The lightning phenomenon

People have always been fascinated by lightning. However, it was only in the mid-18th century that the link between lightning and electricity was established, thanks in particular to experiments by Benjamin Franklin. Since the 1980s, the development of lightning detection networks has helped us to improve our understanding, analyse and quantify the phenomenon. For the past 25 years, Météorage has been one of the leaders in the operation of such networks and associated services.

Introduction

 

Since the dawn of humanity, people have been fascinated by lightning. At that time the thunderstorm phenomenon was seen as having a mystic dimension, expressing the anger of the gods.

It was in the 20th century that the study of lightning became a science. It was at that point that thunderstorms were found to be essential to life, that they maintained our planet’s electrical field.

From the mid-20th century to the present day, the development of energy distribution and transport, IT and electronic networks has been a considerable driver for research into ways of protecting against lightning and thus study of the phenomenon itself. 

 

  1. The phenomenon
  2. Lightning around the world 
  3. Effects and consequences
  4. Safety regulations
  5. Terminology

 

 

1- Physical phenomenon

 

Although flashes of lightning can be observed in sandstorms, snowstorms or burning clouds of ash from volcanic eruptions, the main generator of thunderstorms is the cumulonimbus.

 

LaFoudre1

 

This cloud can develop to an altitude of more than 10 kilometres, and its base, which is roughly 2 to 3 kilometres above ground level, can be tens of square kilometres in area. It is the focus for strong ascending and descending winds, causing collisions between ice crystals surrounded by supercooled water. It is the friction between those particles which causes the cloud to become electrically charged, and the charges move apart. The heaviest particles (water droplets) are negatively charged and can be found at the base of the cloud, whilst the lighter positively-charged particles (ice crystals) can be found at the top of the cloud. Sometimes a positively-charged cell can be found caught within the negatively charged mass. 

 

LaFoudre2

 

Under the influence of the negative charges found at the base of the cloud, the atmospheric electric field at ground level, which is usually in the order of 100 volts/metre becomes inverted, reaching values in the region of -15 to -20 kV/m. That potential difference between the cloud and the ground enables an electrical discharge to be triggered.

There are two types of lightning: intra- (or inter-)cloud where the discharge occurs within a single cloud (or between two clouds), and cloud-to-ground, or lightning strike, where the discharge is between the cloud and the ground. These last ones are responsible for a great deal of damage and loss to the environment, buildings and people.                                                    

In the majority of cases, the lightning strike (or cloud-to-ground) is downward-moving. The discharge phenomenon is initiated by a success of pre-discharges from the cloud to the ground, progressing in a series of steps (stepped leader). 

LaFoudreschéma

  

When one of the points on the leader is sufficiently close to the ground, upward-moving pre-discharges (tracers) form and meet up with the downward leader. A conductive channel is then established between the cloud and the ground, allowing a high-current electrical discharge to travel along it. This current consists of the surface charge from the ground which travels up the ionised channel, neutralising the charges from the cloud: this is the return stroke. This current heats up the air, causing it to expand explosively:  thunder.
The first discharge leaves a conductive path behind it, connecting the cloud to the ground.

Moments later (between 10 and 70 ms), new downward-moving leaders and return strokes will follow the same path, travelling much faster. On average, there will be 4 discharges back and forth along a single path. The human eye generally sees only one flash due to what is known as persistence of vision.
In the minority of cases, upward-moving bolts of lightning may be seen; in such cases, the leader develops from a prominent feature on the ground. The phenomenon may be observed in the mountains, on metallic towers or high-rise buildings.

In addition to their upward or downward movement, lightning strikes are also described in terms of their polarity. In fact, it is possible to have negative bolts of lightning, from a negatively-charged cell, or positive bolts of lightning from a positively-charged cell. 

 

 

2- Lightning around the World: Some figures

 

Lightning can strike anywhere in the world, but much more so in hot, humid regions. A map produced from data collected by the LIS and OTD satellites over an 8-year period shows that very little lightning occurs near the poles, and relatively little in mid-ocean, but the inter-tropical regions of Africa and Latin America are very prone to lightning strikes.

nasa

It is estimated that at any given moment there are 2,000 thunderstorms occurring around the world. Those storms produce between 30 and 100 cloud-to-ground lightning strikes per second, or roughly 5 million bolts of lightning per day.

