10/25/2014

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So much rain fell in Boulder we can measure in milk cartons

So much rain fell in Boulder we can measure in milk cartons
How much 8.62" of rain in a standard rain gauge fills a gallon jug of milk -- just short of half way!
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The Boulder, Colorado area just suffered through some historic flash flooding at the hands of some incredible rainfall that for many around here would be impossible to fathom.

If you look at this map below, it is a 24 hour rainfall total measurement from the volunteer CoCoRahs rainfall network taken the morning after the torrential downpours in Boulder County:


You can see a cluster of measurements there reporting over 8 inches of rain, with one spotter reporting an amazing 8.62 inches -- much of that fell over the span of just a few hours.

I myself had a difficult time trying to picture how much rain that is -- to give a very broad comparison, the thunderstorms we had on Aug. 29 that caused flooding problems around the Seattle area amounted to about 1.25" in one hour.

But I am a member of the CoCoRahs network here in Washington and have the same standard rain gauge in my backyard that those spotters in Boulder were using. So I wondered, just how much is 8.62 inches of rain?

First, a picture of the rain gauge:



It has a 4" diameter at the top that collects the rain and funnels it into a cylinder that is marked up to 1 inch of rain. There is an overflow hole at the top that spills rainfall over 1" into the outer container, which can hold an additional 10 inches of rain.

So I went and measured out 8.62 inches of rain to see what it would look like in the gauge:



Remember, that includes the inner cylinder full of rain as well. Wow!

So, that's neat. But how much rain *is* that really?

Rummaging through our recycle bin (luckily it's been nearly 2 weeks since pickup day) I found an old 1.5 liter bottle of Diet Pepsi and an old milk carton.

First up, the soda bottle. I carefully poured the rain gauge water in there to find it filled up the entire 1.5 liters and had nearly a half inch of rain to spare:



How about a gallon milk jug? Filled it nearly 1/2 the way! I put the top of the rain gauge next to it so you could see how much rain fell over that small of an area (again 4" diameter) to produce that much water:



So imagine dumping millions of those soda bottles over an entire region, or roughly half a gallon of a milk carton spaced out every 4 inches or so. And you can see how that could cause some serious flooding, especially in the mountainous areas where much of that water just ran off, combining with water at lower elevations.

I also tried to simulate this a bit with a hose showing how much water that was. Not sure it has quite the impact I was hoping for, but here you go:



Hopefully their weather pattern shifts soon and to help them recover.

P.S. I went and checked and that same spotter that reported 8.62" of rain on Sept. 12 reported an additional 4.33" of rain on Sept. 13 -- for over a foot of rain in two days (12.95").

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