Lightning

Lightning is a natural phenomenon consisting of electrostatic discharges occurring through the atmosphere between two electrically charged regions. It usually begins inside a thundercloud, where ice crystals and softer graupel pellets collide as they tumble through air cooled to between −15 and −25 °C. Those collisions strip electrons from one particle and deposit them on another, gradually separating positive charge near the top of the cloud from negative charge near its base until the surrounding air can no longer hold the two apart.

When the field finally breaks down, a faint ionised channel called a stepped leader gropes its way toward the ground in jerky increments, drawing an upward streamer from a tree, rooftop, or lightning rod to meet it. The moment the two touch, a return stroke races back up the channel at roughly a third the speed of light, carrying tens of thousands of amperes and heating the surrounding air to about 30,000 °C—close to five times the temperature of the surface of the Sun. That sudden heating is what we hear as thunder: the air expands so violently that it produces a shockwave which decays into a rolling acoustic boom.

Globally, lightning flashes around 44 times every second, which works out to nearly 1.4 billion strikes per year, with roughly seventy percent of that activity concentrated over tropical land. Only about a quarter of all discharges actually reach the ground; the rest stay aloft as intra-cloud or cloud-to-cloud flashes that illuminate the storm from within. The phenomenon has been recognised and feared since antiquity, but it was not until Benjamin Franklin’s eighteenth-century experiments that it was firmly tied to ordinary electricity—an insight that gave us the lightning rod and, indirectly, much of the language we still use to describe electric circuits today.