How does a cloud that floats turn into rain that falls?

A cloud is a visible mass of miniature liquid droplets and ice crystals suspended in the air, and it turns into rain when those droplets grow heavy enough to fall. Cloud droplets are tiny, around 0.02 mm across. A typical raindrop is about 2 mm. Rain is what happens when the small stuff finds a way to become much bigger.

Everything starts with a speck. Water vapour needs a non-gaseous surface to turn into liquid, and in the atmosphere that surface is a tiny solid or liquid particle called a cloud condensation nucleus. These can be dust, soot from fires, sea salt from ocean spray or sulfate from volcanoes, and a typical one is only about 0.0001 mm across.

Why doesn't a cloud just drop its water at once? Air resistance typically keeps the droplets in a cloud stationary. When air turbulence occurs, droplets collide and produce larger droplets. As these descend, coalescence continues until the drops are heavy enough to overcome air resistance and fall as rain. This is called the warm rain process, and it happens most often in clouds that stay above freezing all the way to their top.

In clouds below freezing there is a second route. Ice crystals that gain enough mass begin to fall, and they can melt into raindrops on the way down if the lower air is warm enough.

One myth is worth dropping on the way. Contrary to popular belief, raindrops do not look like teardrops. Large ones are flattened on the bottom like hamburger buns, and a 5 mm drop hits the ground at around 9 m/s.