Lightning is a common phenomenon—millions of lightning flashes occur every day. Scientists understand the general conditions that are necessary within a storm cloud to produce lightning, but the explanation is incomplete, and the details of lightning formation remain a mystery.

According to a long-held basic theory, lightning is produced by the separation of positively and negatively charged particles within a cloud, which creates an electric field. Scientists are not certain how the charges separate, although it is believed that both the process of freezing and the collision of ice and water particles play a role. It is likely that the strong updrafts and downdrafts within a storm cloud cause water droplets, hail, and graupel (a form of frozen precipitation) to collide with such force that some electrons—the negatively charged components of an atom—are knocked free. This leaves some particles positively charged and gives other particles excess negative charge. The heavier particles with excess negative charge gather at the bottom of the cloud and the positively charged particles gather at the top. The cloud becomes polarized as positive charge builds up on one side and negative charge builds up on the other.

Typically, air is not electrically conductive; it does not allow the flow of electric charges. However, a strong electric field can cause air to "break down"—the electric field frees electrons from air molecules, ionizing the air so that it becomes conductive. The electrical breakdown of air forms a conductive path for electrostatic discharge, the flow of electrons between areas at different electrical potential. As the difference in charge between the top and bottom of the cloud grows, a strong electric field forms in the cloud, which could lead to an electrical discharge in the form of lightning.

However, research has found that the observed electric fields of storm clouds are too weak to produce lightning. One new theory for lightning formation suggests that high-energy particles from outer space may trigger a chain reaction that makes the air conductive. Cosmic rays are subatomic particles that travel close to the speed of light and originate outside of Earth's atmosphere. They can come from a variety of sources, including processes in the Sun, supernova remnants, black holes, or quasars. In a process called "runaway breakdown," cosmic rays entering the atmosphere trigger the production of many high-energy electrons by first colliding with particles to produce a few high-energy electrons. Within the strong electric field of a storm cloud, these electrons further accelerate and collide with other atoms, generating even more high-energy electrons, as well as X-rays and gamma rays, which then collide with other atoms. This process creates an avalanche of high-energy particles, resulting in a breakdown region and a conductive path for lightning.

Print Background Essay