In real-life, full-size raindrops don’t form perfect spheres because they are composed of water which is fluid and are only held together by surface tension.
- In normal atmospheric conditions, the air a raindrop moves through is itself in constant motion and even at a cubic metre scale or smaller, it is composed of areas at different airflows, temperatures and pressure.
- As a result of turbulence, a raindrop is rarely in free-fall because it is buffeted by the air around it, accelerating or slowing as conditions change from moment to moment.
- Raindrops start to form high in the atmosphere around tiny particles called condensation nuclei — these can be composed of little pieces of salt left over after seawater evaporates, or particles of dust or smoke.
- Raindrops form around condensation nuclei as water vapour cools producing clouds of tiny droplets that start off roughly spherical.
- Surface tension is the tendency of liquids to shrink to the minimum possible surface area.
- At water-air interfaces, the surface tension that holds water molecules together results from them being attracted to one another more than to the nitrogen, oxygen, argon or carbon dioxide molecules that make up our atmosphere.
- As clouds of water droplets begin to form, they are between 0.0001 and 0.005 centimetres in diameter.
- As soon as droplets form they start to encounter more vapour and collide with one another. As larger droplets bump into other smaller droplets they increase in size — this is called coalescence.
- Once they are big and heavy enough they begin to fall and continue to grow. Droplets can be thought to be raindrops once they reach 0.5mm in diameter.
- Sometimes, gusts of wind (updraught) force raindrops back into the clouds and coalescence starts over.
- As full-size raindrops fall they lose some of their rounded shape. The bottom becomes flattened due to wind resistance whilst the top remains rounded.
- Large raindrops are the least stable, so once a raindrop is over 4 millimetres it may break apart to form smaller more regularly shaped drops.
- In general terms, raindrops are different sizes for two primary reasons, initial differences in particle (condensation nuclei) size and different rates of coalescence.
- As raindrops near the ground, the biggest are the ones that bumped into and coalesced with the most droplets.