What is Lightning?

Lightning is a natural atmospheric phenomenon, which is caused by the instability of charge distribution within a cloud. It also occurs due to charge separation in masses of ash and dust ejected in a volcanic eruption or a nuclear explosion.

Generally, a lightning flash originates inside a cloud, several kilometres above the ground level. Except for ball lightning, which is a very rare phenomenon, lightning is simply an electric spark between a cloud and ground, between two clouds or between two parts of a cloud.

The spark that jumps between the ends of two wires, which are connected to the terminals of a car battery, is a very basic form of lightning. In the first stage of the lightning strike, a channel of charge flows towards ground from the cloud.

When this channel is about 50-100 metres above, earthbound objects in the vicinity (e.g. trees, buildings, human beings, animals etc.) start sending upward channels of opposite charge to meet the downward channel from the cloud.

One of these upward channels succeeds in meeting the downward channel and subsequently large current flows through the object and the object is said to be lightning struck.

The lightning current is a short term transient which lasts for about 100 us. However in a single lightning flash such electric current may flow several times into ground.

Current

The lightning current reaches a maximum value of about 30,000 amperes on average but currents in the range of 800,000 amperes have also been reported. Compared to the current that is consumed by our day-to-day electrical appliances, 5-10 amperes, the lightning current is enormous.

The lightning current, while travelling in air, heats its path to a temperature of about 40,000 degree Celsius. Even comparison with the surface temperature of the Sun, which is about 6,000 degrees Celsius, the temperature increment caused by the passage of lightning is extremely high.

This large increment in temperature causes a sudden expansion of air trapped in the passage of lightning current. As the surrounding air is at a much lower temperature this rapid expansion of the air column along the path of lightning current gives rise to a shock wave (similar to what happens in a bomb explosion).

This shock wave, once travels few tens of meters becomes a sound wave which we call ‘thunder’. As sound waves travel at a speed (about 330 m/s) much smaller than the speed of light (3 x 10 8 m/s), we see the light of lightning before we hear the sound of thunder.

This enables us to calculate the approximate distance to the lightning. The number of seconds between the light and the thunder, divided by 3 is approximately equal to the distance to lightning in km. For example a 9s time interval between light and thunder implies that the lightening has struck 3 km away.

Object

Once the lightning current flows in an earth bound object (a tree, flag pole, building etc.) it generates a short term potential difference between two parts of the object along the path of the current.

The magnitude of this potential difference depends on the resistance between the two points of the object. For example when the lightning current flows along highly resistive (let’s ignore the inductance for the convenience) material such as the wood of a tree, the potential difference generated will be very high.

Mega Volts

For lightning with large currents this potential difference may reach values exceeding Mega Volts if the two points of concern are far apart. In such cases not only the potential difference but also the heat generated will be massive.

On the other hand when the lightning current flows along a good conductor, such as a copper tape the potential difference between two points separated by a similar distance, as in the previous case, will be much less, thus the heat dissipation will also be very small. This observation is the basic concept of structural protection systems which will not be covered in this book.

The potential at the point where the lightning current enters the ground is usually at a large value, typically in the order of several tens of kilo Volts. This potential rapidly decreases as one moves radically away from the point of injection of current to ground, giving rise to a so called “ground potential gradient”. This potential gradient becomes significantly large if the earth resistance of the soil is high.

A potential gradient causes surface currents to flow in many directions from the point of strike.

In some experiments done on artificial lightning (so called triggered lightning), it has been observed that lightning may generate surface flashovers (current that flows on the surface of the ground in the form of sparks) that extends to more than 20 m. Therefore, in the subject of lightning safety this potential gradient plays a significant role.

Speed

The lightning inception and propagation. The self ionizing stepped leader, initiated at cloud level propagates towards the earth at an average speed of about 100,000 m/s.

When it is about 50m-100m above the ground level, earth bound objects send oppositely charged streamers to meet the stepped leader.

One of them becomes successful in meeting the stepped leader before the others do so that a large current will flow through the object that sent the successful oppositely charged streamer. Then that object (coconut tree in this case) is said to be lightning struck.

When the lightning current flows along the tree a large potential develops up along the tree, so that with respect to the base of the tree the potential towards the top of the tree will be very large.

Similarly the potential decreases away from the point at which the current enters the ground (base of the tree). When a person is standing in such a way that his feet are apart along the potential gradient, a potential difference develops between the two feet.

As a result, a current will flow through the body of the person which may injure or even kill the person. This is termed stepped potential.

The step potential is more dangerous to four legged animals as their leg span is several times larger than of a human being.In a direct strike, the lightning current may enter the body from the upper parts, flow inside the body (or if the victim is lucky enough, outside the body) and enters the ground.

One could expect the lightning current to flow outside the body if he is wearing wet clothes (due to low resistance) Courtesy: Handbook on Lightning Safety by Dr. Chandima Gomes