Lightning occurs as a sequelae to, 1. Accumulated electrically charged particles on the surfaces of cloud, 2. Snow storm, 3. Dust storm, 4. Volcanic eruption and 5. Nuclear explosion. Lightning of cloud origin, is most common.
Lightning stroke from cloud —
The upper surface of the cloud is positively charged and the lower surface is negatively charged. From the cloud source three varieties of lightning strokes may occur. These are:
1. Intra-cloud lightning – Here lightning occurs due to flow of the negative charge at the lower surface of the cloud, towards the positive charge at the upper surface of the same cloud. These lightnings are not usually visible. Only a glow of the cloud may be seen.
2. Inter-cloud lightning – Here lightning occurs due to flow of the negative charge of one cloud to the positively charged part of another cloud. This variety is appreciable particularly during night, when the spark is seen to move from one region of the sky to another.
3. Cloud to earth lightning – This occurs when the negative charge from the undersurface of a cloud passes to the positively charged earth’s surface. Ordinarily the earth’s atmosphere acts as a good insulator to prevent the passage of the electrical charge from the cloud to pass to the earth. But when the potentiality of the charge on the lower surface of the cloud increases and the potential difference between the cloud and the earth’s surface is great, the negative charge from the cloud crosses the atmospheric barrier and establishes a channel upto the earth’s surface. The phenomenon of lightning is not a single, one-way process of downward stream of negative charge.
The phenomenon of lightning –
When accumulation of the negative charge at the lower surface of a cloud is sufficient to cross the insulating capacity of the atmosphere, it moves downward making an ionised path or channel. The flow is not usually visible due to low luminosity. The speed of this flow is something more than 100 km/second. When this charge is sufficiently strong, it descends down, which occurs in steps and the initial part of the phenomenon (descent) is known as ‘stepped leader’. When the stepped leader is within 30/40 feet of the earth’s surface, it creates an electrical field, sufficient to cause electrical discharge of positive charge from the surface of the earth or any tall structure on its surface. This discharge is known as ‘streamers’. Streamers have a upward movement through the same ionised channel created by the stepped leader. When the streamers meet the stepped leader, a tremendous wave of electric current occurs, which moves downward at a speed of 100000 km/second, known as ‘return stroke’. The return stroke may be of the strength of several hundreds of million volts and several thousand amperes. The breadth of the spark may be 15/20 feet and may affect electrically, anything up to 100 feet apart. The spark may travel a distance of about 1 Km or more before getting neutralized in the earth. The duration of each such stroke is only a few thousandth of a second. In only a few circumstances the process ceases after only one series of phenomena. But in most of the times, the series of phenomena between the cloud and the surface of the earth, is repeated for 3 – 4 times through the same ionized channel or path. The first lightning stroke branches in its return stroke, nearer to the surface of the earth. But the repeating strokes do not branch and reach the earth through the main channel.
Effects of lightnings and thunderstorms –
Vulnerability – During thunderstorm and lightning, the current strike the ground direct, as in case of lightning occurring on a large open field. But the current has a tendency to pass to the earth through a nearby structure of maximum height. This may be a pillar, a tree or a building. In any case the electrical charge tends to pass
to the earth through the surface of these structures. This also happens in case of a house which is not conducted (“grounded”) to the earth, for conduction of lightning electricity. Hence a person inside a house is usually safe from this effect of lightning. A person taking shelter under a solitary tree during thunderstorm, or a person on an open field is quite unsafe. When under a tree, if the lightning strikes the tree then the current passing through the surface of the trunk of the tree, may deviate at any height of the man who has taken shelter under the tree and thus, a part of the electric current may pass through the body of the person. Even if there will not be deviation of the electric current to hit the person, other effects of lightning may be sufficient to kill him.
Effects of lightning and thunderstorm –
1. Due to high voltage electric current.
2. Due to heated air or flash and fire generated.
3. Due to successive expansion and regression of the heated air surrounding the spark.
4. Due to compression of the air occurring constantly throughout the path, ahead of the high speed return stroke (lightning stroke).
