Selan, of Highland Park School, asks :-

How can you measure the size of raindrops?

Gavin Fisher, a meteorological physicist with the National Institute of Water and Atmosphere, responded.

Raindrops come in all sizes from about half a millimetre to about 3 mm in diameter.

Their size can be measured in all sorts of ways. The easiest way is to photograph them as they fall, but we need a very fast camera. We can catch single drops (or a known bunch of them) and measure the volume of water - then calculate what size they must have been.

When an object falls, for example a free-falling parachutist, they initially speed up and the air resistance force increases until it matches the gravitational force. When this happens they fall at a constant speed (their terminal velocity) which depends on how much resistance they present to air flow. By streamlining their body, ie arms alongside their body and legs closed, they fall much faster than if they arch their body and spread their arms and legs. When they open their parachute they fall even slower ie at a safe landing speed.

We can measure the speed at which raindrops fall and do some physics calculations involving drops falling in air to figure out the size drops which fall with that particular terminal velocity.

One of the fancy techniques involves using a thing called an `acoustic raindrop distrometer'. These work by listening to the sound made when a raindrop hits a microphone pointing upwards that is held out in the rain. Bigger drops make bigger thumps. If we then calibrate the microphone by letting drops of known size hit it (say by using an eye dropper), then we can figure out how big the raindrops were.