The reporting standard for rainfall is in millimetres; this may sound counter-intuitive, because what we're actually interested in is the volume or amount of water surely?

Well that's certainly true, but recording rainfall as a depth of water simplifies the process because it removes surface area from the equation. So if two millimetres of rain have fallen, then your pond, swimming pool, bird-bath will all increase in depth by two millimetres. And if you want to calculate the actual volume of water you still can by multiplying the surface area of the body of water by the depth increase. But don't be caught out by your water-butt. These are collecting the rainfall from a large roof-area, so their apparent depth increase will be much greater!

Water butts are good for finding micrometeorites though, which land on the roof and are washed off during a rainstorm! The iron-rich ones can be recovered by dunking a magnet into the sludge at the bottom of the tank...