type of radiation (e.g., radio waves, microwaves, infrared waves, visible waves, ultraviolet rays, X-rays, gamma rays, and cosmic rays). Teams should state the type of radiation, its wavelength and frequency, and its impact or use on Earth, if any. In addition, have each team represent its chosen wavelength in a drawing. Tape students' work to the board in sequence from low- to high-frequency radiation. Ask students from each team to discuss their information. Point out that X-rays are at the high-energy end of the spectrum and that they may be responsible for "sparking" lightning.

  3. The program states that lightning strikes about four million times each day. In the United States, about 73 people are killed by lightning each year, and thousands are injured. Have student teams prepare a lightning-safety handout to present and share with younger students at your school. Questions that might be answered in a typical handout include:

    • What is lightning? (A discharge of electricity)

    • When is lightning likely to occur? (Before, during, and just after thunderstorms)

    • How can you tell if a thunderstorm may be approaching? (The sound of thunder)

    • Why and how can lightning be dangerous? (Lightning is an extremely large electrical discharge that can damage our bodies.)

    • What should you do if you are outside with friends during a thunderstorm? (Go indoors when you hear thunder and stay away from tall objects, which may attract lightning.)

    • What should you NOT do during a thunderstorm? (Do not stand in an open area, boat, fish, swim, stand near or on metal equipment, take a shower, or use the telephone.)

    • How do you figure out how far away lightning is? (While light travels a mile nearly instantaneously, sound takes approximately five seconds to travel a mile. After you see lightning, count the number of seconds until you hear the thunder. Divide this number by five, which will give you the approximate distance in miles you are from the lightning strike. Divide by three to find the distance in kilometers.)

    • When is it safe to return to outdoor activities after a lightning storm? (After the thunderstorm ends.)


Links and Books

Web Sites

Characteristics of a Storm
thunder.msfc.nasa.gov/primer/primer2.html
Diagrams the lightning discharge process and tells about different kinds of lightning.

Lightning!
fusedweb.pppl.gov/CPEP/Chart_pages/5.Plasmas/Lightning.html
Describes how lightning is created.

Lightning Awareness Week Urges Preparedness
www.redcross.org/article/0,1072,0_332_1183,00.html
Provides basic information about lightning safety and the dangers of lightning.


Books

All About Lightning by Martin A. Uman. Dover Publications, 1986.
Describes the causes of lightning and thunderstorms as far as they are known.

Lightning by Seymour Simon. HarperCollins Publisher, 1997.
Examines the nature of lightning and contains photographs of different kinds of lightning. (picture book)

Lightning Strikes by Jeff Renner. Mountaineers Books, 2002.
Includes safety information for people who engage in recreation or work outdoors.

Teacher's Guide
NOVA scienceNOW: Lightning
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