Video narration
How do clouds produce rain? Think of the beads that appear on a cold window: invisible water vapor cools and becomes tiny visible drops. A cloud is made of droplets like these, and sometimes ice crystals, all suspended in air. Most cloud droplets are too small to fall as rain on their own. In warmer clouds, tiny drops drift around, bump into one another, and join. As they grow, they become heavier. In colder clouds, ice crystals grow instead. If they fall into warmer air below, they melt into raindrops. Rain isn't a cloud overflowing. The particles have to grow until air currents can no longer hold them up. Some drops still evaporate before reaching the ground. But when they survive the fall, the answer is simple: droplets merge, or ice grows and melts, until rain can fall.