Sandaysoft

I'm thinking in the UK only!

So how many mm would you put on the following thresholds for daily rain?

light
medium
heavy
very heavy
exceptional

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Mark
Wilmslow Astro Weather

Hmmm, this is a tricky one Mark. The highest rainfall in a day doesn't necessarily equate to it being the most intense.

Take my records as an example. The highest rain rate I've recorded is 9.76" (248mm) / hour but the total rain for that day was a mere 1.09" (27.7mm).

Compare that to a daily total of 2.06" (52.3mm) of which 1.1" (28mm) fell in one hour. Which is the higher record?

I've always thought that 1" (25.4mm) in 24 hours represents a wet day but that could come via a 20 minute cloudburst or constant rain over 24 hours.

What are you trying to determine?

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Cheers,
Ray, Cheshire.

Hi Ray, I am not considering the rate, just the total for the day. Extreme is probably anything over 100mm? - given UK records seem to top out about 200mm/day.

My terms in the first post are probably a bit confusing as they imply a rate, so here is my first stab - in the absence of Google turning up any 'official' so far. The terms are not important other than as a general indicator, I'm looking for suitable ranges of values.

damp < 5mm
wet 5-10mm
rain 10-20mm
heavy rain 20-50mm
very heavy 50-100mm
extreme > 100mm

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Mark
Wilmslow Astro Weather

Hi Mark,

Ah right, that clarifies things a bit. You do realise that anything we class as very wet or extreme will be mocked by those in warmer climes.

I'd say anything over 2" (50mm) in 24 hours is fairly rare for a given location away from hilly or mountainous areas in the UK. As for the rest...

Damp - up to 0.1" (2.5mm)
Wet - up to 1" (25mm)
Very Wet - up to 2" (50mm)
Extremely Wet - up to 3" (75mm)
Exceptionally Wet - over 3" (75mm)
Once in a lifetime - over 4" (100mm)

Back in August 1972 I can remember taking an exam in a glass-roofed building in Norwich and the noise was deafening. I had a rain gauge which recorded over 5" in 24 hours but some parts of the area had over 7".

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Cheers,
Ray, Cheshire.

I'm sure there will be quite a few variations but I think Ray's is pretty good apart from changes to the following

Exceptionally Wet - over 4" (100mm)
Once in a lifetime - over 5" (125mm) I note that Ray actually recorded over this in 1972

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Neil
Hurstpierpoint Weather