A lightning strike is a massive, rapid discharge of static electricity that occurs when the electrical potential difference between a thundercloud and the ground becomes too great. This natural event unleashes a tremendous amount of energy in a fraction of a second, producing observable and hidden phenomena at the point of contact with Earth. The spectacular flash and subsequent sound are the most immediate effects, but the strike also leaves lasting physical changes and creates a significant, often unseen, electrical hazard across the surface.
The Visible Sequence of the Return Stroke
The process begins invisibly within the cloud, sending a faint, negatively charged channel, known as the stepped leader, toward the ground in rapid bursts. This downward-moving leader travels at approximately 200,000 miles per hour and branches out as it seeks the path of least resistance. As the stepped leader nears the surface, its intense negative charge induces a strong positive charge on the ground and on elevated objects beneath it.
This ground charge responds by launching an upward-traveling positive streamer from the highest point in the local area, such as a tree, building, or even a blade of grass. When one of the stepped leader’s branches connects with an upward streamer, a complete conductive path is established between the cloud and the ground. This connection initiates the return stroke, which is the overwhelmingly bright flash recognized as lightning.
The return stroke is the flow of massive electrical current moving at immense speed—up to one-third the speed of light—back up the established channel toward the cloud. This rapid current superheats the air in the channel, turning it into a brilliant, intensely luminous plasma. Because this illumination propagates upward from the ground connection point, the human eye perceives the flash as traveling from the ground skyward, even though the process started in the cloud.
Physical Marks at the Point of Contact
The extreme energy of the return stroke instantly vaporizes moisture and materials at the point of contact, often resulting in a small crater or depression in the ground. The immense, localized heat, which can reach 50,000 degrees Fahrenheit (30,000 degrees Celsius)—five times hotter than the surface of the sun—melts the surrounding soil or rock. The explosive expansion of vaporized water and superheated air can throw soil and debris outward from the strike point.
Fulgurites
In sandy or silica-rich environments, this intense, rapid heating and cooling creates a unique geological structure known as a fulgurite, or “fossilized lightning.” Fulgurites are hollow, glassy tubes formed when the sand melts and fuses instantly along the path the electrical current takes into the earth. These structures often resemble branching, root-like systems extending several feet underground and provide permanent physical evidence of the strike.
The Spread of Ground Current
Upon striking the ground, the immense electrical charge rapidly disperses radially outward through the earth’s surface. This phenomenon, known as ground current, represents a significant, invisible electrical hazard. The electrical potential, or voltage, of the ground decreases as the distance from the strike point increases.
Step Voltage Hazard
This gradient in electrical potential creates a danger known as step voltage, which can be lethal to humans and animals. Step voltage is the difference in voltage between two points on the ground separated by a short distance, such as a person’s stride. If a person is standing near the strike point, one foot may be at a significantly higher electrical potential than the other, causing a substantial current to flow across the body. The magnitude of this hazard is strongly influenced by the soil’s resistivity; high-resistivity soil like dry sand or rock causes the voltage to drop off more slowly and spread the danger farther.
Auditory and Thermal Consequences
The final, and perhaps most recognizable, sensory consequence of the strike is the sound of thunder. Thunder is produced by the sudden, extreme heating of the air within the lightning channel during the return stroke. The air along the channel is heated to temperatures exceeding 50,000 degrees Fahrenheit almost instantaneously.
This rapid heating causes the air to expand explosively at supersonic speed, creating a powerful shockwave. The characteristic sound depends on the distance and the complexity of the lightning channel. A strike nearby generates a sharp, loud crack as the shockwave arrives quickly, while a distant strike produces a low, prolonged rumble as the sound from different parts of the channel reaches the observer at varying times. The sound always arrives after the flash because light travels vastly faster than the sound wave generated by the superheated air.