how to make a rain gauge
- Thread starter ridesafe
- Start date
Eric-
As long as the diameter of the container is the same at the top (where the rain enters) as it is though out the rest of the collection container, then you just simply measure it like anything else. An inch is an inch, etc. If you wanted to build a device which incorporates a funnel to be able to get more accurate readings more easily, then you would need to use geometry and trigonometry to come up with the proper ratio for the size of the top of the funnel vs. your markings on the collection container. I don't have those numbers for you and quite honestly you can get the collection containers that have the funnel and insert collection tube already there for pretty cheap (20 bucks or so). That's what I use as the automated one with my weather station did not last even a half year before it stopped working. It is also what the NWS observers use.
-John
As long as the diameter of the container is the same at the top (where the rain enters) as it is though out the rest of the collection container, then you just simply measure it like anything else. An inch is an inch, etc. If you wanted to build a device which incorporates a funnel to be able to get more accurate readings more easily, then you would need to use geometry and trigonometry to come up with the proper ratio for the size of the top of the funnel vs. your markings on the collection container. I don't have those numbers for you and quite honestly you can get the collection containers that have the funnel and insert collection tube already there for pretty cheap (20 bucks or so). That's what I use as the automated one with my weather station did not last even a half year before it stopped working. It is also what the NWS observers use.
-John
[FONT=Arial, sans-serif]Okay, y'all got my curiosity up, my inner weather geek all excited and my mathemagical juices flowing, so here goes:[/FONT]
<hr size="2" width="75%">[FONT=Arial, sans-serif]There are probably at least two reasons to construct a rain gauge with a funnel collector atop a smaller diameter measurement cylinder: 1) To gather a more representative sample of rainfall over a bit larger area than that of a relatively narrow cylinder of perhaps 2 or 3 inch diameter, and 2) to make it easier to precisely measure relatively small amounts of rainfall, perhaps to the nearest 0.01", by "amplifying" the measurement.[/FONT]
[FONT=Arial, sans-serif]It would be rather difficult to inscribe 0.01" markings on a small gauge measurement cylinder, and reading the water level in such small increments would be difficult as well.[/FONT]
[FONT=Arial, sans-serif]One solution would be to collect the rainfall in a collection vessel with a cross sectional area of, say, 10 times the cross sectional area of the measurement cylinder, funneling the collected rain water into the narrower cylinder for measurement, thus increasing the height of the water in the measurement cylinder by a factor of 10.[/FONT]
[FONT=Arial, sans-serif]In that case if the graduations on the measurement cylinder were marked off in 0.10" intervals, each 0.10" would represent 0.01" of true rainfall.[/FONT]
[FONT=Arial, sans-serif]Since there is a multiplication factor of ten involved in this design, that does lead to another problem, namely the measurement of much heavier rainfall, since the multiplication would cause a really major downpour of even 3 inches to fill the measurement cylinder to a height of at least 30". How tall a measurement cylinder do you want?[/FONT]
[FONT=Arial, sans-serif]Perhaps a multiplication factor of five would be a good compromise, so graduations scribed on the measurement cylinder at 0.10" intervals would each represent 0.02" of true rainfall, while the same 30" measurement cylinder would accommodate up to 6 inches of rainfall. You would still be able to "eyeball" the water level midway between two graduations to effectively read the water level in the measurement cylinder to determine the true rainfall amount to the nearest 0.01". This would require a collection vessel with a cross sectional area five times that of the measurement cylinder, with the collected water funneled into the narrower measurement cylinder.[/FONT]
[FONT=Arial, sans-serif]This brings us to the mathematics of cross sectional areas.[/FONT]
[FONT=Arial, sans-serif]The most commonly available vessels that might be used as either the collection vessel or the measurement vessel are indeed cylindrical. And the formula for the area of a circular cross section is:[/FONT]
[FONT=Arial, sans-serif]Where (r) is the radius (half of the diameter) of the cylinder. So the cross sectional area of a cylinder with an internal diameter (ID) of, say 3 inches is:[/FONT]
[FONT=Arial, sans-serif]or approximately 7.07 square inches. For a multiplier of five, you'll need a collection vessel with a cross sectional area of 5 * 7.07 = 35.35 square inches, and the diameter (d) of a circle with an area (A) of 35.35 square inches is:[/FONT]
[FONT=Arial, sans-serif]So the diameter of the collection funnel must be:[/FONT]
[FONT=Arial, sans-serif]or approximately 6.71 inches.[/FONT]
[FONT=Arial, sans-serif]The slope of the sides of the funnel is not relevant, just the cross sectional area of the top of the funnel and also the cross sectional area and height of the measurement cylinder that it is fitted to.[/FONT]
[FONT=Arial, sans-serif]So all you need for this example is a collection funnel with a top diameter of 6.71 inches fitted to a measurement cylinder with a diameter of 3 inches. A measurement cylinder with a height of 30 inches would accommodate a total rainfall of 6 inches.
