Practical raindrop counts for the 400 cm² rain gauge

This reference translates rain intensity into approximate drop counts by drop size for 400 cm² rain gauge. It is intended for amateur meteorologists, operational weather personnel, smart-city teams, and hydrologists who want a practical feel for what actually enters a rain gauge during a one-minute or ten-minute reporting interval.

400 cm²gauge opening area

22.6 cm / 8.887 inopening diameter

0.2 mm, 0.1 mm, 0.254 mm (0.1 in)common nominal resolution(s)

1 min and 10 minintervals shown separately

How to read the tables

The entries are expected drops hitting the gauge opening. A value of 100 does not mean every minute will contain exactly 100 drops; it means the long-term average for that rain type and drop-size bin is about 100 drops in the stated interval.

The drop labels are approximate spherical-equivalent diameters. For example, D1.5 means drops around 1.5 mm diameter. A dash means the bin is so rare in this practical rain type that the expected count is less than one drop in ten minutes for this gauge.

The Rain in 1 min (mm) column is the rainfall depth over one minute. It is the same depth over any gauge size for a given rain type; what changes with gauge area is the captured water volume in millilitres and therefore the number of drops.

dash = <1 drop in 10 minutes <1 in shown window 1–10 10–100 100–1,000 >1,000

Important: these are not monodisperse rain tables. Real rain is a mixture of drop sizes. The tables assign a practical water fraction to each drop-size bin for each rain type, then convert that water fraction into expected drop counts.

Note on rounding. Cell counts here are reference values rounded to one decimal place (or to the nearest whole number above 1 000). They were generated from higher-precision internal DSD fractions, so recomputing the cells by hand from the rounded percentages shown later in the article will produce small systematic differences of roughly 1–7%, depending on the bin. See the methodology rounding note for details.

Tip: for a visual sense of how a raindrop's volume and shape change with diameter, see BARANI's interactive raindrop shape & size calculator.

Representative rain types

Tip: swipe sideways on smaller screens to view all drop-size bins.

Rain types used in the reference tables
Rain type	Typical intensity	Representative intensity	Practical DSD family	Parameter cue
Drizzle / light stratiform	below about 2–3 mm/h	1.5 mm/h	narrow gamma / small-drop regime	Dm about 0.8–1.0 mm; high small-drop fraction
Moderate stratiform / weak warm rain	about 3–10 mm/h	6 mm/h	gamma or exponential-like	Dm about 1.1–1.4 mm
Heavy frontal / organized stratiform	about 10–25 mm/h	20 mm/h	broader gamma, event-dependent	larger D0 or Dm, but not giant-drop dominated
Convective / thunderstorm	about 10–100+ mm/h	50 mm/h	normalized gamma / broad spectrum	Dm roughly 1.4–2.1+ mm depending regime
Extreme cloudburst	above about 100–200 mm/h	200 mm/h	extreme envelope gamma	use envelope only; breakup-limited large-drop tail

One-minute reference table

Tip: swipe sideways on smaller screens to view all drop-size bins.

Expected drops hitting the 400 cm² rain gauge in 1 minute
Rain type	Rep. intensity (mm/h)	Water in 1 min (mL)	Rain in 1 min (mm)	D0.5	D1	D1.5	D2	D2.5	D3	D3.5	D4	D4.5	D5	D5.5	D6
Drizzle / light stratiform	1.5	1.0	0.025	3,526	1,112	96.1	3.8	—	—	—	—	—	—	—	—
Moderate stratiform / weak warm rain	6	4.0	0.1	4,974	2,662	767.7	155.1	25.5	3.7	0.5	—	—	—	—	—
Heavy frontal / organized stratiform	20	13.3	0.333	8,996	5,922	2,385	721.0	184.8	42.7	9.2	1.9	0.4	—	—	—
Convective / thunderstorm	50	33.3	0.833	3,760	5,716	4,306	2,137	826.4	272.2	80.3	21.9	5.6	1.4	0.3	—
Extreme cloudburst	200	133.3	3.333	4,970	9,919	10,019	6,752	3,568	1,611	652.2	244.2	86.2	29.1	9.4	1.8

Ten-minute reference table

The ten-minute table is the same rain physics multiplied over ten minutes. It is shown separately so it can be copied directly into a field note, training slide, or blog post.

Tip: swipe sideways on smaller screens to view all drop-size bins.

Expected drops hitting the 400 cm² rain gauge in 10 minutes
Rain type	Rep. intensity (mm/h)	Water in 10 min (mL)	D0.5	D1	D1.5	D2	D2.5	D3	D3.5	D4	D4.5	D5	D5.5	D6
Drizzle / light stratiform	1.5	10.0	35,256	11,124	961.2	38.5	—	—	—	—	—	—	—	—
Moderate stratiform / weak warm rain	6	40.0	49,735	26,616	7,677	1,551	255.3	37.0	4.9	—	—	—	—	—
Heavy frontal / organized stratiform	20	133.3	89,959	59,223	23,853	7,210	1,848	426.8	91.9	18.9	3.7	—	—	—
Convective / thunderstorm	50	333.3	37,599	57,160	43,060	21,369	8,264	2,722	802.6	218.6	56.1	13.7	3.2	—
Extreme cloudburst	200	1,333.3	49,704	99,189	100,189	67,522	35,679	16,106	6,522	2,442	861.7	290.5	94.4	18.3

Plain-language explanation

A rain gauge measures water depth, not individual drops. A rainfall intensity of 1 mm/h means that, if it lasted one hour, every square metre would receive 1 litre of water. For the 400 cm² rain gauge, moderate stratiform rain at 6 mm/h corresponds to about 0.1 mm of rain in one minute and collects about 4.0 mL of water in that minute.

