Lightning is a massive, natural discharge of static electricity that equalizes electrical imbalances in the atmosphere. The most frequently observed type is the negative lightning strike, accounting for approximately 90 to 95 percent of all cloud-to-ground discharges. This electrical event involves the transfer of a negative charge from a thundercloud to the earth below. Understanding this primary form of lightning requires examining the intense charge separation that precedes the flash.
Understanding Electrical Charge Separation
The formation of a negative lightning strike begins with charge separation within a mature cumulonimbus thundercloud. Collisions between ice crystals and soft hail, known as graupel, generate a separation of electrical charges. Lighter, positively charged ice crystals are carried upward by updrafts, accumulating in the upper regions of the storm.
Heavier, negatively charged graupel particles sink, causing the middle and lower parts of the cloud to acquire a net negative charge. This concentration of negative charge at the cloud base induces a corresponding positive charge on the ground directly beneath the storm. The electrical potential difference created between the negatively charged cloud base and the positively charged ground creates the condition for a lightning discharge.
The Three Stages of a Negative Strike
The first stage is the development of the stepped leader, a faint path of negative charge descending from the cloud base. This leader advances toward the ground in short, rapid bursts, or steps, each spanning about 50 meters and lasting only a fraction of a millisecond. The stepped leader branches out as it searches for the path of least resistance to the ground.
As the stepped leader nears the ground, the intense electric field causes a current of positive charge to launch upward from the ground or tall objects, forming an upward streamer. This streamer attempts to neutralize the approaching negative charge. The instant the downward stepped leader connects with the upward streamer, the electrical circuit is complete and the main discharge occurs.
The final stage is the return stroke, the brilliant flash of light that travels rapidly back up the ionized path toward the cloud. This surge of current, which can reach 30,000 amperes, neutralizes the negative charge deposited along the leader channel. The rapid heating of the air within the lightning channel to nearly 30,000 degrees Kelvin creates the intense light and the shockwave heard as thunder.
How Negative Lightning Differs from Positive Lightning
While negative lightning is the standard, positive lightning presents distinct characteristics. Negative strikes originate from the lower or middle sections of the cloud, while positive lightning originates from the upper regions or the anvil of the storm. Because it travels a greater distance, positive lightning can strike the ground many miles away from the main storm cell.
Positive strikes are rarer, typically making up less than five to ten percent of all cloud-to-ground flashes. They carry far greater power and danger than negative strikes. A typical negative strike carries around 30,000 amperes, but a positive strike can generate currents up to 300,000 amperes and voltages up to one billion volts, making them up to ten times stronger.
The energy delivered by positive lightning is sustained for a longer duration, resulting in more destructive power. This increased intensity is often responsible for starting wildfires and causing severe damage to infrastructure. Positive lightning represents a more potent and unpredictable hazard than the common negative strike.