Rain Gauge calculation

wilson1(7 G'boro NC)June 8, 2009

My rain gauge cracked so I bought a new one. Just for comparison, I put 3 inches from the old one into the new one and it measured 2 inches. So one gauge is obviously wrong. I thought I would use a measuring cup to see which is accurate, but of course the measuring cup is in milliliters while the rain gauges are in millimeters. Does anyone know how to convert liters to meters? Or how many millimeters are in an ounce of water? Thanks.

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zigzag

Short of getting a NWS fancy set-up, I don't think it's that important to be uber-accurate. All you have to calibrate is how much rain falls in your yard, versus the TV reports. Often I hear of a measurement at the airport (which seems to be the standard reporting place) that is way above or way below what registered in my yard. I just pay attention to the tube in my yard.

I get really annoyed w/the ml calibrations - for as long as I can remember, threats of conversion to metric measures have loomed, but have never happened. Inches and parts thereof work better for me.

    Bookmark   June 8, 2009 at 10:49PM
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wilson1(7 G'boro NC)

Yes, but that is exactly my problem. I want to know whether it is really three inches or two, and measuring cups are in ounces, not inches. How do you convert? How do I know which rain gauge is accurate?

    Bookmark   June 9, 2009 at 8:09AM
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jimtnc(7b Raleigh tttf)

Why couldn't you fill both up to 2 or 3" (whatever) then measure the outside of both with a measuring tape or stick, find the one that is correct and then pour it into your measuring cup to satisfy your liquid measure?

    Bookmark   June 9, 2009 at 11:01AM
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ichabod(z8GA)

Are both gauges straight tubes, or does either or both have funnel type affairs on the open end? Sounds like what's what's going on is one or the other has a funnel. Easy way to tell is hold a ruler against the tube and compare the ruler to the gauge. If the gauge numbers are farther apart than the ruler it has a funnel. In that case you would have to do some fancy math to get the numbers to come out the same. The funnel makes the rain gauge measurements stretch out so that it is more accurate.

Ichabod

    Bookmark   June 9, 2009 at 11:23AM
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wilson1(7 G'boro NC)

Ichabod, that is interesting. Does that mean that I cannot simply pour an ounce of water in each one and compare for accuracy? Does that mean that they are both possibly accurate?

    Bookmark   June 9, 2009 at 12:58PM
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wilson1(7 G'boro NC)

I forgot to add that yes, one is a straight tube and one is a funnel.

    Bookmark   June 9, 2009 at 1:01PM
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lecorbeau(7b)

Here's a website to tell you how to calibrate any container to be a rain gauge. Depending on the size of the opening, your inch marks will be closer together or further apart.

k.

Here is a link that might be useful: calibrate a rain gauge

    Bookmark   June 11, 2009 at 11:18PM
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dottie_in_charlotte(z7-8 NC)

carolinakate..interesting link but too complicated for my poor brain to absorb at 2AM.
All my rain gauges split in winter when I forget to empty them.
I recall reading many times that the easiest way to determine when you've watered your lawn an inch was to put a regular tunafish can out under the sprinkler to measure.
When all else fails, I call the guys across the street at the golf course and ask them how much rain we got.
They keep accurate measurements.
The more that falls down from up above means the less they have to pump from the wells.

Hard rains on dry soil tend to run off so keeping a spot open (unplanted) so you can dig down and see how much moisture the soil absorbed is probably wise.

    Bookmark   June 12, 2009 at 2:35AM
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zigzag

I use a tube type, but did have a funnel type which registered much different, tho they were inches apart. Got rid of the funnel, went with the tube constant. I do try to remember to take the glass tube in for the winter so it doesn't crack, but am not always successful. ( smile to Dottie ). I just go w/what my tube tells me - since I don't have a golf course across the street ( smile to Dottie, again! ). If all else fails, just use a tunafish can (third smile to Dottie!).

In short, best guesstimate any of us can make is using whatever guage in our own yard and letting our plants tell the real story - ounces, inches or ml's, they don't care as long as you keep them hydrated!

