Lightning is a massive and rapid discharge of static electricity within the atmosphere, occurring when the electrical potential difference between two points exceeds the air’s insulating capacity. This sudden equalization of charge generates an intense, superheated channel of plasma perceived as a flash. The power contained within a single flash is immense, but the danger varies significantly depending on the strike’s origin and electrical polarity. For assessing risk to life and property, the focus remains almost exclusively on strikes that make contact with the Earth’s surface.
Classifying Lightning Strikes Based on Location
Lightning is typically categorized by where the electrical discharge occurs: Intra-cloud (IC), Cloud-to-Cloud (CC), and Cloud-to-Ground (CG) strikes. Intra-cloud lightning is the most frequent type, occurring entirely within a single storm cloud as the electrical charge equalizes between different regions. Cloud-to-Cloud lightning bridges the gap between two separate thunderclouds.
IC and CC forms illuminate the sky and produce thunder but pose virtually no direct threat to people or structures on the ground, as the discharge remains contained high in the atmosphere. The only category relevant for surface danger is Cloud-to-Ground (CG) lightning. CG accounts for a small percentage of all flashes but is responsible for nearly all fatalities, injuries, and property damage.
The Critical Distinction: Negative vs. Positive Strikes
Cloud-to-Ground (CG) lightning is further divided into two primary types based on the polarity of the electrical charge transferred to the ground. Negative Cloud-to-Ground (-CG) is the most common, accounting for approximately 90 to 95% of all CG strikes. This type originates from the negatively charged lower region at the base of the storm cloud.
As the negative charge in the cloud base builds, it induces a positive charge on the ground beneath it, leading to a discharge path. The stepped leader, carrying a negative charge, descends toward the ground, connecting with an upward-moving positive streamer to complete the circuit. These common flashes are the familiar lightning seen during most thunderstorms.
Positive Cloud-to-Ground (+CG) strikes are much rarer, making up less than 5% of all CG events. They originate from the positively charged region in the upper part of the thundercloud, often in the anvil. To reach the ground, the electrical leader must traverse a much greater horizontal and vertical distance through the cloud and atmosphere.
This extended path requires the strike to build up significantly more energy before the dielectric breakdown of the air occurs. Traversing a larger distance fundamentally elevates the strike’s power. This increased potential energy results in a far more intense discharge when it finally connects with the ground.
Why Positive Cloud-to-Ground Lightning Is the Greatest Threat
The positive Cloud-to-Ground (+CG) flash is considered the most dangerous type of lightning due to its distinct physical properties. Its origin in the upper regions of the cloud means it can carry a peak current six to ten times greater than a typical negative strike. While an average -CG flash transfers about 30,000 amperes, a large +CG strike can reach peak currents of up to 300,000 amperes and deliver a potential of one billion volts.
This dramatically higher charge delivery is compounded by the flash duration. Positive strikes often have a longer continuing current, meaning the energy is delivered over a greater period of time. This extended duration significantly increases the heat transfer to the struck object, making +CG strikes the primary cause of catastrophic damage and the ignition of wildfires.
The greatest danger is often the lack of warning associated with a +CG strike, a phenomenon known as a “bolt from the blue.” Because these strikes originate high in the storm and travel horizontally out from the cloud’s anvil before descending, they can strike miles away from the main storm cell. These bolts can reach areas with relatively clear skies, sometimes striking 10 to 25 miles away from where thunder is audible or rain is falling. This absence of visible storm activity provides no indication of the immediate threat, catching victims by surprise.
Secondary Dangers and Indirect Injuries
While a direct strike from any type of lightning is devastating, the majority of lightning-related injuries and fatalities result from indirect effects as the electrical energy dissipates across the surface. The most common cause of human and livestock fatalities is the ground current, which occurs when a lightning strike hits the ground or an object and the current spreads outward.
This spreading current creates a dangerous potential difference, known as step voltage, between two points on the ground, such as a person’s two feet. If a person is standing near the strike point, the electrical current can enter one leg and exit the other, traveling through the body’s lower half. Another common indirect mechanism is the side flash, where lightning jumps from a taller struck object, such as a tree or pole, to a nearby person.
The person effectively becomes a shorter, more conductive path for the current to continue its journey to the ground. The combined power of the highly energetic positive Cloud-to-Ground strike, coupled with indirect injury mechanisms like ground current, explains why lightning remains such a significant natural threat. The overwhelming power of the +CG strike delivers a massive amount of energy that spreads over a wide area, intensifying secondary dangers for anyone in the vicinity.