y a few microseconds. As the leader charge avalanches toward the ground, the return stroke luminosity propagates toward the cloud base at an average speed of 5 × 107 to 2 × 108 metres per second, or approximately one-third the speed of light, and the high-current-carrying core expands to a diameter of a few centimetres. Laboratory experiments suggest that when pressure equilibrium is attained between the return stroke and the surrounding air, the channel approximates a high-current arc characterized by a current density of 1,000 amperes per square centimetre.
Subsequent return strokes
In the rapid passage from ground to cloud, the luminous return stroke is observed to pause at points where large branches join the main channel, and the channel is observed to brighten as charge from the branch flows into the channel. The stroke then continues its upward propagation, reaching the level of the atmosphere where the temperature is 0 °C (typically at an altitude of 5 km [3 miles] above sea level) in approximately 100 microseconds; the downward-propagating stepped leader traverses the same distance in about 30 milliseconds (thousandths of a second). There is then a pause for tens of milliseconds, and the channel cools to a few thousand degrees Celsius. If a second stroke occurs, it begins with the appearance of a dart of light, perhaps 30 to 50 metres (100 to 160 feet) in length, propagating down the channel of the previous return stroke. The dart leader moves downward at a speed of 2 × 106 metres per second (about one one-hundredth the speed of light) and carries a current of the order of 1,000 amperes toward the ground. Once again, when the leader effectively short-circuits a charge centre in the cloud to the ground, another return stroke occurs. After the first stroke, the dart leader may follow the lightning channel only partway before taking a new path to the ground. This gives rise to the common forked appearance of lightning as it strikes the ground.
This sequence of dart leader-return stroke typically occurs three to four times, although a flash to the ground that had 26 strokes and lasted two seconds has been reported. When a flash does have more than one stroke, the subsequent return strokes draw charge from different regions of the parent thunderstorm. Multiple strokes of lightning appear to flicker because the human eye is just capable of resolving the time interval between them.
Dissipation of energy
During the return-stroke stage, approximately 105 joules of energy per metre are dissipated within the lightning channel. This energy is divided among the dissociation, ionization, excitation, and kinetic energy of the particles, the energy of expansion of the channel, and radiation. Spectroscopic measurements reveal that the air molecules, principally those of nitrogen, oxygen, and water, are split into their respective atoms and that on the average one electron is removed from each atom. The conversion from neutral air molecules to a completely ionized plasma occurs in a few microseconds.
When the stroke plasma is created, its temperature is at least 30,000 °C (50,000 °F), and the pressure is greater than 1,000 kilopascals (10 atmospheres). The channel pressure greatly exceeds the ambient (surrounding) pressure, and the return-stroke channel expands at a supersonic rate. The resultant shock wave decays rapidly with distance and is eventually heard as thunder once it slows to the speed of sound. Because it is estimated that only 1 percent of the input energy is stored in the particles and less than 1 percent is emitted as radiation in the visible and infrared region (4,000 to 11,000 angstroms [Å], where Å = 10−10 metre), it is probable that most of the energy dissipated goes into the energy of channel expansion, a process requiring no more than 10 to 20 microseconds.
Since light travels at about 300,000 km (186,000 miles) per second and the speed of sound is only about 0.33 km (0.2 mile) per second, the light from a discharge will always be seen before the sound arrives at an observer. The time delay between the bright