Low Rain Recording

[khr]

I've seen posts and threads about getting erroneous rain gauge readings, but they've always been from the standpoint of high readings. Has anyone gotten consistently low readings? I have a rain gauge in the back yard that reads the same as others around town and the official weather station on the outskirts of town. My new WS-1080 is reading about 25% low every time it rains. Any ideas???

Thanks,
Ken

[tjaliwalpa]

I've seen posts and threads about getting erroneous rain gauge readings, but they've always been from the standpoint of high readings. Has anyone gotten consistently low readings? I have a rain gauge in the back yard that reads the same as others around town and the official weather station on the outskirts of town. My new WS-1080 is reading about 25% low every time it rains. Any ideas???

Thanks,
Ken

Ken,

If you are certain the gauge is consistently 25% less, then you could put in a correction in Cumulus under Configuration > Calibration. Try a multiplier of 1.33 in Rainfall (0.7mm x 1.33 = .9975mm which when rounded is 1.0mm). You can always adjust this multiplier up or down to get your station readings as close as possible to your manual guage.

Bob

[steve]

The design of the Fine Offset rain gauge, with its shallow 'funnel' would tend to make it under-read. Mine does, in comparison to my Davis gauge, and I seem to remember others saying theirs under-read by 20-25%.

[Gina]

I can confirm that.

[DaveNZ]

The design of the Fine Offset rain gauge, with its shallow 'funnel' would tend to make it under-read. Mine does, in comparison to my Davis gauge, and I seem to remember others saying theirs under-read by 20-25%.

After becoming concerned my Davis gauge is under-reading, I have been currently measuring with a manual gauge alongside my Davis. I think it may be out by 10%. But 20-25% is ridiculous considering their gauges are supposed to be calibrated at the factory.

[steve]

I'd be very surprised if Fine Offset gauges are calibrated at the factory, and I'm not at all surprised that they can under-read by 25%.

[tjaliwalpa]

The design of the Fine Offset rain gauge, with its shallow 'funnel' would tend to make it under-read. Mine does, in comparison to my Davis gauge, and I seem to remember others saying theirs under-read by 20-25%.

The shallow funnel of the Fine Offset rain gauge concerned me. I made a modification (as did Phil alias hills) by adding a larger plastic funnel as a collector. We have used the Cumulus calibration, checking it against a manual gauge to get it as accurate as possible. The modification I have made has eliminated wind interference plus the likihood of vibration of bird perching issues.

Bob

[mraviator]

I just had my first rain experience and I saw 50% low readings when compared to my 4" CocoRahs gauge. These were heavy downpour events, and I'm not surprised that almost half of the rain water is likely bouncing off the shallow funnel insides back outside of the rain "box". I suspect the measurement is more accurate with slow, steady rain. I will probably try and add some sort of funnel to the box, or use Cumulus' adjustments.

[Gina]

A funnel makes a big difference.

[werribee_au]

Here's a thread with some mods
https://cumulus.hosiene.co.uk/viewtopic.php?f=16&t=1307

[hills]

I found after the mods Bob and I made, my rain gauge was really accurate except for when it really bucketed down when it would report under my manual gauge.

I strongly suspect this was because the funnel was overloading the shallow collector and it was overflowing and thus not collecting all the rain, so I have made a further mod of increasing the height of the walls at the side of the original collector by a couple of centimeters.

Hopefully now when we get heavy rain it will be able to keep up. So far it looks good, but it hasn't really bucketed down since the mod.

[tjaliwalpa]

Hills,

Great minds think alike. I've noticed the same thing and had planned to add sides to the original collector this weekend, but it's so windy and wet at the moment it will have to wait.

Bob

[hills]

Haha!

I used a plastic outlet that is used to install an new downpipe in an existing gutter. I turned it upside down so that the flange sat over the exiting walls of the gauge and trimmed the flange so that there was little overhang and used silicon to stick it in place.

If the weather was better I'd take a photo but I'm not going on the roof in this weather.

EDIT: Actually I bought a spare so I can show you what I'm talking about. I have trimmed the corners of this one so it would fit in my funnel housing. (you can even see the Mitre10 code if you want one )

[tjaliwalpa]

Haha!
EDIT: Actually I bought a spare so I can show you what I'm talking about. I have trimmed the corners of this one so it would fit in my funnel housing. (you can even see the Mitre10 code if you want one )

Fantastic - Mitre 10, here we come.

Bob

[wd40]

This NOAA article suggest all tip bucket rain gauges have problems in heavy down pours.
http://www.wrh.noaa.gov/otx/outreach/ttalk/raing.php

I found this along the way.

1 inch of rain is more than seventeen MILLION gallons of water per square mile!