As a result of the high voltage electric current, there is spasmodic contraction of the cerebral vessels, resulting instantaneous death due to cerebral anaemia. Cerebral anaemia causes instantaneous stoppage of respiration and there is also instantaneous contraction of the heart muscle due to direct action of the current on the heart.
In less severe cases, there is immediate unconsciousness, from which the person may or may not revive.
When the effect of electrocution is very superficial, arborescent marks, also known as filigree marks or lightning marks, may appear on the body surface, due to prominence of superficial vessels, particularly the veins which are very much congested due to passage of electric current through them. They appear like branching stem of a tree and pass off in a few days if the person survives.
Due to the effect of the high voltage electric current, the metallic wearing materials like the metallic wrist watch band, metallic rings, cause deep burn injuries at their sites, as extreme heat is generated when they attract the current. These metallic belongings also get magnetised due to passage of high voltage electric current through them.
Due to the effect of the generated heat, flash and fire, any degree burn injury may be sustained. Depending on the approximity of the flash, the burn injuries will vary from superficial epidermal burn to deep charring. The clothes may be totally burned.
As a result of expansion of the heated air, there will be a blast effect. This causes extensive mechanical injuries like lacerations and bruises. There may be gross distortion and mutilation of the body. The body may even tear into pieces. There may also be multiple tears on the clothes. Apart from the injuries directly caused, expansion of air may also forcefully throw the body of the victim on the ground against a wall, causing further mechanical injuries.
Regression of the expanded air due to cooling has similar effects but is less intensive than the effects of expansion of air.
The effects due to compression of air in front of the moving spark, is no less severe in its action to cause mechanical injuries.
Postmortem findings —
External findings – Any type and any extent of mechanical and burn injuries may be noticed. With milder electrocution, there may be arborescent marks on the skin. These marks may also be present in severe electrocution, when only a small part of the current passes through the surface of the body, rest passing in deep. In some dead bodies no external signs of the effects of electrocution or fire or any mechanical injury may be present on the body. In these cases, death does not occur due to the direct effects of the lightning, but occurs due to shock when lightning occurs nearby, making tremendous sound.
Internal findings – Where death occurs due to high voltage electrocution and where there has not been much mutilation of the body, haemorrhagic points may be seen on the surface and in the matter of many organs including the brain. There may be tearing of the arteries. There may be shrinkage of the tissue surrounding the arteries. In addition, the brain may be oedematous.
Circumstances of accidental lightning stroke: –
The vulnerable circumstances are –
1. Open field
2. Top of a building or a structure
3. Under a solitary tree
4. Non-conductive or “ungrounded” buildings, where iron and iron strip for earthing of the lightning current has not been used as protective device.
5. An aeroplane passing across a cloud may face or even precipitate a lightning stroke. But mostly no harm is caused as the electric current passes through the surface of the plane. Sometimes there may be some damage to the weak points of the body of the plane and very very occasionally, the fuel tank may be ignited by the heat generated, in which case the plane may crash.
6. Swimming during lightning is specially dangerous because, if the pool suffers the lightning stroke then, the swimmer may get electrocuted by the charged water of the pool. Salt water itself being more conductive, is less dangerous to the swimmer in such a circumstance.
7. “Ungrounded” window grill or even “grounded” window grill in buildings, without protective devices for the entire house, is also dangerous.
Recommended precautionary steps to avoid lightning stroke: –
1. Not to go to an open place during lightning.
2. Not to go on the open roof of a building during lightning.
3. Not to take shelter under a solitary tree.
4. Not to touch a metallic fencing or substance.
5. Not to be near an “un-arrested” telephone wire or antenna.
6. Not to handle any inflammable fuel during lightning.
7. In unprotected buildings, not to go near the metallic window grill or rods, irrespective of whether the grills and the rods are grounded or not. In protected or grounded buildings, not to go near ungrounded window rods or grills.