The question of whether a lightning strike travels up or down is a common misconception about thunderstorms. Lightning is a massive electrostatic discharge that neutralizes electrical tension built up within the atmosphere. This natural phenomenon is not a single, instantaneous event but a multi-stage process involving both downward and upward components. The direction we perceive is often misleading because the different stages of the strike travel at vastly different speeds.
How Thunderstorms Generate Electricity
The foundation for any lightning strike is the separation of electrical charges within a cumulonimbus cloud. This process is driven by the storm’s internal turbulence, specifically the collision of various ice particles. Strong updrafts lift lighter ice crystals toward the top of the cloud, while heavier, softer hail, known as graupel, falls downward.
The lighter ice crystals acquire a positive charge and are carried upward, while the heavier graupel accumulates a negative charge and settles in the lower portion of the cloud. This creates a powerful electrical dipole, with a large negative charge concentration near the base and a positive charge aloft. The massive negative charge at the cloud base induces an opposite, positive charge on the ground directly beneath the storm. Air, which is normally an excellent insulator, becomes stressed by this intense electric field. When the voltage difference becomes too large for the air to withstand, an electrical discharge is set to bridge the insulating gap.
The Invisible Path (Stepped Leader)
The lightning process begins with a downward movement from the cloud, known as the stepped leader. This leader is a channel of negative charge that ionizes the air, converting it into a conductive plasma path. It progresses toward the ground in rapid, short bursts, or steps, each approximately 50 meters in length.
The stepped leader is relatively slow, traveling at roughly 200,000 miles per hour, and is generally too faint to be seen. It branches out as it descends, seeking the path of least resistance, which gives lightning its characteristic zigzag appearance. The leader pauses briefly between steps until it nears the ground or an object on the ground. Its goal is to establish a continuous, conductive channel connecting the negative charge in the cloud to the positive charge on the Earth’s surface.
The Visible Surge (Return Stroke)
When the stepped leader nears the ground, typically within 30 to 100 meters, its strong negative charge intensifies the electric field at the surface. This intense field causes positive charges to launch upward from the ground or from tall objects like trees and buildings, forming upward streamers. These streamers attempt to meet the descending leader, often initiating from the highest points to shorten the gap.
When an upward streamer connects with the downward leader, a complete circuit is established, and the return stroke is initiated. The return stroke is a massive electrical current that surges upward along the ionized path created by the stepped leader. This upward surge is the brilliant, blinding flash the human eye perceives as the lightning strike.
It travels at an incredible speed, up to one-third the speed of light, superheating the air in the channel. Because this upward movement is so fast, the entire channel appears to illuminate instantaneously. Thus, while the initial leader moves down to set the path, the spectacular, visible discharge surges back up to neutralize the charge.
When Lightning Strikes Upward
While the most common cloud-to-ground flash involves a downward leader and an upward return stroke, lightning can also originate from the ground and travel upward from the start. This ground-to-cloud lightning is often initiated by extremely tall structures, such as skyscrapers, radio towers, or wind turbines. These objects effectively reduce the distance to the cloud, allowing the electric field to concentrate at their tips. Under the influence of the cloud’s charge, these tall structures can spontaneously launch an upward-moving leader, traveling toward the cloud to complete the circuit.
Another variation is positive cloud-to-ground lightning, which originates from the positively charged upper regions of the cloud, sometimes called the anvil. Positive lightning is less common, making up less than 5% of all strikes, but it is significantly more powerful and carries much higher currents than negative lightning. These strikes often occur late in a storm’s life cycle or are associated with the outer edges of the storm.