# What are typical ranges of rainfall drop sizes, speeds, and areal or volume densities?

I'd like to look into the possibility of using a Raspberry Pi and the PiCam module to detect rain, and to try to make some measurements of droplet size and rate. This could be done using an LED flash or a (modified/pulsed) laser pointer equipped with a fan-out element, synchronizing the flash/pulse with the Pi camera's electronic shutter for a "freeze frame" effect, followed by some Python image analysis (e.g. PIL).

This is for fun, and not meant to be a quantitative rain gauge necessarily.

In order to better estimate the challenge, I'd like to get some feel for the distribution of sizes, speeds, and either areal rates (drops per sec per square meter) or number densities (drops per cubic meter). I can convert between various units and histograms, but I don't know where to find a good survey to understand over what ranges these can vary.

See also Will long-term viewing of a sunny sky hurt the Pi Camera? for pics of a cool Pi sky camera by @ThomasJacquin as described here.

Edit: per this comment I should point out my Raspberry Pi will be within a few meters of the Earth's surface, where I can keep an eye on it.

• check out the work done under the TRMM project. There are some great references on this. – gansub Mar 26 '18 at 8:00
• @gansub Thanks for the tip! I'm not sure how closely those space-based passive microwave and IR measurements correlate with properties of individual raindrop properties at the surface, but I'll read up on it. – uhoh Mar 26 '18 at 8:08
• in your question you should mention surface rain drops if that is the case. – gansub Mar 26 '18 at 8:29
• @gansub good point, done! – uhoh Mar 26 '18 at 9:05
• references on surface radars should be a good way to start on this. – gansub Mar 26 '18 at 9:22