NATURE PHYSICS: AN INTRODUCTION TO THUNDER AND LIGHTNING

Transcription

1 NATURE PHYSICS: AN INTRODUCTION TO THUNDER AND LIGHTNING A Greek philosopher and scientist named Aristotle thought the sound was caused by clouds colliding. Some Native Americans believed the sound was produced when an enormous sacred bird flapped its wings. And ancient Scandinavians thought the sound resulted from one of their gods, Thor, riding a chariot through the sky while wielding a magical hammer. But what really causes thunder? Modern scientists think they have the answer. To understand, though, we need to know a bit about lightning, because thunder can t occur without lightning. Our next Phenomenal Physics article will focus on the cause of lightning. But for now, we ll keep things simple. With that in mind, think of lightning as a larger version of the process that creates the tiny shock you feel from static electricity after shuffling across a carpet in your stocking feet and then touching a metal doorknob. Figure 1. Cumulonimbus clouds build as an afternoon thunderstorm develops (photo courtesy of GeekPhilosopher.com). In the case of thunderstorms, water evaporates on warm days, resulting in moist air that rises quickly and forms large cumulonimbus clouds (Figure 1). Updrafts and downdrafts within these clouds cause water droplets and ice particles to bang together as they swirl on the air high above the ground (Clouds R Us 2014). All those collisions create an electrical charge, just like the one that built up in you when you crossed the carpet, according to the University Corporation for Atmospheric Research. After a while, the whole cloud fills up with electrical charges (usually with a negative charge closest to the earth) (UCAR 2000). Because opposites attract each other, a positive charge builds on the ground beneath the cloud and concentrates around anything that s elevated, such as mountains, lone trees, people, or even blades of grass. As with most things that continue to build over time, something has to give. 1 P age

2 Figure 2. Lightning results when the negative charge in a thundercloud and the positive charge on the ground grow too strong to be insulated by the atmosphere. The website SciJinks (2014) describes the process this way: Static charges are always looking for the first opportunity to escape, or discharge. Lightning begins as static charges in a rain cloud. These electrical fields become incredibly strong, with the atmosphere acting as an insulator between them in the cloud. When the strength of the charge overpowers the insulating properties of the atmosphere, Z-Z-Z-ZAP! Lightning happens. (See Figure 2.) So, although it s a bit more complicated than this, you can think of yourself as the cloud when you shuffle across the carpet, the metal doorknob as the ground, and the spark given off when you touch the doorknob as lightning. Up next: thunder, nature s base drum. In only a few milliseconds, lightning superheats the surrounding air to as much as 54,000F. That s five times hotter than the surface of the sun (Weather Wiz Kids 2014). And what happens to air when it s heated? Exactly it expands. Because lightning heats and expands the surrounding air so quickly, it s like an explosion. This explosion creates a shockwave that slams into the surrounding air and compresses it. The rapid expansion and compression create an initial crack. The rumbling noise heard afterward happens because the column of air continues to vibrate. How Far? If lightning and thunder happen nearly simultaneously, why do we often see lightning before we hear the thunder (Figure 3)? Because light travels faster than sound. It takes light a little more than 5 microseconds to travel 1 mile; that means light travels about 65,000 miles in the time it takes an average person to blink. Compared to light, sound is a slowpoke traveling about 760 miles an hour, or about 1 mile every 5 seconds. Knowing this, you can calculate how far away lightning is. The next time you see a bolt of lightning, count the number of seconds between the time you see the lightning and hear the thunder. Divide the Figure 3. Count the seconds between the time you see lightning and hear the thunder, and then divide the number of seconds by 5 to determine how many miles away the lightning is (photo courtesy of GeekPhilosopher.com). 2 P age

