<p>I listed <a href="https://www.paperpublications.org/upload/book/Harnessing%20of%20Kinetic%20Energy%20of%20Raindrops-380.pdf" rel="nofollow noreferrer"><em>Harnessing of Kinetic Energy of Raindrops</em></a> as a reference when I answered one of your recent questions, <em><a href="https://earthscience.stackexchange.com/questions/22555/ways-to-make-a-how-hard-is-it-raining-detector-for-personal-use">Ways to make a “How hard is it raining?” detector for personal use?</a></em>.</p>
<p>Serendipitously, that reference has information relevant to this question.</p>
<p>The size and velocities of rain drops varies according to the type of rainfall event.</p>
<p>For <strong>Light Stratiform Rain</strong>, light rain had drops 0.5 mm in size (diameter) and a terminal velocity of 2.06 m/s. The size of large drops is 2.0 mm and their terminal velocity is 6.49 m/s.</p>
<p>For <strong>Moderate Stratiform Rain</strong>, light rain had drops 1.0 mm in size and a terminal velocity of 4.03 m/s. The size of large drops is 2.6 mm and their terminal velocity is 7.57 m/s.</p>
<p>For <strong>Heavy Thundeshowers</strong>, light rain had drops 1.2 mm in size and a terminal velocity of 4.64 m/s. The size of large drops is 4.0 mm and their terminal velocity is 8.83 m/s.</p>
<p>When rain drops start to get to be larger than 4.5 mm in size they split in two, as illustrated by this diagram, which relates rain drop shape to its size. Below 1 mm in diameter, rain drops &quot;are almost spherical&quot;.</p>
<p><a href="https://i.sstatic.net/F9c6U.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/F9c6U.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2021-07-26T11:48:21.320Z
2021-07-26T11:48:21.320Z
Fred
<p>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. <a href="https://pypi.python.org/pypi/PIL/1.1.6" rel="nofollow noreferrer"><code>PIL</code></a>).</p>

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

<p>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.</p>

<p>See also <a href="https://raspberrypi.stackexchange.com/q/76116/35753">Will long-term viewing of a sunny sky hurt the Pi Camera?</a> for pics of a cool Pi sky camera by @ThomasJacquin as described <a href="https://raspberrypi.stackexchange.com/a/78317/35753">here</a>.</p>

<p><strong>Edit:</strong> per this <a href="https://earthscience.stackexchange.com/questions/13727/what-are-typical-ranges-of-rainfall-drop-sizes-speeds-and-areal-or-volume-dens?noredirect=1#comment25899_13727">comment</a> 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.</p>

CC BY-SA 3.0
2018-03-26T08:08:32.787Z
@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
CC BY-SA 3.0
2018-03-26T09:05:35.683Z
@gansub good point, done!
uhoh
CC BY-SA 4.0
2021-07-26T11:51:07.683Z
I highly suggest Chapter 2.6 and 2.7 of "Physics and Dynamics of Clouds and Precipitation" - Pao K. Wang. If I find some time I may do a write up. While this does not answer all of your questions, you will find typical dropsize distributions and will be introduced to some of the major problems arising when finding the quantities you specified.
Joscha Fregin
CC BY-SA 4.0
2021-07-26T12:55:26.833Z
Some frozen raindrops get... [REALLY big](https://www.weather.gov/images/abr/EventSummaries/Thunderstorms_Tornadoes/20100723/Diameter.jpg) (ok, that's likely helped by some collision\agglomerating... of course so is rain)  Let's see how the Pi sensor handles that :-p
JeopardyTempest
<meteorology><rainfall><rain>
What are typical ranges of rainfall drop sizes, speeds, and areal or volume densities?
uhoh
# What are typical ranges of rainfall drop sizes, speeds, and areal or volume densities?

(Asked by: username_0 on 2018-03-26)

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`](https://pypi.python.org/pypi/PIL/1.1.6)).

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?](https://raspberrypi.stackexchange.com/q/76116/35753) for pics of a cool Pi sky camera by @ThomasJacquin as described [here](https://raspberrypi.stackexchange.com/a/78317/35753).

**Edit:** per this [comment](https://earthscience.stackexchange.com/questions/13727/what-are-typical-ranges-of-rainfall-drop-sizes-speeds-and-areal-or-volume-dens?noredirect=1#comment25899_13727) 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.

**Tags:** meteorology, rainfall, rain

> **Comments:**
> **username_0**: @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.  
> **username_0**: @gansub good point, done!  
> **username_1**: I highly suggest Chapter 2.6 and 2.7 of "Physics and Dynamics of Clouds and Precipitation" - Pao K. Wang. If I find some time I may do a write up. While this does not answer all of your questions, you will find typical dropsize distributions and will be introduced to some of the major problems arising when finding the quantities you specified.  
> **username_2**: Some frozen raindrops get... [REALLY big](https://www.weather.gov/images/abr/EventSummaries/Thunderstorms_Tornadoes/20100723/Diameter.jpg) (ok, that's likely helped by some collision\agglomerating... of course so is rain)  Let's see how the Pi sensor handles that :-p  

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## 1 answers

### Answer by username_3 on 2021-07-26. Score: 1 | Accepted answer

I listed [*Harnessing of Kinetic Energy of Raindrops*](https://www.paperpublications.org/upload/book/Harnessing%20of%20Kinetic%20Energy%20of%20Raindrops-380.pdf) as a reference when I answered one of your recent questions, *[Ways to make a “How hard is it raining?” detector for personal use?](https://earthscience.stackexchange.com/questions/22555/ways-to-make-a-how-hard-is-it-raining-detector-for-personal-use)*.

Serendipitously, that reference has information relevant to this question.

The size and velocities of rain drops varies according to the type of rainfall event.

For **Light Stratiform Rain**, light rain had drops 0.5 mm in size (diameter) and a terminal velocity of 2.06 m/s. The size of large drops is 2.0 mm and their terminal velocity is 6.49 m/s.

For **Moderate Stratiform Rain**, light rain had drops 1.0 mm in size and a terminal velocity of 4.03 m/s. The size of large drops is 2.6 mm and their terminal velocity is 7.57 m/s.

For **Heavy Thundeshowers**, light rain had drops 1.2 mm in size and a terminal velocity of 4.64 m/s. The size of large drops is 4.0 mm and their terminal velocity is 8.83 m/s.

When rain drops start to get to be larger than 4.5 mm in size they split in two, as illustrated by this diagram, which relates rain drop shape to its size. Below 1 mm in diameter, rain drops "are almost spherical".

[![enter image description here](https://i.sstatic.net/F9c6U.png)](https://i.sstatic.net/F9c6U.png)


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