March 23, 1998

The nature of sound
Thunderstorm basics

At any given time during the day, over 2,000 thunderstorms are occurring on the Earth. Thunderstorms are the atmosphere's way of discharging e xcess electrical energy by means of dissipating that energy into heat, light, and sound energy. In this lecture, we will begin with a physical discussion of the nature of sound, which is indirectly related to our next topic, thunder and lightning.

Sound

Sound waves, like all other waves, are means of transporting energy. Sound waves occur when air is forced to vibrate. Perhaps one of the best examples to consider is a guitar string. A guitar string, fixed at both end s, will vibrate when it is plucked. Let's take a closer look at what the results will be.

Consider the vibrating guitar string above. Each time it vibrates, it sets the adjacent air into motion o ne way as the string approaches, and the other as the string leaves. A sound wave has been formed. See the illustration below.

The number of these wave cycles that pass a point in a given time is th e frequency of the sound wave.

Most human beings have a hearing threshold between 20 and 20,000 Hertz. A Hertz (abbreviated Hz) is one wave cycle per second. So 20 Hertz is 20 wave cycles per second.

The intensity or loudn ess of a sound wave is determined by its amplitude. The amplitude is a function of how far the air is displaced as the wave passes.

Since sound waves cause displacements, they do work on the mediums they travel through. Work occurs whe n an object or material is moved through some distance. As is the case with light, sound waves travel at different speeds in different mediums. The speed of sound is proportional to the density and temperature of the fluid it is traveling through.

Thunderstorm basics

Thunderstorms are spectacular displays of sound and light that occur in our atmosphere. The exact mechan isms of a thunderstorm will always be under constant study because they are so interesting and difficult to understand.

In a cloud, there are particles of many different shapes and sizes composed of both liquid water and ice. Since water is a p olar molecule, (this means that it is geometrically arranged in such a way that it is more negatively charged at one end than the other) each ice or water particle has a charge on it. Some particles will be more negatively charged than others. So a particle with less of a negative charge than a neighboring particle will be positively charged relative to that other particle.

The expression "opposites attract" refers to the fact that unlike electrical charges (positive versus negative) a re attracted to one another. When objects are attracted to one another, yet they are separated by a distance, a potential energy exists. Potential energy refers to energy that is waiting and available to do work.

Often, in cumulonimbus clo uds that extend high up into the atmosphere (where it is very cold), there are large amounts of ice. Remember that ice crystals take on different shapes and forms depending upon the temperature they form at. We have plates and rods and a number of varia tions in between.

There tend to be different electrical charges that develop on different shaped ice crystals. Such that at one level of a cloud where the temperature is -15 C, the ice crystals there may take on a charge of -1 whereas higher in a cloud where the temperature may be -20 C, the ice crystals may take on a charge of -2. The upper part of the cloud will be negatively charged with respect to the lower part of the cloud. There will exist an electrical attraction between the two cloud le vels which is potential energy, energy which is waiting to be released.

The cloud levels do not merge together, instead, there is a discharge of this electrical potential energy. The potential energy is converted into heat, light, and sound energy , i.e., thunder and lightning. So thunder and lightning is simply a discharge of electrical potential energy between two different parts of a cloud or, as will be now explained, between different parts of a cloud and the ground.

Eart h's surface is composed of a number of different materials. Organic compounds, silicates, minerals, etc. The Earth as a whole has a slight positive charge. Therefore, if because of the ice crystals developing in a cumulonimbus cloud, parts of that clou d have a positive charge relative to the ground, there will begin to be a buildup of electrical potential energy between the ground and the cloud. Once the energy gets to a certain point, there is a discharge and thunder and lightning are the result.

Charges

In the first few days of the semester, we talked about the atom and its primary components: electrons, protons, and neutrons. Electrons are negatively charged, protons are positively charged, and neutrons are neutral. El ectricity is simply a stream of electrons. Lightning itself is a means of transferring electrons. When a buildup of electrical potential energy occurs, there is an attraction between positive and negative charges. In a discharge (such as lightning), th ere is a transfer of electrons from the negatively charged region to the positively charged region. If an object is negatively charged, that means it has a relative excess of electrons. Electrons are negatively charged and that is how something can beco me negatively charged, if it acquires electrons.

