# Is there an upper bound to the amount of rain that can fall in an hour?

The record for most rainfall in a single hour is 12 inches (30.5 cm), set in Holt, Missouri, in 1947. What is the maximum amount of rainfall that could theoretically occur in an hour?

• An answer might want to mention implausible amounts of rainfall predicted by NWP models in 'grid point storms'. There is a good discussion in Bechtold, Numerical Weather Prediction Parameterization of diabatic processes, ECMWF Commented Aug 2, 2015 at 10:18
• I don't think any upper bound will be found to be "realistic," because even what we think of as extremely heavy rain events are rare in most individuals' experiences.
– WBT
Commented Sep 17, 2015 at 12:34
• Turns out there are lots of empirical models around. Seems like the world-record data could help constrain some upper limit calculation. I think the best answer will combine these two approaches. Commented Sep 23, 2015 at 12:02
• The limiting factor would purport to be how much water vapor can be brought into one area and condensed... but that can outperform general winds by converging/spilling over from updrafts from another area of storm? I'm wondering whether a supercell dropping giant hail and barely moving (which can happen) would break records if properly measured. Grapefruit hail with a 4.5 inch diameter would bring 3 inches of ice a stone area. At a 10:1 ice:water ratio, that's 0.3" water per stone. Plus it's often raining at the same time. So a rare prolific storm like this may well bring such records??? Commented Jan 8, 2022 at 18:01
• Just for note/comparison, somewhere in the past couple years there was discussion (may've even been here on SE) about precipitable water, which is how much water vapor is in the entire column of air over a location... and its records... I believe it was found to be around 3.10" for the US (perhaps in Houston during Harvey?, but certainly a tropical system). So even in 5 inch per hour thunderstorms, there is major moisture advection and convergence (moisture gathering in from surrounding areas) Commented Jan 13, 2022 at 7:39

A very moist airmass was in place across Missouri during the afternoon and evening of 22 June 1947, as evident by dewpoint temperatures in the middle 70s across Missouri during the afternoon hours

Advances in meteorological science led Locatelli to speculate that the thunderstorm that produced the historic rainfall of 22 June 1947, did not develop along a classic surface cold front, but to the east of a surface dryline (drytrough) and underneath a cold front aloft that had origins in the Mexican plateau. Regardless of the causative surface and upper air features, a complex of storms at a scale not measured well in the data-spare period of the 1940s developed near Kansas City and moved across Holt, Missouri, depositing 12 inches of rain in less than one hour. Rain continued to fall on Holt after the 12 inch deluge for the next couple hours, but only amounted to an additional one-half inch.

A rare meteorological event occurred that day. I offer a semi-empirical frame of reference that takes into account that the most extreme rainfall rates occur for a short duration (e.g. a few minutes).

In "A methodology to classify extreme rainfall events in the western mediterranean area" by Casas et al., (2004): http://link.springer.com/article/10.1007%2Fs00704-003-0003-x they show rain rates of up to 0.5 cm/min sustained for several minutes (based on a large network of rain monitors in Barcelona). IF that could somehow be sustained for an hour, that would give a rate of 30 cm/hr (which is what Holt saw).

For comparison, the most extreme rainfall rate measured in one minute in the USA is 3.1 cm (1.2 in; see http://wmo.asu.edu/world-greatest-one-minute-rainfall ). Actual rain doesn't continue at such an extreme rate for much longer, but would equate to 186 cm per hour (6 times what Holt, Missouri saw in 1946). The link that kwinkunks provides http://www.bom.gov.au/water/designRainfalls/rainfallEvents/worldRecRainfall.shtml shows a world rate record of 3.8 cm/min which, if sustained, would equate to 228 cm per hour.

• These statistics are really interesting. It strikes me that considering global records should get us closer to the limit. The Australian Bureau of Meteorology suggests 38 mm/min is the record, and 401 mm in one hour. It seems reasonable that these numbers would be close to — just below — the physical limit... So I guess the challenge is to estimate how close with some theory. An agreement within a few percent would be compelling. Commented Sep 17, 2015 at 12:10
• @kwinkunks thanks for linking to this list that has examples that are higher than the "records" listed above.
– f.thorpe
Commented Sep 17, 2015 at 16:09
• I just want to go on the record and say that just because the bounty was auto-awarded does not mean I think this question has been answered. See my comment on the question. (I'm not saying this is a bad answer, just that it's not the answer I was looking for with the bounty.) Commented Sep 25, 2015 at 12:52

The maximum one-hour rainfall may be 305 mm in America, or 401 mm in Australia, but it certainly isn't the world record measurement, which is in the region of 450 to 500 mm (based upon the global rainfall intensity envelope). The maximum possible is governed by three factors, namely 1) the precipitable moisture in the air column, which is a non-linear function of temperature, assuming no limiting evaporation upwind, 2) the dynamics of condensation, assuming no limitation on the availability of raindrop nuclei, and 3) the lateral advective delivery of moisture. This means that the maximum rainfall is likely to be on the windward side of a high mountain in a tropical oceanic island. We will probably never be able to measure the maximum rainfall because, by its nature, it will be in a very windy exposed site, where it is notoriously difficult if not impossible to measure the rainfall accurately. Also, consider the open area of a rain-gauge funnel, and the probability of that just happening to be in the right place to measure the maximum rainfall over many square kilometers. A good description of the probable maximum precipitation is given by Greiser in: http://www.juergen-grieser.de/extremerain_standalone.pdf

• The record 1-hour rainfall was in Shangdi, Nei Monggol, China — not Australia — in 1975. I don't know what the 'global rainfall intensity envelope' is, but 401 mm does in fact seem to be the 1-hour record... unless you have other data? Commented Sep 26, 2015 at 21:40
• Nowhere in Australia are conditions optimal for achieving the world maximum 1-hour rainfall, so I am very suspicious of of the 401 mm claim. None of the likely candidates, such as the Meghalaya hills, Asuncion island, or 'cloudbursts' in monsoon SE Asia, have either ideal instrumentation, nor adequate raingauge network density to have much chance of measuring the maximum accurately. Commented Oct 4, 2015 at 1:35
• As I said in my comment, if you look at the table you'll see that it was not in Australia, it was in China. If you have better data, please link to it. Commented Oct 4, 2015 at 11:31

According to the World Meteorological Organization, a record 1,825 mm (71.8 in) of rain fell in 24 hours at Foc-Foc (elevation: 2,990 m; 9,810 ft) on the French island territory of Réunion in the Indian Ocean on 7–8 January 1966. The event occurred during the passage of tropical cyclone Denise.