Joshua Neilson, of Russley School, asks :-

How fast do raindrops fall?

Gavin Fisher, a physicist with the National Institute for Water and Atmosphere (NIWA) in Auckland, responded.

It all depends on how big they are. Rain always comes in a mixture of sizes. Most raindrops we get in New Zealand will be between 0.2 to 2 millimetres in diameter. We don't get rain drops much smaller than this because they are just too small to fall out of the sky, and they stay as cloud droplets.

The smallest drops fall more slowly because the air resistance holds them back. A droplet of 0.1 mm diameter falls at 0.3 metres per second (less than 1 km/hr), or about the same as a feather. Drops about a millimetre will fall at 4 metres per second (about 15 km/hr), or about the same as a paperclip falling off a desk. Drops of 6 millimetres will fall at 10 metres per second (about 36 km/hr), or about as fast as you could go downhill on a bike.

When we see rain falling, we tend to mostly notice only the bigger ones and it appears as if its all falling at the same speed.

There is a way that you can answer your question yourself, provided there is no wind and you are in a car or on a bike with a speedometer. Notice that when you are moving the rain appears to be falling at an angle. If you can get the driver to adjust the car speed until the angle is halfway between horizontal and vertical then the rain is falling at the same speed as that shown on the car's (or bike's) speedometer. (For small or large drops you may have to measure the angle and use a calculator to multiply the car's (or bike's) speed by the tangent of the angle.)

You could even work back to see what size drops these are. Perhaps you could think of other ways to measure the speed of raindrops?