Thunder is the rapid expansion of air, caused by the lightning heating the channel through which it forms. Sound of course travels more slowly than light and hence we hear the thunder after the flash from the lightning. Only when directly overhead is the thunder clash and flash experienced in unison.
Lightning is when a difference of atmospheric temperatures changes dramatically, it needs three components to form: moisture, instability and rising air. The air rising forms cumulonimbus clouds, around 10,700 to 18,300 meters (35,106 to 60,000 feet) in height, where ice crystals develop. It is probable that as the crystals of ice rise and fall, bumping into each other they cause charged particles to separate, building the positive charge at the top of the cloud and a negative charge at the base of the cloud.
The convection movement within the clouds is affected by the amount of heat of the air, building up separated electrical charges. This develops a corresponding build-up of positively charged particles on the ground, climbing anything above ground level, such as trees, poles and occasionally people.
The electrical potential builds and the insulating air layer is broken down. A stepped leader is formed of ionized particles around 80-100 meters (263 - 328 feet) long, from the negatively charged area in the cloud, heading to the positive ground at typically 100 meters (328 feet) per second. This stepped leader is invisible to human eyes, and races towards earth at less than a second per step. It attracts positive streamers from ground level, and just before it connects with the ground, these streamers pass up the channel made by the stepped leader. The ionized air completes the jump to earth with a conductive path, electrical current flows between cloud and earth at 1x108 meters per second, heating the air. This is forked lightning. A single bolt of lightning can reach temperatures of up to 50,000ºC, hold a charge equivalent of 100 million volts and stretch for more than eight kilometers (five miles). When a thunderstorm is raging lightning attacks can occur up to nineteen kilometers (ten miles) away. Discharges within or between clouds causes dramatic flashes of light and is sheet lightning.
Positive lightning is when a positive charge is discharged from the top of a cloud, often striking many kilometers from the storm, is often longer in duration and has typically a higher current.
In Britain on average, thirty to sixty people are struck by lightning per year of which a few are killed, mostly due to heart-related injuries sustained from the strike. Once struck by the lightning, the victims often suffer long term after effects such as headaches, insomnia, deafness, paralysis, memory problems and other brain-damage illnesses.
U.S. statistics place a person's chance of being struck in America at around 700,000 to one, but after living eighty years, this has dropped to only 3,000 to one.
Lightning detectors, used by meteorological offices, detect low frequency electromagnetic waves caused by lightning. Measuring the differences in time it takes for the signal to arrive at the different detector stations, enables meteorologists to pinpoint the location of the storm and track its course.
Benjamin Franklin, an American postmaster, uncovered how lightning works, when he experimented with his kite and keys.
Miles to Kilometers: multiply miles by 1.609