Why doesn’t water fall in one go from a cloud?

A: A cloud is not a big pool of liquid water but consists of minuscule droplets (~10 microns each) and sometimes ice crystals. These particles are so small and light that they are easily suspended by rising air currents and turbulence in the atmosphere.

Each droplet is subject to gravity but because it’s so small, air resistance almost perfectly balances its weight. The falling speed of a 10-micron droplet is only around 1 cm/s, so it would take hours to fall through 1 km of air. Updrafts in clouds are often stronger than this.

As droplets collide and coalesce into larger drops or as ice crystals grow and melt, their mass increases much faster than air drag. A 2-mm-wide raindrop can fall at around 7 m/s, which is faster than updrafts. So once droplets reach that size, gravity wins and the droplets fall as rain.

A bucket of water is a continuous body. Surface tension holds all the molecules together, so if you overturn it, the water pours out in a sheet. A cloud has no such cohesion: it’s just a diffuse suspension of independent droplets scattered through kilometres of air.