Lightning is a massive, rapid discharge of static electricity in the atmosphere. Determining its charge depends entirely on the direction of the electrical current. This phenomenon is a sudden attempt to rebalance a significant electrical imbalance that builds up in a thundercloud. The resulting strike is classified based on the polarity of the charge transferred to the ground, meaning lightning can transfer either negative or positive charge to the earth’s surface.
How Electrical Charge Separates in a Storm
The foundation for any lightning strike begins with the development of intense electrical potential within a cumulonimbus cloud. This process, known as electrification, relies on the turbulent movement of air currents within the storm system. Strong updrafts lift moisture high into the atmosphere where it freezes into various forms of ice, including small ice crystals and soft hail, known as graupel.
Charge separation occurs through the collision between these ice particles, a process called triboelectrification. When lighter ice crystals collide with larger, denser graupel, a charge transfer occurs. The lighter ice crystals acquire a positive charge, while the heavier graupel gains a negative charge.
Vertical winds physically sort these charged particles. Powerful updrafts carry the smaller, positively charged ice crystals upward toward the cloud’s top, or anvil. Conversely, the heavier, negatively charged graupel falls toward the lower portion of the cloud. This results in a massive charge distribution, with a concentrated negative charge at the cloud base and a positive charge at the cloud top, creating the electrical field necessary for a lightning strike.
Understanding Negative Cloud-to-Ground Lightning
Negative cloud-to-ground (CG) lightning is the most frequent type, accounting for about 90 to 95% of all strikes. This discharge begins when the concentrated negative charge at the cloud base overcomes the insulating properties of the surrounding air. A faint, ionized channel called a “stepped leader” emerges from the cloud, carrying negative charge toward the ground in quick, discrete segments.
As the stepped leader nears the ground, the intense electric field attracts a corresponding positive charge from the earth’s surface. This positive charge rises to meet the leader through an upward-moving channel known as a “streamer,” often emanating from tall or conductive objects. Once the descending stepped leader and the ascending streamer connect, a highly conductive path is established.
This connection triggers the “return stroke,” which is the visible, brilliant flash of lightning. The return stroke is a massive surge of electrical current that travels rapidly back up the ionized channel from the ground toward the cloud. This surge drains the negative charge and neutralizes the area, briefly heating the air along its path to temperatures hotter than the surface of the sun.
The Distinct Characteristics of Positive Lightning
While less common, positive cloud-to-ground lightning transfers a net positive charge to the earth. This type originates from the positively charged upper regions of the storm cloud, often the expansive anvil. Because it must travel a greater distance from the cloud top, the electrical potential required to initiate the strike is significantly higher than for a negative strike.
Positive lightning often strikes the ground far away from the main storm cell, sometimes in areas that appear to be clear sky. This distant capability, combined with its longer travel path, results in a discharge that is much more powerful than its negative counterpart. The peak current and charge transferred by a positive flash can be up to ten times greater, sometimes exceeding 300,000 amperes.
The increased energy and longer duration of the positive flash make it more destructive and dangerous. These powerful strikes are frequently linked to significant damage and are a major cause of forest fires. Positive lightning often occurs during the dissipating stages of a thunderstorm or in winter storms, meaning a strike can occur even when the main threat seems to have passed.