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The physics of lightning and why ‘if it roars, go indoors’ is good advice

Tornadoes, wind, and hail grab the headlines but it’s lightning 
that typically poses the biggest danger

To wrap up our look at severe thunderstorms, we’re going to revisit how lightning is thought to form and then take a look at some lightning facts and safety tips.

Tornadoes, wind, and hail often make the headlines when it comes to thunderstorms, but it is lightning that is often the biggest threat.

For this discussion we’ll look at a typical lightning strike that starts in the cloud and hits the ground.

First of all, lightning is caused by a buildup of electrical charge within a thunderstorm. It is believed that strong up- and downdrafts within a thunderstorm cause particles of dust, water, and ice to hit each other. These millions of collisions allow for electrons to be transferred between particles, causing these particles to become charged. This is a very similar process to the one that gives you a charge when you drag your feet across a carpet in the winter.

Within the thunderstorm, these same up- and downdrafts separate the charged particles into regions, so that some areas of the storm become negatively charged, while other areas become positively charged. Exactly how this happens is still not completely understood.

When an area of the storm gains a strong enough charge, it will act on the air around it and cause it to ionize (the air molecules break apart, forming positive and negative atoms). This ionized air can now conduct electricity.

Meanwhile, on the ground, the strong negative charge in the clouds above the Earth pushes electrons (which are negatively charged) away. In objects that are good conductors, such as metals, the electrons move easily, so these objects become strongly positive — this makes them more attractive to the negative charge in the cloud. You can sometimes see this when you are outside just before a thunderstorm. If the hair on someone starts to stand up, that means that the individual strands of hair have become charged, and since they have the same charge they repel each other and begin standing up. By the way, this is not a good thing, so even though it seems cool and funny it should be telling you that there is a very strong charge in the clouds above you and you need to take cover!

As objects on the ground become more and more positive, they begin to send out what is known as positive streamers. These positive streamers reach out towards the cloud trying to make a connection.

At the same time, a ‘stepped leader’ is moving down from the cloud. This is a narrow channel that is coming down from the base of the cloud, forming a zigzag pattern as it builds towards the ground. This stepped leader or channel is filling with electrons as it makes its way to the ground. Once the stepped leader gets close to the ground, the positive streamers try to connect up with it. Once one of them makes the connection the channel is complete and all the electrons can now flow. This whole process, up to this point, will typically take about one second.

The electrons in the channel that are closest to the ground will begin to flow first, followed by electrons further and further up the channel. As these electrons flow they bump into particles of air, transferring some of their energy in the form of heat. This causes the air to heat up and glow.

Since the electrons flow from the bottom up, it may appear that the lightning originated from the ground even though it originated in the cloud. If there is a large enough charge in the cloud we may see two or three “dumps” of electrons down the original channel. On the ground we would see this as a multiple flash of lightning.

Now on to some lightning facts and safety information.

The average lightning bolt is about two centimetres wide and 7.5 kilometres long. But lightning bolts can be as long as 40 kilometres, with the record length being 190 kilometres. These long bolts of lightning often come out of the side of the storm and can hit the ground a long distance from the storm. This is where the term “bolt out of the blue” comes from.

As far as safety is concerned, there is no truly safe place from lightning if you get caught outdoors.

When I last wrote about this topic four or five years ago, I received several stern emails saying I took lightning safety too casually. The current public information being given out by both Environment Canada and the U.S. National Weather Service is: If it roars, go indoors.

That is, if you see lightning or hear thunder you should head for appropriate cover immediately, which is indoors. I agree that this is your safest and best course of action. It is simple and straightforward.

I always thought lightning safety was common sense, but after watching people around a lake take cover under a group of large trees when they heard a relatively close clap of thunder, I realized that most people don’t have much common sense when it comes to lightning!

About the author

AF Contributor

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the University of Winnipeg. He operates a computerized weather station near Birds Hill Park, Manitoba.


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