Joseph Graham, of Ardgowan School, asks :-

How many litres of water are in a cloud?

Gavin Fisher, an atmospheric scientist with the National Institute for Water and Atmosphere, responded.

We could go and measure it, by flying through the clouds with an aircraft which has some fancy instruments. But it is cheaper to work it out using a few facts that scientist have already measured ie the size of water droplets in clouds and the number of them per cubic metre.

The sizes were measured by covering a small piece of glass with oil, moving it through a cloud and looking at the trapped droplets through a microscope. We find that droplets larger than about a millimetre have fallen out as rain. The smallest cloud droplets are about 5 micrometres in diameter (a micrometre is one millionth of a metre), the most common are around 15 to 25 micrometres and the biggest remaining are about 40 micrometres.

We can measure how many droplets there are per cubic metre using special cameras or scattering from lasers. These show that very thin clouds might have 30 million droplets in a cubic metre, average clouds in NZ have around 300 million, but some large thunderstorm clouds can have up to 3000 million drops per cubic metre.

So a typical cloud has about 300 million droplets per cubic metre each with a volume of about 200 million millionths of a cubic metre giving a total of about 0.6 thousandths of a litre of water per cubic metre of air. That doesn't sound like much! If you had a cloud in your bedroom, there would be about two teaspoons of water in it.

A small cumulus cloud is about 100m long, 100m wide and 100m deep so contains about 600 litres which would fill a bath a couple of times.

A medium sized raincloud can be 2km long, 1km wide and 3km deep. This contains about 3.6 million litres of water, enough to fill a school swimming pool.

The largest tropical thunderstorms can be 50km by 50km and 25km deep thus holds about 40 thousand million litres - which is a lot of water! If it was all dumped on a small town, it would cause a flood about half a metre deep!