Why do some clouds produce lightning?

Lightning is produced almost exclusively by cumulonimbus clouds, the towering thunderclouds that reach high into the cold upper atmosphere. Inside these clouds, powerful updrafts hurl water droplets and ice crystals against each other, and these collisions transfer electric charge. Lighter, positively charged particles tend to gather near the top of the cloud while heavier, negatively charged ones sink to the bottom, creating a huge voltage difference. When the electrical stress becomes too great for the air to resist, it breaks down and a lightning bolt discharges the charge, heating the air to around 30,000 degrees Celsius. That sudden heating makes the air expand explosively, producing the shock wave we hear as thunder.