:o)

    Bookmark   June 12, 2009 at 5:03AM
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jimtnc(7b Raleigh tttf)

You mathematician types are gonna have to answer this question for me. I have a rain gauge, and my dog has a dog bowl about 6" round that sits on the patio. When we get a pretty good rain, I'll check to see what the rain gauge shows, but ironically that dog bowl is showing what looks to be about the same level of water. Soooo, why can't I just stick in a yardsick to measure it instead of worrying about what the area is, etc? I haven't done that yet, but it sure looks like about the same amount in both.

    Bookmark   June 12, 2009 at 8:33AM
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zigzag

jimtnc, I think you've got it about right .... it's all relative and water in the gauge vs. water in the dog bowl will be pretty much the same. So, let your plants be your guide - after all, they're the only real reason we mere mortals are trying to track rainfall! (except for the tv weather-people, they're just trying to earn a paycheck!)

    Bookmark   June 12, 2009 at 4:02PM
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lecorbeau(7b)

Well, yes, I admit the link was a bit much. But if the sides of your container are straight like the dog bowl and the tuna can, then you should just be able to measure the depth. The link was for if anyone wanted to use a different shape.

    Bookmark   June 12, 2009 at 8:56PM
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dottie_in_charlotte(z7-8 NC)

So, what do the weather man and the people at the airports use? We always hear what the airport reports as receiving for rain but that airport is 20 miles away and the area impacted by highways,monstrous runways (heat sinks), jet turbulence and a river, lake and 2 nuclear power plants that discharge cooling water and raise the temp of the river.
All that combines to make more than a microclimate for harsher weather so their readings aren't accurate for my area.
I still think the tried and true(as far as gardening is concerned)method is to dig a small hole and see what has absorbed to root level.

Anyone know where I can score a turf core sampler tool?
It's a hand tool used by landscapers and nursery people to take soil samples of about 8" deep.

    Bookmark   June 13, 2009 at 1:58PM
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jqpublic(7b/8a Wake County NC)

"I forgot to add that yes, one is a straight tube and one is a funnel."

Well I'm guessing the funnel one would catch more water, but that doesn't have anything to do with your current problem.

The problem with what you're trying to do is that liters are a measurement of volume, while inches and meters are length measurements. It all depends on the size of the tube. The funnel one may collect more water b/c its grabbing more water out of the sky, while the tube one is only grabbing the water falling right into the opening.

    Bookmark   June 14, 2009 at 4:29AM
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wilson1(7 G'boro NC)

This has become quite an interesting thread. Thank you all for your input. CarolinaKate, I loved that link! But it will take me a long time to do those calculations - Jim you are too funny! I love the idea of putting a measuring stick in the dog's bowl. JQ, that of course, was my original dilemma because rain gauges are in inches while measuring cups are in ounces and I was taking water from a measuring cup and pouring it into the rain gauge to see which gauge was accurate. I think Ichabod is probably closest to finding a solution for me. The conclusion I have come to is that both are right: the funnel collects more water from a wider area, but the marking adjust for that while the straight tube's marking take into account its method of collecting water. Thanks to all of you for the great discussion.

    Bookmark   June 14, 2009 at 9:47PM
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coorscat(6 and 9)

FYI the link below can be used to convert anything. I set the link to volume (since that is the measurement in which the thread is interested)
Incidently, I discovered that a US hogshead and a UK hogshead are different and I didn't even know that a hogshead was a unit of measure

Here is a link that might be useful: Online Conversion

    Bookmark   June 16, 2009 at 3:16PM
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solaknc(7b8a NC)

I'm a member of CoCoRaHS.org, and all 8,000+ members use a 4" rain gauge ($20.00) for consistancy across the country. Here's a link to some (20,000) reporting stations around NC and surrounding areas. Just enter your parameters for the station closest to you. Or, become a member of CoCoRaHS yourself :-)

Here is a link that might be useful: Database

    Bookmark   June 26, 2009 at 11:16AM
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wilson1(7 G'boro NC)

Solaknc, that is really interesting. Are those specific gauges and where do you buy them? We are always checking our rain gauge, so maybe we should become a part of CoCoRaHS. What an great site!

    Bookmark   June 26, 2009 at 4:53PM
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solaknc(7b8a NC)

Wilson1, there's a link on the CoCoRaHS.org main page, right hand side. Mine hasn't been working lately---no rain to measure. :-)

    Bookmark   June 28, 2009 at 9:35AM
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