Lightning, Beautiful but Dangerous

From GeoClasses

Media:Lightning, Beautiful but Dangerous by Kristen Gill.WAV

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1 Introduction
2 Explanation
3 Types of Lightning
4 Personal Experience
5 References

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Introduction

Lightning, as defined by our book Understanding Weather & Climate, results from the discharge of electricity within clouds. This accounts for about 80% of the lightning we see across the sky called cloud to cloud as shown in figure 1. The other 20% is lightning that we see from clouds to the ground as seen in figure 2.

Figure 1. Image of cloud to cloud lightning.

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Explanation

Lightning is charged particles in the atmosphere that accumulate in a fair weather electric field and discharge through lightning in a storm. The cloud-to-ground lightning occurs with negative charges in the clouds and positive charges on the ground. The air becomes conductive with the positive ions and electrons and a bolt of lightning is discharged. There has been much early research done involving lightning and electricity in the United States and Europe. The first documented experiments were by Dalibard and De Lors in France and Benjamin Franklin in Philadelphia, Pennsylvania. These occurred as early as the 1700’s. Franklin performed his experiment with a kite and a key during a storm. He put his hand near the key and it caused a spark probably from the charged particles in the air. Many experiments with lightning have been fatal since of course, lightning is dangerous. An average of 2,000 storms occur at any moment throughout the world. A bolt of lightning is usually around 3-4 miles long.

Figure 2. Image of cloud to ground lightning.

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Types of Lightning

There are many types of lightning as described in our text. A stepped leader occurs before the actual lightning. It is a shaft of negatively charged air and is seen as a branch from the main shaft of lightning. This shaft when near the ground combines the spark with the leader and a stroke is formed. These are the two common occurrences noted with cloud to ground lightning. Figure 3 shows the sequence of a stepped leader and the shaft of lightning as it occurs.

Figure 3. Image of lighning sequence.

With ball lightning, a mass of light composed of electrified air can be seen before the lightning. Other types include the rare St. Elmo’s fire and sprites which are bursts of bright light. As noted in our text, lightning kills an average 69 people in the United States and 7 in Canada annually. We should take cover when storms occur since lightning can flow through trees, water, and other conductive surfaces. One should be aware of weather conditions at all times. A NOAA weather radio is a great instrument to have on hand to monitor the weather. Channel 5 has recently been promoting their weather radio for around $30 and programming it for free. This is not much to pay for our safety. Unsafe places during storms include any area near metal or water; on hills; next to electrical equipment; and under trees. The safest place recommended by the National Lightning Safety Institute is a large permanent building or metal vehicle. It is considered safe only after at least 30 minutes without any thunder or lightning observed.

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Personal Experience

We had a very large oak tree in our yard hit by lightning. My mother came home from working at the hospital 3p-11p and was looking out the kitchen window during the storm. She saw the top of the tree as it was struck by the lightning and observed a large ball of lightning at the base and the tree was on fire. When it was struck, bark from the tree flew in all directions. It burned for a short time and a large limb of the tree fell to the ground. We had to have it removed for fear of damaging our house. You can see in figure 5 the burned area where the lightning went through the tree.