How do thunderstorms build up charge in the sky?

Inside a powerful cumulonimbus cloud, turbulence causes collisions between graupel, ice crystals, and supercooled water droplets. These collisions separate charge, with lighter particles often carrying positive charge upward and heavier graupel tending to accumulate negative charge lower in the cloud. That separation can create electric fields strong enough to ionize air and trigger a lightning strike. The process is still an active area of research, but it is well established that mixed-phase ice clouds are key to storm electrification. Most lightning lasts less than a second, even though the electrical build-up happens over many minutes.