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# rain in milimeters

• 23-12-2020 9:36pm
Closed Accounts Posts: 957 ✭✭✭

What exactly does it mean when it says there will be 40mm of rain? Is it that is rain lands on a level surface like concrete it would be one and a half inches high?

• Registered Users Posts: 788 ✭✭✭

What exactly does it mean when it says there will be 40mm of rain? Is it that is rain lands on a level surface like concrete it would be one and a half inches high?

One millimeter of rain" is actually one cubic millimeter per square millimeter. On average, over the area you're talking about, each square millimeter has received one cubic millimeter of rain. If you divide n mm3 by 1 mm2, you get -- n mm!

• Closed Accounts Posts: 957 ✭✭✭

Snowbiee21 wrote: »
One millimeter of rain" is actually one cubic millimeter per square millimeter. On average, over the area you're talking about, each square millimeter has received one cubic millimeter of rain. If you divide n mm3 by 1 mm2, you get -- n mm!
Thank. in English:D how much water would 40 mm mean? Would there be surface water on a road?

Is 40 mm a lot? I read it in some yellow warning so I assume it is considerable

• Registered Users Posts: 1,088 ✭✭✭

In simpler terms, if you left any straight sided, open topped container out thats how much it would fill by.

• Closed Accounts Posts: 957 ✭✭✭

KAGY wrote: »
In simpler terms, if you left any straight sided, open topped container out thats how much it would fill by.
in how much time?

• Registered Users Posts: 1,088 ✭✭✭

Thank. in English:D how much water would 40 mm mean? Would there be surface water on a road?

Is 40 mm a lot? I read it in some yellow warning so I assume it is considerable

40mm in an hour would be a lot, definite flooding, even over a day we'd consider it a wet day. On average id say we get about 200mm over a whole month

• Registered Users Posts: 8,316 ✭✭✭

in how much time?

25mm doesn't seem like much, but when you consider it's 25mm over every surface over miles and miles, usually funneled into drains and rivers then it's a lot of water.

• Closed Accounts Posts: 957 ✭✭✭

KAGY wrote: »
40mm in an hour would be a lot, definite flooding, even over a day we'd consider it a wet day. On average id say we get about 200mm over a whole month
OK thanks.the 40 mm or X mm is over an hour is it? Is that how it is measured X mm per hour?

• Registered Users Posts: 8,316 ✭✭✭

OK thanks.the 40 mm or X mm is over an hour is it? Is that how it is measured X mm per hour?

It doesn't have to be an hour. It's just a measurement of rainfall over any period.

For example 25mm over a 24 hour period would be a very wet day.
24mm in an hour would be a deluge.

• Closed Accounts Posts: 957 ✭✭✭

cml387 wrote: »
It doesn't have to be an hour. It's just a measurement of rainfall over any period.

For example 25mm over a 24 hour period would be a very wet day.
24mm in an hour would be a deluge.
OK thanks guys;)

• Registered Users Posts: 1,088 ✭✭✭

Just checked met eireann, in Birr there was only 10 days with over 40mm in one day from 1956 to 2009

• Registered Users Posts: 1,088 ✭✭✭

OK thanks.the 40 mm or X mm is over an hour is it? Is that how it is measured X mm per hour?

Normally they say over what time, e.g. hour, day. But to make things a little more confusing sometimes you might see the instantaneous or average rate expressed in mm per hour, even if the shower only lasts a few minutes. But that's the same as driving, your speed is measured in km per hr even if you're only popping down to the shops.

40mm per day is 1.7mm per hour, which doesn't seem like a lot until you realise that's on every surface over a whole region.

• Registered Users Posts: 1,026 ✭✭✭

KAGY wrote: »
40mm in an hour would be a lot, definite flooding, even over a day we'd consider it a wet day. On average id say we get about 200mm over a whole month

Try half that. If even.

And in the East you'd average about 60 to 70mm max a month.

• Moderators, Society & Culture Moderators Posts: 12,569 Mod ✭✭✭✭

Snowbiee21 wrote: »
One millimeter of rain" is actually one cubic millimeter per square millimeter. On average, over the area you're talking about, each square millimeter has received one cubic millimeter of rain. If you divide n mm3 by 1 mm2, you get -- n mm!

Not a fan of that explanation tbh, it's a bit more complicated than it needs to be imo. 1mm of rain being 1 litre per square metre is much easier to visualise!
cml387 wrote: »
25mm doesn't seem like much, but when you consider it's 25mm over every surface over miles and miles, usually funneled into drains and rivers then it's a lot of water.

This is why I think it's good to think of mm in terms of litres per square metre a lot of the time, as you start to connect the important units for flooding, litres of discharge and catchment area.

• Registered Users Posts: 8,219 ✭✭✭

The German Met Service only use the term "liter pro Quadratmeter" (litre/m²) and never mm. It makes more sense, though is a little more cumbersome to write than simlple mm.

• Registered Users Posts: 1,361 ✭✭✭

The German Met Service only use the term "liter pro Quadratmeter" (litre/m²) and never mm. It makes more sense, though is a little more cumbersome to write than simlple mm.

Yes, this one makes it much more obvious the amount of rain that is due (or has) fallen, and makes it a bit easier for the average person to visualise.

It really makes me realise how physically heavy rain is as well, if I think of the 10mx30m lawn here, that's 300m² and 4mm of rain on that lawn is over a ton of water that has fallen on that lawn. It's no wonder the grass would be a bit flat after a shower!

• Registered Users Posts: 8,913 ✭✭✭

What exactly does it mean when it says there will be 40mm of rain? Is it that is rain lands on a level surface like concrete it would be one and a half inches high?

I use the 5" gauge to take measurements of rainfall for Met Eireann - the exact same as used in the first part of the video.

In the second segment, the tipping bucket - is part of my Davis Vantage Pro station - a much smaller and less elaborate gauge, but nonetheless effective in measuring rainfall rate.

Hope the video above helps in your learning!