Rain Totals: Buyer Beware

On Monday morning as I was on my way to a speaking engagement at Kennedy Elementary, I heard on the radio that the airport recorded 1.85 inches of rain with other reports near 2 inches and “one report of 3.5 inches of rain in West Fargo”.   You may ask yourself, how did some buddy get 1.5 inches more than the other reports?  There are certainly instances when 1 mile can be the difference between 2 inches and 3.5 inches because of high variations within thunderstorms, but this past weekend was more than likely an example of a poor rain gauge, poor rain gauge location, or both.

There are several CoCoRaHS  observers in town.  Here is a list of the weekend totals at those sites (locations with only one report have been crossed out).  The NDAWN site at NDSU recorded 1.80 inches of rain as another reference.

Cocorahs

CoCoRaHS observers use standard 4 inch rain gauges, are trained with knowledge of basic set up and location of said gauge.  You will notice some variation in totals (typical), yet all the reports were generally in the 1.75 to 2.00 inch range in the city (just like the Airport and NDAWN site at NDSU).  Another observer in north Moorhead (not listed above), reported total rain of just over 2 inches at his location.  So the totals were all within the normal range of variation for a situation that did not involved thunderstorms.  Yes, there was a bit of thunder with the rain on Sunday, but it was a widespread rain event, not individual discreet thunderstorms that impacted the area, therefore, rain totals tend not to differ widely, just more subtly like what the CoCoRaHS observers reported from this latest event.

Therefore, you can begin to question the 3.5 inch report.  As previously noted, the CoCoRaHS observers all use a 4 inch diameter gauge, the NDAWN station uses a 6 inch gauge and the airport uses a 12 inch diameter funnel.  Generally, the larger the funnel, the more accurate your reading with be.  A small diameter funnel can be skewed sometimes by just a few additional large drops falling into that space.  From past experience, small diameter gauges (like the one in the image below) can be significantly higher than a larger gauge.

 

photo

 

Plus, rain gauge placement is very important.   In open areas try to place the gauge twice as far from obstacles as they are high (a building or tree).   In urban areas strive to be as far from a building or a tree as they are high.  Rain gauges are frequently placed very near homes for convenience is checking, but that placement often leads to higher totals as the structure can add to the totals.

Therefore, that 3.5 inch report was almost certainly a bad report for any of the above reasoning.  Probably a small diameter gauged mixed with poor siting, plus, often times people forget to empty their gauges which also adds to poor reporting.  The NDAWN team highly recommends purchasing a 4 inch diameter gauge.  These gauges can be found at some local hardware stores or you can go to this website for more information:

http://www.weatheryourway.com/cocorahs/rgcoco.htm

Here is an example of what the gauge looks like properly mounted (courtesy of the CoCoRaHS website):

 

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The great thing if you live in the upper Midwest, these gauges come from Fergus Falls, MN.

Daryl Ritchison