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Rainfall Intensity

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nking
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Rainfall Intensity

Post by nking »

I have been searching for a definitive guide for describing rainfall intensity – along the lines of the recognised Beaufort scale. All I have found so far is the following:-

Very light rain (Drizzle) describes rainfall with a precipitation rate of less than 0.25 millimetres (0.0098 in) per hour.
Light rain describes rainfall which falls at a rate of between 0.25 millimetres (0.0098 in) and 1 millimetre (0.039 in) per hour.
Moderate rain describes rainfall with a precipitation rate of between 1 millimetre (0.039 in) and 4 millimetres (0.16 in) per hour.
Heavy rain describes rainfall with a precipitation rate of between 4 millimetres (0.16 in) and 16 millimetres (0.63 in) per hour.
Very heavy rain terminology can be used when the precipitation rate is between 16 millimetres (0.63 in) and 50 millimetres (2.0 in) per hour.
Extreme rain (Cloud Burst/Downpour) can describe rainfall with precipitation rates exceeding 50 millimetres (2.0 in) per hour.

Is there a universal accepted scale or is it country specific?

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steve
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Re: Rainfall Intensity

Post by steve »

I think that like most of these things, there's no universal standard. The UK Met Office use the following:

For Drizzle (= rain drops less than 0.5 mm diameter (!))

Slight = negligible run off from roofs
Moderate = up to 1 mm/hr
Heavy = more than 1 mm/hr

For rain other than showers:

Slight = less than 0.5 mm/hr
Moderate = 0.5 to 4 mm/hr
Heavy = more than 4 mm/hr

For showers:

Slight = 0 to 2 mm/hr
Moderate = 2 to 10 mm/hr
Heavy = 10 to 50 mm/hr
Violent = more than 50 mm/hr
Steve

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nking
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Re: Rainfall Intensity

Post by nking »

Thanks Steve. I tried a search on the MO site and all I got was data concerning radar rain intensity etc. What's your secret for searching the MO as you always seem to find just what's needed ;)

Would you post the link? I hope others may also post links as it will be intersting to see how the definitions/terms differ.

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steve
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Re: Rainfall Intensity

Post by steve »

Here's the document I got it from: http://www.metoffice.gov.uk/corporate/l ... heet03.pdf

There are a number of other useful documents there: http://www.metoffice.gov.uk/corporate/l ... heets.html
Steve

DaveNZ
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Re: Rainfall Intensity

Post by DaveNZ »

The definition I know of for heavy rain is at least 7.6mm/hr.

For my site I use

>0.0 light rain/drizzle
>2.5 moderate
>7.5 heavy
>20 very heavy
>50 torrential

Some of it is based on actual descriptions of rain rates I found online; some is just picked by me.
For my location rain rates above 20mm/hr are achieved probably about half the time it rains (though not sustained for long, usually)

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