Sunday, July 1, 2012

Rain Gauges... What Works And What Doesn't

A while back I posted about rain gauges as we started testing some new ones with the goal of figuring out which one(s) we liked and thought were accurate.

Now I have final data for the 2011-2012 rain year I can present my conclusions.  To help with that, here's the original picture of the gauges in question:

From left to right, here's what I measured with each gauge last year and what they are called:
  • 32.86" - A Stratus gauge, 12" capacity
  • 32.15" - A 6" gauge
  • 33.02" - A wedge shaped 6" gauge
  • 35.20" - Our ancient, yellow, plastic 5" capacity gauge
  • 75.81" - A decorative, glass 8" capacity gauge
I think they were all close enough together for the test that they should have read about the same total rainfall over the season, but clearly there are some obvious differences.  Going through them one at a time, from left to right again, here are my thoughts:
  • The Stratus gauge is far and away the easiest to read if the amount of precipitation is less than 1".  The central tube holds 1" of rain and is easy to read in linear 0.01" increments.  None of the other gauges were nearly as easy to read, nor were they as accurate (as far as I can tell).  That said, however, if the amount of rain goes over 1" it overflows into the outer tube, and the reading process gets more complicated and error prone.  Still, this gauge works very well, and after having used it for a season I trust it.  It's also the standard in use by anyone doing anything serious with the weather as far as I can tell.  There are many places to purchase it.  Here's one, for example:
  • The 6" gauge read about 2% less than the Stratus gauge.  It is difficult to read in amounts of less than 0.1", so the fact that it was that close is probably due to my rounding readings off in opposite directions enough to even out the error.  There is a fundamental design problem with this gauge, though: it slides into a support that is screwed down, but the wind could blow the gauge right out of that support in some cases.  While I never had that happen, it got close a couple of times.  As a result of those issues I cannot really recommend this gauge.  You can find on, though, if you are interested.
  • The 6" wedge gauge presents a mixed bag.  It read only about 0.5% more than the Stratus gauge - essentially the same - but it is hard to read because the markings are small and faint.  Any condensation on the side of the gauge and it is nearly impossible to read without wiping it down, holding it up in the light, and squinting.  It is also the case that the scale isn't linear, so the more it rains the less accurate your reading will be.  And it has only 6" of capacity, so if you're getting a lot of rain you'll be out in it, emptying out the gauge before it overflows.  This is the second best gauge here, though, and for some it may be more than adequate.  It can be purchased from several online vendors, or directly from the manufacturer: Tru-Chek.  It's not particularly expensive either, so consider it if the Stratus isn't to your liking.
  • The yellow 5" gauge is really old, and was a hardware store special back when we bought it 20 years ago.  It read about 7% higher than the Stratus, and though it is heavily embossed and thus easier to read than the wedge, it is marked only in increments of  0.1".  It isn't quite linear either, with a slight taper to the shape.  And finally, the 5" capacity is too limiting if you get heavy rains.  Despite living with it this long I really cannot recommend it, but you can find something essentially identical at if you really want one.
  • The butterfly gauge is essentially worthless, having been created by someone who thought they knew how a rain gauge works but actually got all the details wrong.  This gauge read 2.3 times the rainfall logged by the Stratus gauge.  Yes, really.  It's supposed to have 8" of capacity but the impossibly bad design means you actually have less than 4" if you want to bother correcting for it.  I could write a tome about all the things they did wrong with this product, but I'm not going to bother.  Just don't buy it.  Here's a link to a very similar product from the same maker that you can ignore unless all you want to know is that it rained.  (You certainly won't have a clue about how much water actually came down if this is the only gauge you use.)  In short, give this one a wide berth.
So what happens next with our rain gauges?  Well, that's a question...

I need to move the Stratus gauge about 40 feet from its current location.  There are some oak trees nearby that are going to start impinging on the rainfall in particularly windy conditions, so I need to get it out of that area.  But when I do that I don't know how much the new location will affect the totals.  40 feet doesn't seem all that far, but I suspect I need to prove it actually isn't important.

I think what I will do is move the Stratus gauge to the new location and leave the 6" wedge gauge where it currently hangs.  The idea is to use the wedge to cross check the Stratus and see if they continue to read about the same or not.  If I can get through one more year and remain convinced that the 40 foot move hasn't affected things, then I will be done and have only one gauge to work with again.

The other gauges will probably be discarded.  None are worth passing on in my opinion, so I will recycle them.

I'll update rain data on my web site - see the links to the right - as things happen in the new rain year, but I'll probably only record the numbers from the Stratus unless I see big differences between it and the wedge.