How to Measure the Speed of Falling Raindrops

Bored on a rainy day? Want to know how fast rain is falling in the hopes that the clouds will soon dry up? It’s pretty difficult to say with exactness how fast the rain outside your window is falling (too many variables), but you can certainly get an approximation. Here is how to measure the speed of falling raindrops.

Difficulty: Moderately Challenging

Instructions

Things You'll Need:

• Wind gauge
• Video camera

1. Determine the prevailing winds. Wind blowing up from below will obviously slow a drop, but even crosswinds will skew the drop shapes, making it less aerodynamic. If there is any wind at all, your estimate will surely be slightly high.

2. Determine how big the drops are. You’ve got to be awfully quick with a ruler to measure raindrops. Most drops fall between 3 mm and 6 mm. You can pretty safely assume “fat rain” rain is on the 6 mm end, and drizzles are on the 3 mm.

3. The smallest drizzle falls at about 6.5 m/s and the largest drops fall at about 9 m/s. You can eyeball the drop size and then adjust it accordingly based on what you think are the prevailing winds.

4. You can use a highly sensitive force meter inside a wind-proof cylinder. When a raindrop falls in you’ll see a spike when it hits, then a lower number (its weight in Newtons). With this data you can use Newtonian movement equations to figure its speed at impact.

5. You can also use a video camera running at a high frame rate. If you can manage the lighting so that you can record a raindrop falling in front of a ruler, you should be able to see the distance it travels over the period of 2 or 3 frames and determine its speed from that.

Tips & Warnings

• Newtonian movement equations can be found online.
• Without an advanced camera, measurement tools and wind control, this isn’t an exact science