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.

Earthbound objects

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 lighting. 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 (eg. trees, buildings, human beings, animals etc.) start sending upwards channels of opposite charge to meet the downward channel from the cloud. One of these upwards channels succeeds in meeting the downward channel and subsequently large current flows through the object and the object is said to be lightning struck.

Electric current

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

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 increments 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 metres becomes a sound wave which we call ‘thunder’.

Lightning before sound

As sound waves travel at a speed (about 330 m/s) much small than the speed of light (3 x 10 8 m/s), we see the light of lightning before we hear the sound of thunder.

Resistive material

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 an example a 9 s time interval between light and thunder implies that the lightning has struck 3 km away.

Once the lightning current flowing 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 an 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.

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.

Good conductor

On the other hand when the lightning current flows along a good conductor, such as a copper take 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.

Higher elevations

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 radially 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.

There are several secondary effects, such as falling from higher elevations due to the momentary shock, falling of heavy materials from structures (detached due to lightning strike) on the person, burns due to volatile materials in the surrounding catching fire, psychological trauma etc.

Nervous system

The passage of current inside the body may lead the person into ventricular fibrillation (unsynchronized muscle operation of the heart), respiratory arrest (inability to breath), burning of vital organs such as brain, liver, kidneys etc. and internal bleeding due to bursting of blood vessels.

The persons may also suffer from nervous system damage, broken bones and loss of hearing or eyesight. Lightning injuries can lead to permanent disabilities or death. On an average, 20 percent of strike victims die and 70 percent of survivors suffer long term disabilities.

Lightning casualties

These injuries are primarily neurological, with a wide range of symptoms, and are sometimes difficult to cure.

A person struck by lightning does not carry any current or charge, hence he should be attended immediately and given first aid. Almost 90 percent of the lightning related deaths due to primary causes could have been avoided if the deceased had been attended within a short time period. The vast majority of lightning casualties can be easily, quickly and cheaply avoided if the proper measures are followed.

Hence, the general public needs increased awareness of the lightning hazards and knowledge of lightning safety including procedures of essential first aid i.e. Cardio Pulmonary Resuscitation (CPR) or at least mouth-to-mouth resuscitation. Courtesy: Handbook on Lightning Safety