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Lightning forms due to the separation of electrical charges within a thunderstorm cloud (cumulonimbus). As ice crystals and water droplets collide inside the cloud, electrons are transferred: smaller, lighter ice crystals become positively charged and rise toward the cloud top, while heavier graupel (soft hail) acquires a negative charge and sinks toward the cloud base. This creates a large electric potential difference — the bottom of the cloud becomes strongly negatively charged, which induces a positive charge on the ground below. When the potential difference becomes large enough to overcome the insulating resistance of the air (roughly 3 million volts per metre), a stepped leader — an invisible channel of ionised air — propagates downward from the cloud in a series of steps. Simultaneously, positive streamers rise from tall objects on the ground. When a stepped leader meets a streamer, a conductive channel is complete. A massive return stroke — the visible lightning bolt — then rushes upward along this channel at roughly one-third the speed of light, heating the surrounding air to around 30,000 K. The rapid expansion of this superheated air produces the shockwave we hear as thunder.
answered by Omniscientia Team · 190 words · 18 Mar 2026