In early September of 2013, the Boulder,
Colorado, area had huge amounts of rain. 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.
This is the story of my attempts to measure
rain during the storm. 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.
By Monday morning (September 16), I had
measured over 16 inches in our backyard rain gauge from the storm which began
September 10. The gauge is the same type
the National Weather Service uses. It has a funnel that deposits rain into an
inner tube with a smaller diameter (like this one), but bigger. 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. Thus, each inch in the tube is equivalent to
0.1 inches (a tenth of an inch) of rainfall.
This makes it easier to read accurately!
My gauge is old. I inherited it from a
weather-observing neighbor who moved away.
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.
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
gauge tilts slightly, so I took a measurement on the uptilt side and the
downtilt side and took an average. That
evening I found that the bottom of the gauge sagged in the middle, leading to
an even deeper measurement than the downtilt side. 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. I recorded measurements to within
the nearest quarter inch (see the graph below).
Were my measurements accurate? On Friday
morning, September 13, I took measurement using a more accurate method to
compare with my estimates. 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. 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. On the graph, this is marked as 1. (The lower shows the
uncorrected values.)
But the rain hadn’t stopped. I awoke on the morning of September 15th and
heard reports that up to 2 inches of rain fell overnight. I went outside to
check our gauge – only to see that it had been knocked over (probably by
raccoons). Fortunately, I have a second
rain gauge in my backyard – a plastic gauge that registered about 0.25 inches.
I added a conservative 0.2 inches, since this gauge was under trees (marked as
2 on the graph).
The final number: 16.37 inches on rain, more or less.
Why do I add “more or less”? Because there is uncertainty in the
measurements. 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. 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. 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. 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. 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. 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.
The exposure of the rain gauge is
undoubtedly the greatest source of error.
According to the (link is external)National Weather Service and (link is
external)CoCoRAHS (both of which use citizen volunteers to measure rainfall),
“exposure” of the rain gauge is important. Rain may be blocked by nearby
obstacles causing the number to be lower than it should. Or, rain may be blown
into or away from the gauge by wind gusts.
The recommendation is that the gauge be about twice the distance from
the height of the nearest obstacles, but still sheltered from the wind.
The gauge was certainly sheltered from the
wind. 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.
There is a much smaller tree to the southwest.
All the obstacles suggest that some rain
could have been blocked from reaching the gauge, which would imply that the
rainfall total is too small. On the
other hand, some rain might have been running down the branch in the picture.
(In fact, because of the large amount, I thought this might be the main effect
before doing some research on exposure)
It is also recommended that the gauge be
level, which it wasn’t. I’m not too
worried about this, since it was nearly vertical.
The conclusion? There was a lot of rain. It could have been an inch more than my
measurement or an inch less. Acknowledging this is called reporting error. It
doesn’t mean that the measurements are wrong, it just gives an idea of how
accurate they are. My total was not the largest; there were at least two other
measurements near 18 inches.
Now that I’ve described all that can go
wrong measuring rainfall, let me add that, putting a rain gauge in the right
place, and taking an accurate rainfall measurement is fairly easy. If you have
a perfect cylinder, simply stick a ruler in and read the depth (make sure to
correct for any offset of the “zero” line and correct for this offset; and see
if the ruler pushes the water level up very much).
If you have a bucket (or glass) with sides
that aren’t straight up and down, you’ll need to do a bit of math to figure it
out. Here’s what you’ll need to do:
1、Measure the
diameter of the bucket at the level of the rain. Subtract out twice the thickness of the
walls.
2、Measure the
diameter of the bucket at the bottom in the same way.
3、Calculate the
average of the two diameters.
4、Divide by two to
find the average radius.
5、Find the average
volume of rain = Depth x radius x radius x 3.14.
6、Find the area at
the top of the bucket (this is the area over which the rain is collected).
1.Measure the diameter
2.Divide the diameter by 2 to get the
radius
3.Area = radius x radius x 3.14
7、Divide the
rainfall volume by this area to get the rainfall.
It would be an interesting exercise to put several buckets (or rain gauges) in different places in a field, your back yard, or your schoolyard to see how much the measurements vary. Soup cans, though not perfect, would work pretty well. I might try this during the next rainstorm. (I hope not too soon!)
