I should know this...

A friend of mine has asked me a question concerning a simple rain gauge he is trying to use to monitor rainfall. It is basically a graduated cylinder with markings in inches going up the side. He is telling me that his simple rain gauge is reading consistently twice the amount of rain being reported via the local weather service. I asked him all the basic questions…is it away from trees, buildings etc. And he said it was…yet consistently he get 2X the amount of rain reported.

Now…he is a mechanical engineer and I am an instrumentation technician and the discussion that broke between us is leaving me confused. If you look up “How to make your own rain gauge”, it appears to be unimportant of which container to use. A 2 liter bottle with ruler seems to be the standard method of making one of these for your back yard. But my question is why isn’t the size/diameter of the collection chamber important when trying to collect rainfall for measurement to a reasonable level of accuracy.

As we were discussing this, I tried to guess my way through it as to why he might be reading double the amount of rainfall. Assuming he indeed was away from structures that could add to his lab beaker, and assuming one of his kids isn’t playing tricks…isn’t the diameter of the collection chamber of any importance? If anything, his 1 or so inch lab cylinder should actually miss more rain than it catches. If he were to pour the rain from that tube into another container, certainly he would get a different height of water even though the volume of water is the same. Can anyone help to explain this?

I really never gave it much thought since I have the Davis tipping bucket and all the work is being done automatically. But I’m also curious to know when is an inch of rain an inch of rain when catching it in a container.

If I put a 5 gallon bucket outside next to a 2 liter bottle, will each container show the same height of water after a rain? I suppose it would seeing that the relative areas are catching the same amount of rain. But then the more I keep thinking about it, it starts to become less clear how that could actually be the case.

interesting. now this engineer… he is comparing his measurement with the local weather site. My question is how far from the “official” site is he ?

Here at my station, the official NOAA station is 11 miles from where I am at . I figure that over that distance, rainfall will be different amounts.

Find out how far he is, where his rain guage sits? on a fence post by the trees or is it out in the open.

I do not think there is a universal standard dimension. Most ‘official’ rain gauges have a collecting area between 4 and 8 inches (100-200mm). So long as you do not go too small there should be little difference, there are enough rain drops randomly falling over that sort of area to even out any sampling error.

And you are correct, the ‘depth’ of water collected is independent of the area, but the volume will vary of course. Manual gauges often use a collection ‘beaker’ which has 1/10th the area of the collecting pan, or is tapered towards the bottom, it just makes the measurements of small amounts of rain easier to read. The measuring scale has to be non-linear for tapering vessels of course!

Well I thought the same thing. He lives 40 miles from my weather station and I have 2.71 inches for the month of June. He’s saying he’s seeing different reported amounts in his local area from 3 to 4 inches. He’s got over 7 inches of rain measured.

I’ll tell him to move it and see what he gets.

40 miles is a huge distance when it comes to weather. what does the local cocorahs say the amounts being in his area? he could check with those areas to see if he is fine or needs to purchase a real man’s weather station… :?
Cocorahs uses a similar guage, perhaps he could invest in one of theirs, but check the daily totals in the area against his guage
http://www.cocorahs.org/States.aspx

Check out “rain gauge” at Wikipedia. As long as the funnel ratio to graduated cylinder size is correct and the graduated markings are also correct then it should measure correctly. Don’t forget that official weather stations days run from 06Z to 06Z while ours tend to be midnight to midnight. There could be quite a difference in precip amounts depending on how far off you are from GMT.

rick

This shouldn’t make much difference because I think the comparison is being done over a month. It would need a very big rain event to add a couple of inches extra in the 6 hours difference at the start or end of the month.

One question that no-one has asked…he is using JUST the cylinder, i.e. he hasn’t put a funnel on top of it to increase the catchment area?

Is there anything with elevation that may be playing a role here? For example I drive 30 miles west to get to work for the past 10 years. About 1/2 way, there is a bit of a ridge line. It is not that big (probably a few hundred feet high), but it is big enough that it effects the weather locally. When fronts blow through, the air has to rise the water is squeezed out. On radar I see many storms loose energy at this point. Driving through it, the rainfall is always heavier than even 2 miles away in either direction.

Well…the reason I referenced my weather station 40 miles away is that my totals are fairly consistent with the totals he’s getting from local reports 40 miles from me. As it has been said, rain fall amounts will not be exact across a region, but a reading of 7 inches opposed to a reported ~3.5 inches is way out there.

I was getting confused about the physics or mechanics or geometry or whatever of the collection container size vs. collecting rain falling from the sky. But after thinking about it and actually typing out the question, it became clear that the diameter of the container is proportional to the amount of rain it captures. So a 5 inch diameter will collect the same amount as a 10 inch diameter and can be measured the same way since the area of the containers is proportional to the amount of rain that can be collected. I know what I’m trying to say but I don’t know if I’m explaining it right. I originally thought his container was too small.

So what remains is tweaking on things to increase accuracy. In this guy’s case, his collection container size is not the issue for numbers this far off. I originally thought there was a ‘standard’ size needed but now I can see it really doesn’t matter TOO much. Something else is throwing it way off whether it be bad placement or his kids playing tricks on him. I think he needs to relocate his gauge to see if that changes anything.

I do appreciate the feedback that I’ve gotten. Thanks.