It is located about 10 feet south of the house, which is about 15 feet high, and to the west of a fence and small trees as well as the tree in the photograph. The gauge was certainly sheltered from the wind. The recommendation is that the gauge be about twice the distance from the height of the nearest obstacles, but still sheltered from the wind. Or, rain may be blown into or away from the gauge by wind gusts. Rain may be blocked by nearby obstacles causing the number to be lower than it should. According to the National Weather Service and CoCoRAHS (both of which use citizen volunteers to measure rainfall), “exposure” of the rain gauge is important. The exposure of the rain gauge is undoubtedly the greatest source of error. Looking up from where the rain gauge was before Time 2. Also, because my rain gauge was open at the top, some of the water could have evaporated, although evaporation was probably minimal, given the high relative humidity. Switching the rain gauges adds uncertainty and so does the previously-mentioned spillage when I poured the remaining water in the gauge into the tube. It is also possible that the 0.98 inches could be high, but I doubt it: I had briefly run a sprinkler hose at a low setting, but I had moved the gauge out of the way and I turned the water off immediately once the rain started. Runoff from this branch could have added to the total before I moved the gauge four feet to the west for the last two measurements. I noticed on September 13 that the tree had intruded again: the end of one branch was about 10-15 feet over the gauge, or slightly to the east. The metal gauge had been in the same place for several years, but I have moved it in the past year away from a growing tree. Why do I add “more or less”? Because there is uncertainty in the measurements. The final number: 16.37 inches on rain, more or less. I added a conservative 0.2 inches, since this gauge was under trees (marked as 2 on the graph). Fortunately, I have a second rain gauge in my backyard – a plastic gauge that registered about 0.25 inches. I went outside to check our gauge – only to see that it had been knocked over (probably by raccoons). I awoke on the morning of September 15 th and heard reports that up to 2 inches of rain fell overnight. (The lower shows the uncorrected values.)īut the rain hadn’t stopped. The result was 0.38 inches more than my rough estimate from the night before - a storm total of 14.52 inches up to this time. After bailing out five full tubes of rain, I poured the remaining water through the funnel into the tube to a depth of 13.5 inches, spilling a little bit during this process. Were my measurements accurate? On Friday morning, September 13, I took measurement using a more accurate method to compare with my estimates. I recorded measurements to within the nearest quarter inch (see the graph below). With these flaws, the lack of the ten-to-one exaggeration of depth, and some measurements being taken in the dark with a flashlight, my data were only approximate. That evening I found that the bottom of the gauge sagged in the middle, leading to an even deeper measurement than the downtilt side. The gauge tilts slightly, so I took a measurement on the uptilt side and the downtilt side and took an average. On the morning of September 12th, the gauge was so full and heavy, with over seven inches of rain, that I decided to stick a yardstick in the gauge to measure the rain amount, and save pouring into the inner tube for the end of the storm. The funnel and inner tube doesn’t quite fit, so, I leave the gauge open and then pour the rain into the inner tube using the funnel. I inherited it from a weather-observing neighbor who moved away. Thus, each inch in the tube is equivalent to 0.1 inches (a tenth of an inch) of rainfall. The inner tube’s diameter is just small enough to make the depth of rain ten times what it would be in a gauge without the tube and funnel. It has a funnel that deposits rain into an inner tube with a smaller diameter (like this one), but bigger. The gauge is the same type the National Weather Service uses. It’s also about the many possible sources of error when making rain measurements – from old rain gauges to growing trees and even, possibly, inquisitive raccoons.īy Monday morning (September 16), I had measured over 16 inches in our backyard rain gauge from the storm which began September 10. This is the story of my attempts to measure rain during the storm. Which brings us to the questions - How do you measure rain? And how accurate are the measurements? Even though I have done weather research for many years, during this storm I was reminded how hard it is to measure rain accurately. In early September of 2013, the Boulder, Colorado, area had huge amounts of rain. Peggy LeMone is an NCAR Senior Scientist who studies weather and cloud formation.
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