Saturday, November 12, 2011

Rain Gauges - actual data from actual rain

As per the earlier post about rain gauges we are testing several to see how they compare.  While this isn't a scientific test, it is fun, and some people even expressed interest in it.  We had some rain yesterday, so I took pictures of the gauges this morning as I read them, and figured I'd write that up.  In summary, and in order of apparent accuracy, the gauges read:
  • 0.33" - official gauge
  • 0.32" - wedge shaped gauge
  • 0.3" - old yellow gauge
  • 0.3" - gauge
  • 0.8" - butterfly gauge
Below are the pictures of each one with some comments.

The "official" gauge, which I keep wanting to (incorrectly) call the NOAA gauge:

What you see here is the easiest gauge of all to read if the total amount of rain is less than one inch.  I didn't even remove it from the mounting bracket or pull out the central tube.  All I did was wipe the outside of the big cylinder to get rid of the condensation, point the camera, and click.  0.33" is pretty simple to read, don't you think?

Note the bird dropping in the bottom of the tube.  That might add a tiny bit too much to the total, but much less than the accuracy of 0.01", so I ignored it.  The funnel has a diameter of three or four inches, so stuff like this will fall in from time to time.

Also note that the scale on the central tube is simple and linear.  Very easy to read, and the meniscus is easy to see, even through the outer tube.

The only problem with this gauge is what happens when you have more than one inch of rainfall.  It overflows into the outer tube and you have to pour it into the central tube in portions to get the total amount.  Accuracy is still good, but convenience is not.  Then again, it can measure up to 12 inches of rain in one shot that way, which is better than anything else I've found so far.

The wedge gauge:

This is the bottom of the wedge shaped gauge.  I'm not quite holding it vertically in the picture, but it claims 0.32" of rain when held properly.

Looks a bit hard to read, don't you think?  It is, for a few reasons.

First, the embossed numbers are pretty small.  If you need reading glasses in general, you will need them to read this gauge.  Not so nice if it's still raining when you're trying to read it.

Second, the embossing isn't all that pronounced.  Getting it to show in the picture was tough.  (You can click on the picture to see the full sized view, which helps, but it still isn't easy to read.)

Third, there is no paint on the embossing.  that would make this a lot easier to read, but also add to the manufacturing costs.

Finally, and most critically in my opinion, the scale on this gauge is not linear.  Since it is wedge shaped there are places where the embossed numbers change from what you might expect.

Here's the same picture that I have hand edited in an image editor to make the English scale more obvious.

Note the numbers getting closer together as you head up the gauge.  Also note that we go from increments of 0.05" to 0.1" after the 0.2" mark.  And there are other places where similar changes happen farther up the gauge.

Reading this one isn't nearly as simple as it should be.

Accuracy is probably pretty good.  The difference between 0.33" and 0.32" is pretty much in the noise range.  Most if not all of this rain fell yesterday and in the evening, but the gauges sat out in the fog all night before they were read.  If the wedge gauge collects less fog than the official gauge, it might read slightly less just for that reason, for example.

Anyway, some paint would sure help this one.

The Old Yellow gauge:

This is the 19 year old gauge we've been using all along.  It's nice and simple, but as you can see it's a bit hard to read.

If you remember your chemistry class, though, you read the bottom of the meniscus, so that (when held vertically) is about 0.3" of rain.

The embossing is easier to read on this one than on the wedge, but still no paint.

And don't plan on reading this to anything more accurate than 0.05".  Even that is a guess in most cases, since even the slightest tilt of your hand will move the meniscus around a fair bit.

The gauge:

This gauge has a strange combination of things that make me wonder about it.

The large numbers on the front (1, 2, etc) are painted and embossed, but useless for anything except to remember which inch you're "in" when reading the thing.

The lines on the sides are painted only - not embossed - which means that they will flake off and the gauge will be unreadable as soon as the sun does it in.

When held vertically - which I am not quite doing in the picture - the meniscus was right at 0.3".  Like the yellow gauge, though, reading anything other than 0.05" increments isn't going to happen.

In all I am not sure why put their name on this thing.  I suspect a season or two and it will be unreadable, and I wonder about its accuracy.  Determining that will require a storm that gives us 3 or 4 inches of rain, so the linearity of the scale and the accuracy of the painted lines can be compared with the official and wedge gauges.

The butterfly gauge:

This is the last - and least accurate - gauge.  Though it is hard to tell from the image that tube is held just about vertically, and yes it really claims we got 0.8" of rain, where the others all said 0.3" - 0.33". 

Like the gauge, the marks on the tube are only painted on, not embossed.  That, however, is because the tube is made of glass, not plastic.  Kind hard to emboss glass like this on the cheap.

The paint will, no doubt, come off with enough UV exposure, so it would probably be useless in a couple of seasons, even it it wasn't wildly inaccurate, which makes it effectively useless now.

But why is it inaccurate?  Simple... look at the top of the tube, which is what it is hung from in that brass holder:

See that lip?  Much of the rain that lands on it runs into the gauge, but the scale is calibrated only for the inner diameter of the tube.  The net result is that the gauge collects a lot more water than it should given the scale, and the numbers are way off.

And since that lip is curved, I suspect wind has interesting affects too.  If there is no wind as the rain falls it is possible that more of the water that hits the lip winds up in the tube.  if there is wind, though, some may blow off the lip and result in a different - though still inaccurate - reading.  This is speculation on my part, but so far this gauge doesn't consistently read as a multiple of any other gauge, so there is something odd going on.

Anyway, good rain gauges have a knife-like edge at the top to clearly define the collection area, not a hazy, rounded boundary like this one.

I will keep this gauge in the set and collecting data from it though I know it is useless for real record keeping.  It is actually kind of fun to see just how far off it can be.  More than 2X in this rain, obviously, but the range of differences is fascinating to a nerd like me.

There you have it... some information on the various gauges so far.  Interesting to me, at least.  Hopefully you too.