e 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 flash of light and the arrival of the associated thunder can often be used to estimate the distance to a discharge. Every three seconds correspond to one kilometre, and every five seconds correspond to one mile.
The total thunder waveform comes from the entire lightning channel and includes the effects of channel branching and tortuosity, sound propagation in the atmosphere, and acoustic reflections from the local topography. The result is a series of sounds that are variously described as peaks, claps, rolls, and rumbles. At distances of a few hundred metres, thunder begins with a sudden clap followed by a long rumble; at larger distances, it begins with a rumble.
A small percentage of discharges between the cloud and ground are actually initiated at the ground and propagate upward to a charged region in the cloud. These discharges often are initiated (or triggered) by tall structures or by towers on hilltops. The upward branching of such discharges makes them visually distinguishable from their “right-side-up” counterparts, giving the impression of a cloud-to-ground lightning flash that is upside down.
Cloud-to-cloud and intracloud lightning
True cloud-to-cloud lightning is rare because most lightning flashes occur within a cloud. The first lightning flash in a thunderstorm is typically an intracloud discharge. When an intracloud discharge occurs, the cloud becomes luminous for approximately 0.2 to 0.5 second. The discharge is initiated by a leader that propagates between regions of opposite charge (or from a charged region to the neutral atmosphere). Luminosity is more or less continuous and has several pulses of higher luminosity of one-millisecond duration superimposed upon it. This situation suggests minor return strokes as the leader contacts pockets of opposite charge, but the similarity ends there. The total amount of the charge transfer is generally similar to the amount involved in a ground discharge: 10 coulombs, with a range from 0.3 to 100 coulombs. The mean velocity of propagation of intracloud li