Rain forms when the air in a cloud gets so saturated (packed full)
of water vapor (a gas) that it can't hold any more. Some of the water
vapor condenses into tiny water droplets (turns into a liquid). You're
probably wondering why doesn't all of the water condense all at once,
all over the place and fall down in a big splash. The reason is that
this condensation happens gradually. And it happens faster in some
spots (spots that are at just the right temperature or have just the
right amount of water) than in others.
So let's say that we're looking at one tiny spot that starts to
condense. We end up with a teeny-tiny water droplet, hardly the size of
a pin-head. So where will the next bit of water condense? Will it be
close to the first droplet or far away? Well, it will probably be right
on top of the first droplet, since it's easier for water vapor to
condense and add itself into another bit of water than to just do it
As water adds itself on to the first tiny droplet, the droplet gets
heavier and heavier. And eventually it's heavy enough to fall. Of
course the same thing has been happening all over the place, starting
with other tiny droplets, so we end up with lots and lots of raindrops.
But it doesn't all fall at once, since each drop starts off as just a
tiny bit of condensation, to which the other water vapor sticks.
(republished on 08/01/06)