Why does rain fall in drops and sheets?

Rain falls on a road in Kuala Lumpur, July 23, 2015.
| Photo Credit: Eutah Mizushima

Many windblown drops can be forced together to form what weather reporters call ‘sheeting rain’, but rain is always born as minuscule drops of condensed water vapour, according to the book ‘Clouds and Weather’ by John A. Day and Vincent J. Schaefer.

The formation of these droplets depends on the right amount of water vapour at the right pressure and temperature, but it also requires the presence of tiny solid particles of matter in the air on which the water vapour can gather and condense.

These bits of dust and salt are called cloud condensation nuclei. Salt starts collecting vapour at about 80% relative humidity, while bits of clay begin to take on water molecules at 100% relative humidity.

As the water molecules slowly collect and condense on the particles, cloud droplets form. They are a million times the volume of the original particle but are still very tiny. It takes perhaps 3,000 droplets to form a small raindrop. The drops in a heavy shower are the size of around 6,000 droplets, according to The New York Times.

The droplets can grow into drops by several processes. First, they can slowly continue to attract vapour. Second, larger droplets fall faster than small ones and collide with them, sometimes joining into larger drops.

Finally, evaporating droplets may collect on ice crystals in clouds. The crystals may warm and melt into rain drops or they may grow ‘branches’ and fall as snowflakes.