Question: Why does rain drop as droplets and not all at once.

Keith Siew answered on 17 Jun 2013:

That’s an interesting question! Not my area of expertises but I’ll try my best to answer it!

So first off I think we need to understand some basic chemistry and physic behind water to figure out what is going on!

Water is made up two hydrogen atoms and one oxygen atom. In chemistry you have a thing called valences for each element which tells us how many electrons are in its outer shell (a shell is the area we call around the nucleus where electrons sit)… so hydrogen has one just one electron and because its so small its outer shell can only hold a maximum of two electrons. Oxygen has 6 electrons in its outermost shell which can hold a maximum of 8 electrons. Both of these atoms want to either fill their outer shell or want to have it completely empty! So oxygen wants two extra electrons and each hydrogen wants to give away its one electron or gain an extra one… so when they bond they essentially do a trade so both are happy! But because electrons have a negative charge and the nucleus has a positive charge this means that the oxygen atom after the trade has a slightly negative charge and the two hydrogens are now slightly positive. The molecule of water is shaped like a “V” with hydrogens on the top of each tip of the V and the oxygen on the bottom point of the V, this therefore makes the molecule of water polar (i.e. it has a positive and negative end… much like a magnet).

So this polar quality comes into play in something we call “van der waals forces”, this is where the positive ends of water molecules become attracted to and form weak links with the negative ends of other water molecules (you can see an illustration here: http://tinyurl.com/kym53yc ). This gives water its “cohesive” properties, which means water molecules like a big bunch of magnets, want to stick together!

So now that we understand that water likes to stick together if you think about a cloud… this is just a big bag of steam (gas form of water) and water vapour which is essentially made up of countless super tiny balls of water… as either the number of them increases or they cool down, they begin to slow down and stop whirling around in the cloud and begin to collide with one another… they then start to clump together and gradually form larger and larger droplets until they become so big and heavy they can’t be kept in the cloud anymore. Once large enough the force of gravity pulls them down to earth and we get rain!

So its a gradual process… the droplets eventually reach a size that are two heavy for the wind/air pressure to keep them suspended in the air and gravity pulls them down. If you were to have no gravity… it’s possible that if given enough time all the water in the cloud would form a giant ball of water… but thankfully that’s not the case!

Hope that answers your question!