Thursday, May 4, 2017

Yes, Size Matters...

...for rain gauges, anyway.  I have no experimental data regarding other subjects.

ProfessorRoush has always been a purchaser of those little cheap 1 inch diameter rain gauges, both for price and for their ability to be mounted easily to a post.  I always wanted them cheap because, as often as not, I leave them open-side-up a little too long and lose one to frozen shatterage nearly every year.  For ages, I had one down at the garden and one up by the house, the nearest for convenience on cold rainy spring mornings and the farthest because the rain in Kansas is so spotty that I thought the second often might have differing readings (though it doesn't).

Then, a couple of years ago, I purchased a 2 inch rain gauge that stuck into the ground on a little metal stand (pictured at left) and I immediately noticed that it commonly registered more rain than the smaller gauges, sometimes double the amount of rain.  What the heck, an inch is an inch in regards to rain, right?

Recently, on an experimental whim, I purchased the rain gauge pictured at the right below this paragraph, which is about halfway between the two previous sizes.  And in the recent rains over several days, the tally was; Biggest gauge, 3.4 inches, medium gauge, 2.7 inches, and two small gauges, 2.1 and 2.2 inches respectively.

What I neglected to previously consider was that rain never falls straight down in Kansas.  It commonly sweeps in at a 30º angle to the ground.  Sometimes, it seems to be completely horizontal and never actually reaches the ground, or thereabouts.  I'm pretty certain that if my face didn't sometimes intercept the path of rain, those individual droplets might make it as far as Missouri before they fell.  So a simple explanation might be that some of the rain is hitting the side of the gauge instead of dropping into it.

Of course, any decent mathematician would have calculated in seconds that the area of a 1 inch circle is πr², or 0.785 square inches.  Held at a 30º angle to oncoming rain (and estimating by eyeball), the apparent opening of the now ellipse is 1 inch X 0.6875 inches.  The formula for the area of an ellipse is πab, or π(semi-major radius)(semi-minor radius).  In this case, that is π(0.5)(0.3438) = 0.54 square inches.  The same amount of rain just doesn't have the same target area, so the gauge doesn't fill as much.  Voila!

Of course, the real "angular diameter" of the gauge to rain that falls at near subtornadic velocity has a more exact formula  (δ=2 arctan(d/2D)), but then you get into arctans and deltas and other things that I don't want to spend time relearning. I'm still confident enough to put the validity of my crude explanation and estimates of rain depth up against the likely validity of a specific 20-year future climate change prediction by any scientist, "settled science" or not.  Bigger IS simply better, regarding rain gauges, and I'm sticking to it.


  1. I measure for the Community Collaborative Rain, Hail, & Snow Network (CoCoRaHS -- sponsored by NOAA, used by National Weather Service and others). You can find a good photo of the gauges we use at this link: I am KS-RL-30. The web site has great info on rainfall.

    1. That's pretty neat, however, determining amounts over one inch sounds tedious with that gauge. I should probably join, but it helps my conscience to see that there are several monitors in my area.


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