Lightning is a natural electrical discharge, but different types exist. While the familiar, downward-striking bolt is common, a far more powerful and less frequent variation also occurs. This rare event is known as positive lightning, and its characteristics make it dangerous. Understanding this intense phenomenon requires looking closely at how electrical charge is organized within a storm cloud.
Defining Positive Versus Negative Lightning
Lightning is categorized based on the electrical charge transferred from the cloud to the ground. The most common type is negative cloud-to-ground lightning, accounting for approximately 90 to 95 percent of all strikes. This lightning originates from the negatively charged base of a thunderstorm cloud. A stepped leader, a channel of ionized air, carries a negative charge down to the Earth, which has a net positive charge induced beneath the storm.
In contrast, positive lightning transfers a net positive charge from the cloud to the ground. It originates from the positively charged upper regions of the storm cloud, often the flat, spreading top known as the anvil. Positive lightning is rare, making up less than five percent of all cloud-to-ground strikes.
The electrical leader in positive lightning carries a positive charge downward, meeting a negative streamer rising from the ground. This reversal in charge source and polarity results in significant physical differences in the electrical discharge. Though both types are generated by the same overall storm system, their points of origin within that system dictate their power and behavior.
The Formation Mechanism
Charge separation within a large cumulonimbus cloud is the underlying mechanism for all lightning. Collisions between ice crystals and graupel (soft hail) cause charge separation within the cloud’s turbulent updrafts. Lighter ice crystals carry a positive charge upward, while heavier graupel carries a negative charge downward, creating a positive layer high up and a dominant negative layer below.
Positive lightning occurs when the upper positive charge region is able to connect directly to the ground. This typically happens when the intermediate negative charge layer is too far away or has dissipated, often in the later stages of a storm. Strong wind shear can also tilt the storm column, exposing the high-altitude positive charge outside the main cloud base.
The leader carrying the positive charge must travel a significantly longer path, often horizontally out from the anvil before descending. This greater distance necessitates a stronger electric field to overcome the insulating properties of the air. The resulting positive leader is highly energetic by the time it initiates a strike, distinct from the shorter path taken by a negative leader from the cloud’s base.
Extreme Power and Unique Hazards
The longer distance and stronger electrical field required to generate positive lightning result in a powerful discharge upon striking the ground. A typical negative lightning strike averages about 30,000 amperes, but a positive strike can carry a peak current five to ten times stronger, sometimes reaching 300,000 amperes. Positive lightning also has a longer flash duration, which increases its destructive potential.
The combination of higher current and extended duration makes positive lightning efficient at igniting fires, such as wildfires, and causing severe damage to infrastructure. This higher energy output can overload electrical systems and communication towers. The force of the discharge also contributes to a higher fatality rate for those struck by this less common form of lightning.
One dangerous characteristic is the “bolt from the blue” phenomenon, where positive lightning strikes far from the main storm cell. Because the charge originates in the cloud’s upper reaches, it can travel horizontally for 10 to 25 miles before descending. This means a strike can occur in an area where the sky appears clear and observers may not perceive any risk, catching people completely unaware.