Topic: How much discrepancy do you typically see between your stations?

[ggsteve]

We are into the heaviest, steadiest rainstorm we have had since I got both my Rainwise Mk III and my Cocorahs gauge.  Over the last 24 hours my Rainwise registered 1.84", my Cocorahs registered 1.80".  That's about a 2% difference.  The Rainwise collection chamber is twice the diameter of the Cocorahs, theoretically that should make it 4 times as accurate.  Both gauges probably suffer from a little shadowing from trees, but this is a pretty straight up and down rain with no wind.  How much difference do you guys see between your stations? 

[ValentineWeather]

Steve I've just had a full month for good comparison also. 8.43 Cocorahs vs 8.28 Davis tipping with 20 days of recorded precipitation, difference (.15). Much of that I attribute to those lost not quite full buckets that never tip and evaporate before next rain.

The 4 " diameter of the Cocorahs should have no affect on accuracy, in fact the field test I saw the Cocorahs actually recorded slightly more than the official 8" NWS gauges in side by side comparisons. So this doesn't explain why the Rainwise is recording more other than calibration or location. 

[ValentineWeather]

Here is the 10 year study 8" vs 4" gauge. http://www.cocorahs.org/Media/docs/SMOI%202.2%20Ten-Year%20Gauge%20Comparison.pdf

Summary I'll post:
 
4. CONCLUSIONS
Based on ten years of side by side
measurements of the NWS 8" diameter Standard
Rain Gauge and the similar design but small 4"
diameter clear plastic gauge, the 4" gauge
consistently showed more precipitation than the 8"
gauge. Most daily difference were 0.00” or 0.01”
but when summed over all precipitation events
each year, the differences amounted to 0.48" or
3% of the annual average of 15.96 inches. Annual
differences ranged from 0.8% to 5.8%. The
majority of the differences accrued during spring
and summer months. Large daily differences of
0.05" or greater were uncommon but typically
occurred during snow, heavy rain, hail, or high
wind. In rare instances of high winds and snow, or
heavy rains with hail, the 4" gauge would show
less precipitation than the 8" gauge.

[Cutty Sark Sailor]

Davis Vue: 23' in height:  CoCoRaHS  34"  ground.  50' WSW of the ISS mount.  Up to 2" rain, slow, steady, slow wind.. virtually no difference... maybe 0.01... with wind, who knows?  At any rate... you ALWAYS use the CoCoRaHS gauge to report to CCR!  Do NOT use the electronic! And you stated some of the reasons... evaporation, bugs, wind, altitude, urban boundary layer, lots of factors.  This topic has been beat to death... arrrrgh...  What the heck?  CCR uses a standard 4" guage, that's the way it works, and all this other 'stuff' is beside the point. 

[Bushman]

Old proverb:  "Man with two watches never knows what time it is". 

[SlowModem]

May:  Cocorahs:  2.60 in, Davis VP2:  2.45.  They're not exactly together but close.

Edit:  I was talking about the location, but after I thought about it, the measurements are not that far off either.

[ggsteve]

Interesting discussion.  Over a long period the Cocorahs gauge does seem to have recorded more rain for me too.

May Cocorahs: .44"   May Rainwise: .41" (very unusual May around here).  These figures were gathered with a measurement checking time of 6:30 am.  So the actual dates measured were from 6:30 am on 4/30 to 6:30 am on 5/31.

[ggsteve]

Here is the 10 year study 8" vs 4" gauge. http://www.cocorahs.org/Media/docs/SMOI%202.2%20Ten-Year%20Gauge%20Comparison.pdf

Summary I'll post:
 
4. CONCLUSIONS
Based on ten years of side by side
measurements of the NWS 8" diameter Standard
Rain Gauge and the similar design but small 4"
diameter clear plastic gauge, the 4" gauge
consistently showed more precipitation than the 8"
gauge. Most daily difference were 0.00” or 0.01”
but when summed over all precipitation events
each year, the differences amounted to 0.48" or
3% of the annual average of 15.96 inches. Annual
differences ranged from 0.8% to 5.8%. The
majority of the differences accrued during spring
and summer months. Large daily differences of
0.05" or greater were uncommon but typically
occurred during snow, heavy rain, hail, or high
wind. In rare instances of high winds and snow, or
heavy rains with hail, the 4" gauge would show
less precipitation than the 8" gauge.

Interesting article.  The final hypothesis that the metal SRG may evaporate more water than the 4" plastic gauge in light shower conditions makes good sense.  I wonder if anyone has tested that?

[AjaxKnights29]

My AcuRite 5 in 1 recorded 30.5 mm during our last big rainstorm here while my CoCoRaHS gauge had 45.2 mm. That's a 39% difference.

[ValentineWeather]

Here is the 10 year study 8" vs 4" gauge. http://www.cocorahs.org/Media/docs/SMOI%202.2%20Ten-Year%20Gauge%20Comparison.pdf

Summary I'll post:
 
4. CONCLUSIONS
Based on ten years of side by side
measurements of the NWS 8" diameter Standard
Rain Gauge and the similar design but small 4"
diameter clear plastic gauge, the 4" gauge
consistently showed more precipitation than the 8"
gauge. Most daily difference were 0.00” or 0.01”
but when summed over all precipitation events
each year, the differences amounted to 0.48" or
3% of the annual average of 15.96 inches. Annual
differences ranged from 0.8% to 5.8%. The
majority of the differences accrued during spring
and summer months. Large daily differences of
0.05" or greater were uncommon but typically
occurred during snow, heavy rain, hail, or high
wind. In rare instances of high winds and snow, or
heavy rains with hail, the 4" gauge would show
less precipitation than the 8" gauge.

Interesting article.  The final hypothesis that the metal SRG may evaporate more water than the 4" plastic gauge in light shower conditions makes good sense.  I wonder if anyone has tested that?

I don't know but a very good point. If proven true the NWS should change over to plastic. 

[jemboogie]

My AcuRite 5 in 1 recorded 30.5 mm during our last big rainstorm here while my CoCoRaHS gauge had 45.2 mm. That's a 39% difference.

I too see differences in my Acurite 5-in-1 and my CoCoRaHS gauge. But it is usually the 5-in-1 measures greater, not my CoCoRaHS gauge.

[AjaxKnights29]

My AcuRite 5 in 1 recorded 30.5 mm during our last big rainstorm here while my CoCoRaHS gauge had 45.2 mm. That's a 39% difference.

I too see differences in my Acurite 5-in-1 and my CoCoRaHS gauge. But it is usually the 5-in-1 measures greater, not my CoCoRaHS gauge.

I feel thats the same here. I am getting my data from Aculink which I think doesnt work properly with 5 in 1 for some reason.

[miraculon]

I lashed a spare outer cylinder to a clothes pole about 15 feet away from the regular CoCoRaHS gauge. We had a nice steady rainfall, nothing spectacular in terms of rain rate. (0.08"/hr at the most according to my WU Davis plots)

This "2nd CoCoRaHS" gauge caught 0.29 inches, where the usual permanent CoCoRaHS gauge measured 0.32 inches.

The Davis VP2 caught 0.28" for the storm and the NovaLynx 0.39". I checked the NovaLynx with a beaker the other day, and got the correct number of tips for the 450mL amount.

My siting is sub-optimal and due to the small yard. The gauge is closer to the recommended 2:1 distance from the buildings, more like 1:1. Even so, the variation especially with the identical gauge was interesting. The spare gauge was a bit higher than the normal gauge and was further away from the buildings a little.

Greg H.