Saturday, November 19, 2011

Unexpected Rainfall Numbers

UPDATE ON 11/20/11: this post contains an error of vast and troubling proportion: 0.20" is not the same as 0.02".  Yours truly apologizes and retracts it here.

We had something that might have been called rain yesterday.  It was very tiny drops, off and on, for hours.  My wife might have called it "measurable fog."  It was still going after dark so I left reading the gauges for this morning.  The results, though, are a surprise:
  • The "official" gauge - the one I trust the most so far - read 0.20".  That seemed to make sense to me on the level of gut feel.  Everything was wet for some time yesterday.
  • The old yellow, weather.com, and wedge gauges, though, were all either empty or showed just a trace.  Nothing measurable in any of them.
  • The butterfly gauge - which usually reads at least twice what the others claim - contained just 0.10".
How to explain that?

Well, my best guess is that we had plenty of evaporation overnight.  Things were dryish this morning, which means the water went somewhere.  And the three gauges that had only a trace also have the largest openings, making it easy for evaporating water to escape.

The butterfly gauge had more in it earlier in the day, yesterday, than 0.10".  I noted it in the afternoon when I picked up the mail, but I wasn't taking readings as it was still raining at the time.  So it must have evaporated out of there.  No one emptied it, I know that.

The official gauge is interesting.  Because it is a small cylinder enclosed in (and protected by) a larger cylinder, and since there is a funnel covering most of the interior cylinder and all of the outer cylinder, I suspect evaporation is slower.  It's a pretty small hole for the water to evaporate out of in any case, so while it can happen, it takes more time.

In short, though it seems counter intuitive, I think the official gauge wins again, and that it is design flaws in all the others that made them read too low this time around.

Not what I anticipated - particularly with the butterfly gauge - but it makes sense.

The spreadsheet has been updated with the new numbers.

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