Lightning strike activity at a given point is expressed in terms of the keraunic level, or “number of days when thunder can be heard”. That number can be 300 in regions such as Indonesia.

A more specific measurement of lightning strike activity can be derived by means of automatic detection networks. Ground impact density is used, which measures the number of impacts per surface unit per unit of time. In some regions of the Congo, it can be as much as 158 impacts per km² per year, but disparities within any given country can be huge.

 

3- Effects et consequences

The main effects of lightning are as follows :

 

Heating effects

These effects are linked to the quantity of charge flowing through the lightning strike. It translates into melting points of varying size where the lightning makes contact with a conductive material, or an increase in temperature at points of poor contact with highly resistive materials. In low-conductivity materials, a huge amount of energy is released in the form of heat; the moisture contained in them can then turn to vapour with a pressure high enough to blow them apart. That process can be seen, for example, when a building suffers a direct lightning strike.

Sound effects

The electrodynamic forces linked to the current flowing through the lightning cause expansion of the air along the lightning channel, increasing the pressure within that channel. The increase in pressure and its sudden disappearance create a shockwave. The distance of the lightning channel and its direction relative to the observer will determine the perceived sound spectrum.

Light effects

The effect on installations is limited to optical equipment. Ocular lesions may occur in people.

Electrical effects

Excess voltage by conduction: when lightning strikes a power line, the electrical pulse travels along the conductor, generating a very high additional current in the power line, which in turn leads to an overvoltage. This phenomenon almost always results in a short-circuit.

Earth potential rise: resistance in the earth means that earth connections are resistant, and that causes a sudden rise in voltage at the installation when the current from the lightning passes through.

Magnetic induction: the lightning strike is accompanied by electromagnetic radiation. If that radiation reaches a conductor (a power line, for example), the electromagnetic flux can generate high induced voltages. 

 

Consequences for people, living organisms and property

Humans and living organisms

People can be exposed to lightning in various ways:

  • - direct lightning strike: the electrical discharge arises from direct impact with the person.
  • - side flash lightning strike: the current travels down something which is a poor conductor, before choosing a path of least resistance, which may be a person standing nearby.
  • - step voltage lightning strike: when the lightning strikes a point on the ground, there is sufficient potential difference to generate a current which travels along the lower limbs of a person or animal.
  • - touch voltage lightning strike: if someone touches two conductors at the same time, one of which is affected by an overvoltage due to lightning, the potential difference between the two conductors is such that the lightning current travels through the person’s body.
  • - induced current lightning strike: lightning strike through the capacitive channelling of a downward-moving fork of lightning.

The major risk from a lightning strike is that of cardiac arrest. As in all cases of electrification, only immediate cardiopulmonary resuscitation will save the victim’s life. Other symptoms are possible and a specialist examination will be needed to identify them.

The injuries which may be encountered include burns or neurological, cardiovascular and pulmonary, traumatic, ear and eye injuries. There are safety regulations which must be followed to guard against such types of accident.

Property

A direct lightning strike may result in:

  • - the destruction of buildings and equipment by fire or explosion,
  • - accidents associated with handling flammable products at the time of the thunderstorm.

Overvoltages travelling along power lines cause damage to all sensitive equipment:

  • - damage to electronic components and other parts,
  • - malfunction of automatic machinery and computer equipment,
  • - reduced service life of electronic components,
  • - industrial production lines brought to a standstill,
  • - lost production

   

4- Safety Regulations

 

Lightning protection is governed by laws, regulations and standards which differ from country to country. Best practice has also been established in certain business segments such as aeronautics and the oil industry.

By way of illustration, here is a non-exhaustive description of the current protection techniques:

Protection against the risk of overvoltage from external networks

  • External conductive networks not used to transmit energy (metal pipework) must be connected to the earth network in order to avoid risks linked to potential difference.