3 number of seconds by 5. The resulting number tells you how many miles away the lightning occurred. If the thunder follows the lightning almost immediately, the lightning was very close (SciJinks 2014). Danger Lightning is dangerous. It kills and injures between 75 and 100 people each year more than hurricanes and tornadoes do. Still, the chances of you being a victim of lightning are small if you play it safe. Just remember the following facts and tips (NOAA 2014): NO PLACE outside is safe when thunderstorms are in the area (Figure 4)! If you hear thunder, lightning is close enough to strike you. When you hear thunder, immediately move to a safe shelter: a substantial building with electricity or plumbing or an enclosed, metal-topped vehicle with the windows up. Stay in a safe shelter for at least 30 minutes after you hear the last sound of thunder. Figure 4. If possible, take shelter inside a building at the first sign of a thunderstorm (photo courtesy of stockvault.net). Indoor Lightning Safety Once you ve taken shelter indoors, remember to: Stay off corded phones, computers, and other electrical items that put you in close contact with electricity. 3 P age

4 Avoid plumbing, including sinks, baths, and faucets. Stay away from windows and doors. Stay off porches. Do not lie on concrete floors or lean against concrete walls. Last-Resort Outdoor Tips to Reduce Risk If you re caught outside with no safe shelter anywhere nearby, the following actions may reduce your risk: Immediately get off of elevated areas such as hills, mountain ridges, or peaks. Never lie flat on the ground. Never take shelter under an isolated tree. Never use a cliff or rocky overhang for shelter. Immediately get out of and away from ponds, lakes, and other bodies of water. Stay away from objects that conduct electricity (barbed wire fences, power lines, windmills, etc.). Demonstrations Try the following demonstrations to better understand how thunder and lightning are created. A Simple Way to Make Thunder Materials: Brown paper lunch bag Process: Blow into the brown paper lunch bag and fill it with air. Twist the open end and close with your hand. Quickly hit the bag with your free hand. Explanation: Hitting the bag causes the air inside the bag to compress so quickly that the pressure breaks the bag. The air rushes out and pushes the air outside away from the bag. The air continues to move forward in a wave. When the moving air reaches your ear, you hear a sound. Thunder is produced in a similar way. As lightning strikes, energy is given off that heats the air through which it passes. This heated air quickly expands producing energetic waves of air resulting in a sound called thunder. Fun Fact Lightning does not always create thunder. In April 1885, five lightning bolts struck the Washington Monument during a thunderstorm, yet no thunder was heard. Make Lightning Materials: Aluminum pie pan Small piece of wool fabric Styrofoam plate 4 P age

5 Pencil with a new eraser Thumbtack Fun Fact Thunder is not only heard during rainstorms. It is uncommon, but not rare, to hear thunder when it is snowing. Process: Push the thumbtack through the center of the aluminum pie pan from the bottom. Push the eraser end of the pencil into the thumbtack. Put the Styrofoam plate upside-down on a table. Quickly rub the underneath side of the plate with the wool for a couple of minutes. Pick up the aluminum pie pan using the pencil as a handle and place it on top of the upsidedown Styrofoam plate that you were just rubbing with the wool. Touch the aluminum pie pan with your finger. You should feel a shock. If you don t feel a shock, try rubbing the Styrofoam plate again. Once you feel the shock, try turning the lights out before you touch the pan again. Check out what you see. You should see a spark. Explanation: Why does this happen? It s all about static electricity. Lightning happens when the negative charges, which are called electrons, in the bottom of the cloud (or, in this experiment, your finger) are attracted to the positive charges, which are called protons, in the ground (or, in this experiment, the aluminum pie pan). The resulting spark is like a mini lightning bolt. References Clouds R Us, 2014, Thunder and Lightning, website visited August 19, NOAA, 2014, National Weather Service, Lightning Safety, website visited August 19, SciJinks, 2014, What causes lightning and thunder?, website visited August 19, UCAR, 2000, Lightning: Just for kids, website visited October 21, Weather Wiz Kids, 2014, Lightning, website visited August 19, Contact Us Website: Physics@portageinc.com 5 P age