That small water volume can still contain many drops because small drops are tiny. In the practical moderate-rain distribution used here, this gauge receives roughly 2,662 D1 drops in one minute, plus many D0.5, D1.5, and D2 drops. The gauge does not wait for a few giant drops; most real rainfall is a distribution of small and midsize drops.

For light rain and drizzle, the smallest bins dominate the count. For convective rain and cloudbursts, larger drops appear more often and carry more of the water, but even extreme rain still contains many small and midsize drops.

Technical methodology, without heavy math

Step 1: Convert rain intensity to rain depth. In one minute, the rain depth is:

rain depth = R / 60 mm

where R is the representative rain intensity in mm/h. This is the same over any horizontal surface.

Step 2: Convert rain intensity to captured water volume. For a gauge area A in cm² and rain rate R in mm/h, the water caught in one minute is:

Vrain = R × A / 600 mL per minute

This is just the rule that 1 mm of rain = 1 litre per square metre, rewritten for cm² and minutes.

Step 3: Convert a drop diameter to drop volume. A spherical-equivalent drop of diameter D in mm has volume:

Vdrop = π × D³ / 6000 mL per drop

Step 4: Assign the water to realistic drop-size bins. Instead of assuming all rain arrives as one drop size, each rain type uses a practical gamma-style drop-size distribution. The distribution gives a water fraction for each bin: D0.5, D1, D1.5, and so on.

Step 5: Convert water fraction to expected drop count. For each drop-size bin:

Expected drops in bin = water fraction × Vrain / Vdrop

The ten-minute value is ten times the one-minute value. The dash rule removes bins that would average less than one drop in ten minutes for this gauge.

Water fractions behind the count tables

This table shows the water-volume fraction assigned to each drop-size class. The fractions are rounded for readability; rows sum to approximately 100%.

Tip: swipe sideways on smaller screens to view all drop-size bins.

Water-volume fraction assigned to each drop-size bin
Rain type	D0.5	D1	D1.5	D2	D2.5	D3	D3.5	D4	D4.5	D5	D5.5	D6
Drizzle / light stratiform	23.1%	58.2%	17.0%	1.6%	0.08%	<0.01%	<0.01%	<0.01%	<0.01%	<0.01%	<0.01%	<0.01%
Moderate stratiform / weak warm rain	8.1%	34.8%	33.9%	16.2%	5.2%	1.3%	0.28%	0.05%	<0.01%	<0.01%	<0.01%	<0.01%
Heavy frontal / organized stratiform	4.4%	23.3%	31.6%	22.7%	11.3%	4.5%	1.5%	0.47%	0.13%	0.04%	<0.01%	<0.01%
Convective / thunderstorm	0.74%	9.0%	22.8%	26.9%	20.3%	11.5%	5.4%	2.2%	0.80%	0.27%	0.08%	0.02%
Extreme cloudburst	0.24%	3.9%	13.3%	21.2%	21.9%	17.1%	11.0%	6.1%	3.1%	1.4%	0.62%	0.15%

What this table does and does not represent

The tables assume a horizontal gauge opening, uniform rain over the gauge, and no wind. They represent sampling/count expectations, not all rain-gauge errors. Real instruments also face wind undercatch, splash-in, splash-out, wetting, retained water, evaporation, blockage, siting, and calibration effects.

The DSD families are simplified references. They are not a replacement for a local disdrometer dataset. They are useful because they prevent unrealistic examples such as treating light rain as if all the water arrived as only giant drops, or treating a cloudburst as if all water arrived as only tiny drops.

References

BARANI Design, rain gauge sizes and diameters. https://www.baranidesign.com/faq-articles/2020/2/26/rain-gauge-sizes-and-diameters
Ma et al. (2019), Hydrology and Earth System Sciences, five-year disdrometer study of stratiform and convective DSD characteristics. https://hess.copernicus.org/articles/23/4153/2019/
Rees and Garrett (2021), Atmospheric Measurement Techniques, disdrometer sampling statistics, Marshall–Palmer DSD example, WMO one-minute context, and 6 mm upper cutoff in simulations. https://amt.copernicus.org/articles/14/7681/2021/amt-14-7681-2021.html
Baltas, Panagos, and Mimikou (2016), Hydrology, goodness-of-fit comparisons of multiple statistical distributions (gamma, lognormal, exponential) to disdrometer observations for stratiform and convective rainfall. https://www.mdpi.com/2306-5338/3/1/9
NASA Global Precipitation Measurement FAQ, raindrop size and large-drop breakup notes. https://gpm.nasa.gov/resources/faq/how-big-can-raindrop-get
NASA GPM, accessible explanation of why drop-size distribution matters for rain estimation. https://gpm.nasa.gov/science/gpm-measures-raindrop-sizes-space
Squarespace Help Center, Code blocks: HTML, Markdown and CSS in code blocks. https://support.squarespace.com/hc/en-us/articles/206543167-Code-blocks
Squarespace Help Center, Markdown blocks: use code blocks for custom HTML/CSS. https://support.squarespace.com/hc/en-us/articles/205813788-Markdown-blocks

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