On average, the Earth receives 40 to 50 lightning strikes per second, or more than 1.4 billion lightning strikes per year (NOAA, 2023). Yet, the mechanism that produces lightning remains largely unknown to the general public — and even to many building professionals.
Understanding how lightning forms is not just a scientific curiosity. It is also the essential basis for correctly sizing a lightning protection system that complies with the IEC 62305, anticipating risks, and justifying technical choices to clients, project owners, and insurers.
What is lightning?
Lightning is a very high-intensity natural electrical discharge that occurs between two areas of different electrical potential: between two clouds, within the same cloud, or between a cloud and the ground. It is this last category—cloud-to-ground lightning—that presents the main risk to structures and people ( National Weather Service, NOAA ).
A cloud-to-ground lightning strike releases energy of up to 1 to 5 gigajoules, with peak currents reaching 200,000 amperes in extreme cases — although the median value is around 30,000 amperes (CIGRE WG C4.407, Technical Brochure 549).
How does a storm cloud form?
The formation of a thunderstorm begins with vertical atmospheric instability. Warm, humid air rises rapidly, forming a cumulonimbus cloud (anvil-shaped cloud) that can reach an altitude of 10 to 15 km. It is within this cloud that the separation of electrical charges occurs.
This separation process results from several mechanisms:
- Collisions between ice crystals and graupels : at temperatures between -10°C and -25°C, light ice particles (crystals) acquire a positive charge and rise, while graupels (soft hailstones) acquire a negative charge and descend.
- Temperature gradient : the temperature difference between the top of the cloud (very cold) and its base creates a movement of charges.
- Electrical effects of water droplets : violent convective movements inside the cloud amplify the separation.
Consequently, a typical configuration is observed: a zone of dominant negative charge at the base of the cloud (between -10°C and -20°C) and a zone of positive charge at the top. Furthermore, a pocket of positive charge can form at the base of the cloud, generating the famous "positive lightning," which is more powerful but less frequent.
How is a cloud-to-ground lightning strike triggered?
When the potential difference between the base of the cloud (which is negatively charged) and the ground becomes sufficient—usually several hundred million volts—the electrical breakdown process begins. However, it does not occur instantaneously: it follows a precise, multi-stage sequence.
Step 1: The stepped leader
An invisible channel of plasma, called a stepped leader , propagates towards the ground in successive jumps of 50 to 100 meters, at a speed of approximately 200,000 m/s (Uman, MA, "The Lightning Discharge," Academic Press). This leader ionizes the air as it passes, creating a channel of negative charge conductivity.
As a result, as the tracer descends, it induces an accumulation of positive charges on the ground surface — particularly concentrated on prominent objects: trees, antennas, lightning rods, buildings.
Step 2: Upward Leaders
Simultaneously, upward leaders rise from the ground from prominent points. This is the phenomenon on which the principle of the Early Streamer Emission (ESE) lightning rod : by emitting an upward leader before surrounding structures, the ESE intercepts the downward leader and captures the lightning first.
When one of these upward strokes joins the downward stroke, the connection is established. This connection (or "return stroke") marks the beginning of the stroke return.
Step 3: The return stroke
Once the connection is established, the conductor channel is traversed by an intense, luminous current that travels from the ground to the cloud at a speed of approximately 100,000 to 150,000 km/s, or 1/3 to 1/2 the speed of light (NOAA NESDIS). It is this return current that we see as lightning.
Next, the temperature in the channel reaches 27,000 to 30,000 K — about 5 times the surface temperature of the Sun. This explosive temperature increase causes the surrounding air to expand rapidly, which we perceive as thunder.
Step 4: Multiple returns (subsequent strokes)
A single lightning flash often comprises several return strikes, separated by a few milliseconds. On average, a cloud-to-ground lightning flash has 3 to 5 return strikes (CIGRE Technical Brochure 549, 2013). This phenomenon explains the characteristic twinkling observed during a thunderstorm.
How long does a lightning flash last?
The total duration of a flash, from the initiation of the tracer to the last return stroke, is generally 0.2 to 1 second. Each individual return stroke lasts less than 1 millisecond. Yet, in this tiny amount of time, considerable energy is released into the plasma channel.
Why is this physics essential for lightning protection?
Understanding the physics of lightning has direct and measurable implications for the protection of structures:
- Calculating the protection radius : IEC 62305-3 uses the rolling sphere method, where the radius depends on the chosen protection level. This calculation is based directly on the physical parameters of the lightning current (peak current, load).
- Lightning strike density (Ng) : The IEC 62305-2 standard uses ground lightning density (Ng, in impacts/km²/year) to calculate the risk. In France, the average value is around 1.5 to 2 impacts/km²/year (Météorage data), but it can exceed 4 in certain regions of the Massif Central or the Pyrenees.
- Conductor sizing : Down conductor cross-sections are calculated to withstand the thermal effects of lightning current, the values of which are defined by IEC 62305-1.
- Surge protection : Lightning current induces surges in nearby electrical and communication networks, requiring surge arresters sized according to the energy levels defined by the standard.
Therefore, the physics of lightning is not an academic abstraction: it is the basis of every engineering choice in a lightning protection project.
The role of LPS Manager in risk data management
In a lightning protection plan compliant with IEC 62305, all this data—lightning density, risk parameters, protection levels—must be documented, archived, and updated regularly. LPS Manager is the professional software designed to centralize all this information by site, simplify risk calculations, and generate regulatory reports in accordance with IEC 62305.
In addition, LPS Manager provides access, via Strike Radar, to lightning density data (NSG) from real-time atmospheric monitoring — thus enabling the refinement of risk analyses with precise local data.
Conclusion: Lightning, a science at the service of safety
In summary, a cloud-to-ground lightning strike is the result of a complex, multi-step physical process: charge separation within the cumulonimbus cloud, stepped propagation of the leader, joining with updraft leaders, and return strike. Each step of this process is quantifiable and standardized.
Understanding the physics of lightning allows for the design of effective protection systems, the justification of technical choices to clients and regulatory bodies, and the optimization of installations based on the actual site parameters. This is why LPS France places scientific rigor at the heart of every lightning protection project, both in Europe and internationally.