So consider a lightning discharge between a cloud and the ground. The ground feels a relative excess of electrons relative to the cloud because it is has a slight negative charge. Since the atmosphere is always trying to attain balances, it will try to remove the charge gradient between the cloud and the ground by transferring electrons from the ground through a lightning bolt to the cloud.

Have you ever, during the winter, gotten a "shock" when you walked acros s a carpet and turned on the light? The spark that you saw and the snap you heard is essentially a micro version of lightning. You, by walking across the carpet, acquired a negative charge relative to the wall. When you touch the metallic, positively c harged, light switch, there was a discharge of electrons from you to the metal. That is a discharge that occurs over a few millimeters. Lightning is a discharge on a scale of a mile or more! Which is one reason why lightning often attains temperatures over 50,000 F! Higher than the surface of the sun!

You may or may not have heard that if you are in a lightning storm, to keep as far away from metal as possible. This is because metals in general are positively charged and are good conductors of electricity. Lightning will often strike metal as it is a good conductor to transfer electrons out of the ground.

Lightning Safety

Each year, many people are killed by lightning. Many of these deaths could have been preventable if some precautions were taken. Consider the following tips next time lightning threatens. And remember, there are more lightning deaths in Florida then in the rest of the country combined.

• Stay away from trees if you are caugh t outdoors. Trees are good targets for lightning.
• Lightning does often strike twice in the same place. Usually, if lightning strikes somewhere, there was a reason and that location will likely be a target again. Keep away from places that yo u know have been lightning targets in the past.
• Seek shelter at the first sign of dark clouds or the first rumble of thunder. Thunderstorms often creep up unexpectedly. If you are out on a golf course, by the time you realize a storm is near, you ma y not have time to make it to the clubhouse.
• Keep away from water. Water is often a target for lightning. Since water conducts electricity very well, if you are in a small lake, even if the other side of the lake is struck, you may still be electroc uted.
• A car is actually a safe place to be in a thunderstorm, despite its metal exterior. Tires ground the car and keep it in relative electrical balance with the ground underneath. The lightning will not often choose a car as its target.
• Do not hold onto metal objects such as golf clubs or aluminum baseball bats. Many people killed in lightning die on golf courses.
• If you are caught out in the open nowhere near shelter, find a spot in as low an elevation and as far away from trees a s possible. Do not lie flat on the ground, however, drop to your knees. The trick is to remain as low to the ground as possible, while having as little of your body in contact with the ground as possible.
• Indoors: keep away from windows, do not take a shower or bath, and turn off and unplug electrical equipment.

Thunder

Thunder and lightning are caused by the same thing, the discharge of electrical potential energy between two parts of a cloud or the cloud and the ground. Let's look at thunder now and we will reflect on our discussion of sound from above. We also will be visiting with our old friend, the Ideal Gas Law in our study.

Consider a lightning bolt. The air within the lightning shaft is heat ed to a temperature of 50,000 F in matter of milliseconds. If we consider a parcel of air in that shaft that is allowed to expand or contract freely such that the pressure remains constant, it will undergo a violent expansion. Remember from the Ideal Ga s Law, that the volume of a gas is proportional to its temperature.

A sonic boom occurs when something travels faster than the speed of sound. Perhaps those of you who live or have been near an Air Force base have heard a sonic boom when a jet fighter flew by as they often cruise about faster then the speed of sound.

With lightning, the air is forced to expand so fast, that the air is expanded faster than the speed of sound. Hence, a sonic boom, i.e. thunder, is created.

It is actually possible to estim ate how far away a lightning strike was with the aid of a simple trick. If you see lightning, immediately begin counting: one one-thounsand, two one -thousand, etc. Divide the number of seconds you count by five, and that is approximately how many miles away the lightning strike was.

In Wednesday's lecture, we will discuss some further aspects of thunderstorms, such as hail and tornadoes.

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