What is lightning?
Lightning is an electrical spark. Sparks occur when the Voltage is great enough to overcome the electrical resistance of the air between those points, allowing current to flow. In a thunder cloud the voltage occurs as a result of a large difference in electrostatic charge between the cloud and the ground.
Air is immensely, mind bogglingly resistant. A single cubic meter of air has an electrical resistance of around 13 Quadrillion Ohms (13,000,000,000,000,000 Ω), and the more air between our two points, the more resistant the space.
Lightning typically discharges at around 30,000 Amps and 100 Million Volts. This massive build-up of voltage is enough to ionise the air into a super conductive, super-heated plasma many times hotter than the sun. It’s this heating and ionising process that reduces the resistance of the air along the path of the lightning bolt to around 3000 Ohms along which the electrical current flows back and forth until the charge is dissipated and the path cools. This superheating and ionisation is also responsible for the bright streak of light which we call lightning.