Weatherz School: Where rain reports come from
When rain is coming down in buckets, curiosity begs the question, how much are we getting? For an accurate answer, a literal bucket won’t cut it.
An official rain gauge used by the National Weather Service is 8” in diameter. It probably looks more heavy duty than what you have out back. The big outside tube is ten times larger than the inside tube that rain gets funneled into.
What looks like one inch of depth measures as a tenth of an inch. This way, we can accurately measure down to a hundredth of an inch.
Rain gauges like this are used at official weather stations, but that only accounts for a couple dozen locations in the Northland. There are a lot of gaps to fill, and the general public can help fill those gaps with a similar gauge on a smaller scale.
A 4” rain gauge still allows for accuracy down to one hundredth of an inch. The inner tube holds up to an inch. If we get more than that, the overflow goes in the big tube. To measure this, we would dump out the first inch, then pour what spills over into the inner tube to measure that.
We add all of the measurements together for the storm total. For example, if the overflow measures an additional inch with more spillover that ends up measuring 0.61”, the storm total would be 1+1+0.61=2.61”.
Any gauge that doesn’t use the two-tube method just isn’t going to have the same guarantee of accuracy. The 4” gauge is the standard for people who participate in CoCoRaHS.
That’s the Community Collaborative Rain, Hail, and Snow network. Anyone can sign up for this to report daily precipitation observations. These are collected by the National Weather Service as official rain totals.
If you see a report that says, “13 NNE Duluth,” that means the CoCoRaHS observer lives 13 miles north northeast of the City’s official coordinates.
If you’ve ever yelled at the TV saying, “that’s not how much rain I had!” You can join CoCoRaHS and the amount you have will be one of the official reports. In fact, if we want to have the best weather data, there should be a CoCoRaHS observer every square mile.