Rain gauge not always accurate for Dewey Beach reporting

Over the past three months I’ve come to a very disturbing conclusion. I’ve discovered that I’ve been reporting inaccurate rainfall totals from my home here in Dewey Beach.

During those three months I’ve been participating in the CoCoRaHS program, where volunteers log their daily rainfall onto a Web site that monitors rainfall patterns across the country. Comparing the rainfall totals I got using the official CoCoRaHS manual rain gauge to my automated weather station, I discovered a large discrepancy. As difficult as it is for me to admit this mistake, it’s time for me to fess up and correct the record.

I’ve been using a Davis Instruments Vantage Pro 2 weather station to measure and record my weather statistics. This is considered one of the best weather stations available, and cost almost $1,000 four years ago.

However this weather station, as most automated weather stations do, uses a tipping bucket method to collect and measure rainfall. Rainfall enters the top of the device through a small hole, drops down into a small catch bucket that is on a fulcrum with a second bucket. When a hundredth of an inch of rain is collected in the bucket, the weight of the rain pushes the bucket down where it makes an electrical connection and empties at the same time. The second bucket is now in position to catch the rain.

It all works like a seesaw. Each time the bucket makes the connection, a hundredth of an inch of rain is recorded on my station monitor.

I had heard rumors that the tipping bucket method of measuring precipitation was not always accurate. However, when I first compared mine to a manual collection gauge I had in my backyard, the results were usually within a hundredth of an inch, so I wasn’t worried. The trouble was, I only compared the two after a brief rainstorm, and not for longer periods of time.

Now that I’ve been taking measurements for the past three months with both my automated gauge, and the official 4-inch wide manual gauge that CoCoRaHS specifies, a larger discrepancy has emerged. It now appears that I’ve been under reporting my monthly Dewey Beach rainfall totals by between 12 and 20 percent.

The CoCoRaHS Web site reports that when they did their tests comparing their manual collection gauge with tipping bucket gauges “the majority of automated rain gauges, when summed over several months or years, report less precipitation than actually fell by a significant amount — sometimes 25 percent or more.”

In doing some research on tipping bucket gauges, I’ve learned that they are less accurate in very light rain conditions, or very heavy rainy conditions. Strong winds can also effect their accuracy. It is not uncommon to have 20 to 35 mph winds blowing across my house.

When CoCoRaHS started its program 10 years ago, they decided that, to get the most accurate results, they need everyone to use the same manual rain gauge. Yes, it is a pain to have to go outside to physically check your gauge each morning. But I now believe the results are more accurate and worth the trouble.

The first month I participated in the program was May. At the end of that month my automated gauge showed a total of 1.71 inches of rain, while my manual CoCoRaHS gauge had a total of 2.16. June showed similar results, with the automatic gauge totaling 1.72 inches, and the manual gauge 2.06. In July we had much less rain, but my manual gauge still showed a higher total with .50 inches compared with only .44 inches from the weather station.

I’ve been reporting that my home on South Fidalgo gets significantly less rain than the downtown fire station, where our official weather statistics are gathered. At the end of 2007 I wrote that downtown rainfall was 27.51 inches, and here at Dewey Beach we only received 20.89 inches.

If you add an additional 15 percent to account for the automated gauge discrepancy, the Dewey Beach total is now just over 24 inches. Still less than downtown, but not by nearly as much. You can rest assured that from now on I will be reporting my manual rain gauge results, and only using the automated gauge as a backup.

The CoCoRaHS program (http://www.cocorahs.com) is an excellent tool for studying rainfall patterns and amounts. To date I am the only volunteer on Fidalgo Island, and one of six in Skagit County. I hope in the near future we get more volunteers on Fidalgo Island so we can accurately determine rainfall amounts in the different regions of our community.

To sign up just go to the CoCoRaHS Web site, click the “join CoCoRaHS” button in the upper right hand corner. All the instructions are there, and you can even purchase your official rain gauge from the site.


July summary

July was the first month this year where sunny days broke the double digit barrier. I counted 18. The next highest total was half that many in June. The weather continued to be cooler than normal, with only half our normal rainfall. The most interesting note about July is that we got .03 of an inch of rain on the 29th, historically the driest day of the year.