Protection of internal networks

  • - Equipotential bonding is a prerequisite for good protection against lightning strikes; it is a reliable way of preventing sparkover or damage to equipment. To achieve it, all earth connectors must be networked and all metallic structures should be interconnected by means of that network.
  • - Cableways: protected cables should be separated from unprotected cables.
  • - Reduced loop areas: despite good equipotential bonding, there is still a risk of overvoltage due to magnetic fields inducing voltages in any looped electrical conductor.
  • - Grouping sensitive equipment together is of value in risk management.
  • - Surge suppressor: limits its amplitude upstream of the installation.
  • - Lightning arrestors and surge protectors: their role is to restrict transitory overvoltages to levels which will not harm equipment and to carry discharge currents to earth.

Protection of buildings

The lightning conductor on the top of a building enables the lightning strike to be captured and the energy channelled safely to earth. There are three types of lightning conductor:

  • - Lightning rod: a metal rod placed on the tallest part of the building. The energy will be carried to earth along a vertical path (such as a Franklin lightning rod – sparkover device).
  • - Cage lightning conductor: consists of constructing a cage around the whole building. Conductors may be fitted with short points on the parts at the top of the building. Each descending conductor is connected to an earth connection.
  • - Taut wire lightning conductor: a system comprising one or more conductive wires strung across the installations they protect. This method is particularly frequently used to protect high voltage power transmission lines.

The earth connection as part of an earth grid means that there is no potential difference between conductive parts or devices. It generally allows energy to be carried away to earth.

 

Recommendations for people to protect themselves against lightning

  • - avoid open-air activities.
  • - avoid using the telephone.
  • - avoid running or taking large steps.
  • - avoid natural shelters (caves etc.).
  • - take shelter in an enclosed building if possible, otherwise in a car.
  • - do not shelter under a lone tree.
  • - if there is no shelter available, crouch down with your feet together and knees bent against your chest.
  • - avoid cycling or horse-riding.
  • - in open spaces, do not carry metal objects (umbrella, golf club, fork etc.).
  • - avoid any contact with metal objects (water pipes), or electrical equipment.
  • - unplug the television aerial and power cord.
  • - in the mountains, do not stand against a rock wall, but move away from it.
  • - on a boat, connect the boat’s metal mast to the hull if it is made of metal, or to a metal chain dangling in the water. 

 

5- Terminology

 

LF : Low frequency  technology (Vaisala LS7001 sensors installed in France and The United Kingdom)

VHF : Very high frequency technology (Vaisala LS800 sensors installed around Paris)

 

Term Description Detection technology Details
Flash Set of breakdowns and current discharges for a lightning event. LF and VHF

Generic term.

A Flash can occur within clouds (CC) or between a cloud and the groune (CG)
A Flash can be composed of one or many Strokes.
A Flash combines current discharges that are detected in LF and electrical breakdowns that are detected in VHF

Stroke or return stroke Electrical discharge in a lightning flash to earth  LF  
First stroke First stroke of a cloud to ground flash LF  
Subsequent Stroke  All the remaining strokes after the 1st one (the 2nd, 3rd, …) in a cloud to ground flash LF  
Flash A shortcut to define a flash by its 1st stroke LF The 1st stroke is used to represent a group of strokes
Intra-cloud Intra-cloud current discharge LF Between two cloud (CC) or within a cloud (IC)
VHF Flash Set of electrical bursts in an event VHF Generic term.
A VHF flash is an intra-cloud phenomenon, it is made of bursts that can be grouped in branches
Source (or burst or breakdown) Emission of an electrical burst VHF Detected only in VHF
Branch or ramification Grouping of the sources belonging to one flash, used to draw their development in space and time VHF A group of VHF sources represented by branches
Flash extent Contour of the sources belonging to a single flash VHF Represented by a polygon or an ellipsis
Source density Number of sources as computed on a grid for a given time period VHF Represented by a grid or surface

 

 

 

 

 

[1] source : C. GARY « la foudre » éditions Masson / Introduction p XIII and METEORAGE data