An abrupt, high-current electric discharge that occurs in the atmospheres of the Earth and other planets and that has a path length ranging from hundreds of feet to tens of miles. Lightning occurs in thunderstorms because vertical air motions and interactions between cloud particles cause a separation of positive and negative charges.

The vast majority of lightning flashes between cloud and ground begin in the cloud with a process known as the preliminary breakdown. After perhaps a tenth of a second, a highly branched discharge, the stepped leader, appears below the cloud base and propagates downward in a succession of intermittent steps. The leader channel is usually negatively charged, and when the tip of a branch of the leader gets to within about 30 m (100 ft) of the ground, the electric field becomes large enough to initiate one or more upward connecting discharges, usually from the tallest objects in the local vicinity of the leader. When contact occurs between an upward discharge and the stepped leader, the first return stroke begins. The return stroke is basically a very intense, positive wave of ionization that propagates up the partially ionized leader channel into the cloud at a speed close to the speed of light. After a pause of 40–80 milliseconds, another leader, the dart leader, forms in the cloud and propagates down the previous return-stroke channel without stepping. When the dart leader makes contact with the ground, a subsequent return stroke propagates back to the cloud. A typical cloud-to-ground flash lasts 0.2–0.3 s and contains about four return strokes; lightning often appears to flicker because the human eye is capable of just resolving the interval between these strokes.

Lightning between cloud and ground is usually classified according to the direction of propagation and polarity of the initial leader. For example, in the most frequent type of cloud-to-ground lightning a negative discharge is initiated by a downward propagating leader as described above. In this case, the total discharge will effectively lower negative charge to ground or, equivalently, will deposit positive charge in the cloud.

A discharge can be initiated by a downward-propagating positive leader. Positive discharges occur less frequently than negative ones, but positive discharges are often quite deleterious. Another type of lightning is a ground-to-cloud discharge that begins with a positive leader propagating upward; this type is relatively rare and is usually initiated by a tall structure or a mountain peak. The rarest form of lightning is a discharge that begins with a negative leader propagating upward.

The electric currents that flow in return strokes have been measured during direct strikes to instrumented towers. The peak current in a negative first stroke is typically 30 kiloamperes, with a zero-to-peak rise time of just a few microseconds. This current decreases to half-peak value in about 50 microseconds, and then low-level currents of hundreds of amperes may flow for a few to hundreds of milliseconds. The long-continuing currents produce charge transfers on the order of tens of coulombs and are frequently the cause of fires. Subsequent return strokes have peak currents that are typically 10–15 kA, and somewhat faster current rise times. Five percent of the negative discharges to ground generate peak currents that exceed 80 kA, and 5% of the positive discharges exceed 250 kA. Positive flashes frequently produce very large charge transfers, with 50% exceeding 80 coulombs and 5% exceeding 350 coulombs.

Red sprites, elves, and blue jets are upper atmospheric optical phenomena associated with thunderstorms and have only recently been documented using low-light-level television technology. Sprites are massive but weak luminous flashes appearing directly above active thunderstorms coincident with cloud-to-ground or intracloud lightning. They extend from the cloud tops to about 95 km (59 mi) and are predominantly red. High-speed photometer measurements show that the duration of sprites is only a few milliseconds. Their brightness is comparable to a moderately bright auroral arc. Elves are associated with sprites. They are optical emissions of approximately 1 millisecond, with a fast lateral, horizontal expansion that emits more red than blue light. They occur at altitudes of 75–95 km (47–59 mi). Blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following the emergence from the top of the thundercloud, they typically propagate upward in narrow cones of about 15° full width at vertical speeds of roughly 100 km/s (60 mi/s), fanning out and disappearing at heights of about 40–50 km (25–30 mi).