The expected number of raindrops hitting the 533 cm² rain gauge in 1 minute and 10 minutes for different rain rates
/Practical raindrop counts for the 533 cm² rain gauge
This reference translates rain intensity into approximate drop counts by drop size for 533 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.
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.
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.
Representative rain types
Tip: swipe sideways on smaller screens to view all drop-size bins.
| 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.
| 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 | 1.3 | 0.025 | 4,698 | 1,482 | 128.1 | 5.1 | 0.1 | — | — | — | — | — | — | — |
| Moderate stratiform / weak warm rain |
6 | 5.3 | 0.1 | 6,627 | 3,547 | 1,023 | 206.6 | 34.0 | 4.9 | 0.7 | — | — | — | — | — |
| Heavy frontal / organized stratiform |
20 | 17.8 | 0.333 | 11,987 | 7,891 | 3,178 | 960.8 | 246.2 | 56.9 | 12.3 | 2.5 | 0.5 | — | — | — |
| Convective / thunderstorm |
50 | 44.4 | 0.833 | 5,010 | 7,617 | 5,738 | 2,847 | 1,101 | 362.7 | 107.0 | 29.1 | 7.5 | 1.8 | 0.4 | — |
| Extreme cloudburst |
200 | 177.7 | 3.333 | 6,623 | 13,217 | 13,350 | 8,997 | 4,754 | 2,146 | 869.1 | 325.4 | 114.8 | 38.7 | 12.6 | 2.4 |
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.
| 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 | 13.3 | 46,979 | 14,822 | 1,281 | 51.2 | 1.3 | — | — | — | — | — | — | — |
| Moderate stratiform / weak warm rain |
6 | 53.3 | 66,272 | 35,466 | 10,230 | 2,066 | 340.2 | 49.3 | 6.5 | — | — | — | — | — |
| Heavy frontal / organized stratiform |
20 | 177.7 | 119,870 | 78,914 | 31,784 | 9,608 | 2,462 | 568.7 | 122.5 | 25.1 | 5.0 | — | — | — |
| Convective / thunderstorm |
50 | 444.2 | 50,101 | 76,165 | 57,377 | 28,474 | 11,012 | 3,627 | 1,070 | 291.2 | 74.7 | 18.3 | 4.3 | — |
| Extreme cloudburst |
200 | 1,776.7 | 66,231 | 132,169 | 133,502 | 89,973 | 47,542 | 21,461 | 8,691 | 3,254 | 1,148 | 387.1 | 125.8 | 24.3 |
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 533 cm² rain gauge, moderate stratiform rain at 6 mm/h corresponds to about 0.1 mm of rain in one minute and collects about 5.3 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 3,547 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.
| 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, statistical distributions fitted 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
- 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