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Meteorological phenomenon can span large ranges in size and strength and categorizations exist based on thresholds, One example is the Saffir–Simpson hurricane wind scale (SSHWS) (see also NOAA)

Tropical Depression TD11 (LUPIT) will be passing by and I saw an Extremely Heavy Rain Advisory with a color-coded map of counties in Taiwan categorized as

  • Heavy Rain (yellow)
  • Extremely Heavy Rain (orange)
  • Torrential Rain (red)
  • Extremely Torrential Rain (purple)

update: It's been pointed out that there is a "descriptions tab" on the page which says:

  • Heavy Rain: 24-hour accumulated rainfall exceeds 80 millimeters, or 1-hour rainfall exceeds 40 millimeters.
  • Extremely Heavy Rain: 24-hour accumulated rainfall exceeds 200 millimeters, or 3-hour accumulated rainfall exceeds 100 millimeters.
  • Torrential Rain: 24-hour accumulated rainfall exceeds 350 millimeters, or 3-hour accumulated rainfall exceeds 200 millimeters.
  • Extremely Torrential Rain: 24-hour accumulated rainfall exceeds 500 millimeters.

I'm curious if these four descriptions are standardized categories or are simply chosen here to give a sense of increasingness to the progression in color and severity of rainfall.

So I'd like to ask:

Are terms like "Extremely Torrential Rain" universal in meteorology? Are these standardized categories widely used in meteorology or are these ad hoc definitions for this weather service?


Screenshot from https://www.cwb.gov.tw/V8/E/P/Warning/W26.html circa 01:30 07-Aug-2021 UTC

Extremely Torrential Rain from https://www.cwb.gov.tw/V8/E/P/Warning/W26.html circa 01:30 07-Aug-2021 UTC

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  • $\begingroup$ Downvoted because the answer to your question is right there, in the link you provided. All you had to do to get an answer to your question was to click on the "Description" button, which contains "Extremely Torrential Rain: 24-hour accumulated rainfall exceeds 500 millimeters." $\endgroup$ Aug 7, 2021 at 8:41
  • $\begingroup$ @DavidHammen is that particular definition only used on that website or is it used in the greater meteorology community as well? One goal of Stack Exchange questions is to provide a space for good answers for the benefit of future readers. $\endgroup$
    – uhoh
    Aug 7, 2021 at 8:47
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    $\begingroup$ Then you should rephrase your question: Is this a definition used only in Taiwan, or does it have a wider acceptance? $\endgroup$ Aug 7, 2021 at 8:48
  • $\begingroup$ @DavidHammen how does that look? $\endgroup$
    – uhoh
    Aug 7, 2021 at 8:54

1 Answer 1

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In New Zealand, heavy rainfall is defined as rainfall greater than 100 mm in 24 hours.

The rainfall categories defined in USGS are as following:

  • Slight rain: Less than 0.5 mm per hour.
  • Moderate rain: Greater than 0.5 mm per hour, but less than 4.0 mm per hour.
  • Heavy rain: Greater than 4 mm per hour, but less than 8 mm per hour.
  • Very heavy rain: Greater than 8 mm per hour.
  • Slight shower: Less than 2 mm per hour.
  • Moderate shower: Greater than 2 mm, but less than 10 mm per hour.
  • Heavy shower: Greater than 10 mm per hour, but less than 50 mm per hour. Violent shower: Greater than 50 mm per hour.

Here is the glossary of rainfall intensity (among other things) as defined and used by the Indian Meteorological Department:

No rain Rainfall amount realised in a day is 0.0 mm
Trace Rainfall amount realised in a day is between 0.01 to 0.04 mm
Very light rain Rainfall amount realised in a day is between 0.1 to 2.4 mm
Light rain Rainfall amount realised in a day is between 2.5 to 7.5 mm
Moderate Rain Rainfall amount realised in a day is between 7.6 to 35.5 mm
Rather Heavy Rainfall amount realised in a day is between 35.6 to 64.4 mm
Heavy rain Rainfall amount realised in a day is between 64.5 to 124.4 mm
Very Heavy rain Rainfall amount realised in a day is between 124.5 to 244.4 mm
Extremely Heavy rain Rainfall amount realised in a day is more than or equal to 244.5 mm
Exceptionally Heavy Rainfall This term is used when the amount realised in a day is a value near about the highest recorded rainfall at or near the station for the month or season. However, this term will be used only when the actual rainfall amount exceeds 12 cm.
Rainy Day Rainfall amount realised in a day is 2.5 mm or more.

In Taiwan, as mentioned in the question, the rainfall categories are:

  1. Heavy Rain: 24-hour accumulated rainfall exceeds 80 millimeters, or 1-hour rainfall exceeds 40 millimeters.
  2. Extremely Heavy Rain: 24-hour accumulated rainfall exceeds 200 millimeters, or 3-hour accumulated rainfall exceeds 100 millimeters.
  3. Torrential Rain: 24-hour accumulated rainfall exceeds 350 millimeters, or 3-hour accumulated rainfall exceeds 200 millimeters.
  4. Extremely Torrential Rain: 24-hour accumulated rainfall exceeds 500 millimeters.

From the above examples, it is clear that there is no standard terminology for rainfall intensity used worldwide, unlike the Saffir–Simpson hurricane scale. Various quantities of rainfall are given various monikers by the meteorological departments of the local governments based on the climate, geography and drainage of the area under consideration.

Further, the words in rainfall classification are so widely used that it is perhaps not possible to standardize them even if one wants to. A typical daily rainfall in Cherrapunji would be considered as heavy rain in Riyadh, for example. On the other hand, Categories 1, 2, 3, 4 and 5 in the Saffir–Simpson scale did not have any meaning before being defined. Whereas, 'moderate', 'heavy', 'torrential', etc. are widely used words whose meaning varies from region to region when used in association to rainfall quantity.

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  • $\begingroup$ (We're really far from consistent on tropical systems either, with differences in wind speed averaging periods and terminology and plenty of other complexities around the globe... basically it looks like the eastern hemisphere has as much variety in gauging what an "intense cyclone" is (or similar words), as there is on what "heavy rain" is. In meteorology, we're far from an integrated global community!) $\endgroup$ Feb 18 at 1:52
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    $\begingroup$ Right @JeopardyTempest, all these terminologies indeed depend on what a typical weather in a region is. Because what is typical for one region, might be severe for another. $\endgroup$
    – joy
    Feb 18 at 7:10
  • $\begingroup$ uhoh, another one :-) $\endgroup$
    – uhoh
    Apr 25 at 1:12
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    $\begingroup$ Thanks again @uhoh :D $\endgroup$
    – joy
    Apr 25 at 13:49

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