It would probably also be best if the collection funnel had a vertical "fence" fitted around its outer edge and extending up perhaps an inch or two to more clearly delimit the edge of the collector surface. That, and a means of scribing the measurement cylinder in 0.10 inch intervals, each of which will represent 0.02” of rainfall..[/FONT]
[FONT=Arial, sans-serif]Do you want to fabricate such a contraption? It might be a whole bunch easier to just buy one![/FONT]
[FONT=Arial, sans-serif](Note: The "tipping bucket" rain gauge does a much better job of solving the fine measurement problem while also accommodating large rainfall amounts. The collection "bucket" has a very small capacity, and each time it fills, it quickly tips, dumping its contents, then immediately resetting, while also tallying the number of "tips".)[/FONT]
<hr size="2" width="75%"> [FONT=Arial, sans-serif]Any wizards out there feel free to critique this presentation, and although I am a degreed "mathemagician", my 'rithmetic is often more like that of the Jethro Bodine School o' Cypherin', so feel free to make any corrections to my cypherin' as well.
[/FONT]
<hr size="2" width="75%">[FONT=Arial, sans-serif]There are probably at least two reasons to construct a rain gauge with a funnel collector atop a smaller diameter measurement cylinder: 1) To gather a more representative sample of rainfall over a bit larger area than that of a relatively narrow cylinder of perhaps 2 or 3 inch diameter, and 2) to make it easier to precisely measure relatively small amounts of rainfall, perhaps to the nearest 0.01", by "amplifying" the measurement.[/FONT]
[FONT=Arial, sans-serif]It would be rather difficult to inscribe 0.01" markings on a small gauge measurement cylinder, and reading the water level in such small increments would be difficult as well.[/FONT]
[FONT=Arial, sans-serif]One solution would be to collect the rainfall in a collection vessel with a cross sectional area of, say, 10 times the cross sectional area of the measurement cylinder, funneling the collected rain water into the narrower cylinder for measurement, thus increasing the height of the water in the measurement cylinder by a factor of 10.[/FONT]
[FONT=Arial, sans-serif]In that case if the graduations on the measurement cylinder were marked off in 0.10" intervals, each 0.10" would represent 0.01" of true rainfall.[/FONT]
[FONT=Arial, sans-serif]Since there is a multiplication factor of ten involved in this design, that does lead to another problem, namely the measurement of much heavier rainfall, since the multiplication would cause a really major downpour of even 3 inches to fill the measurement cylinder to a height of at least 30". How tall a measurement cylinder do you want?[/FONT]
[FONT=Arial, sans-serif]Perhaps a multiplication factor of five would be a good compromise, so graduations scribed on the measurement cylinder at 0.10" intervals would each represent 0.02" of true rainfall, while the same 30" measurement cylinder would accommodate up to 6 inches of rainfall. You would still be able to "eyeball" the water level midway between two graduations to effectively read the water level in the measurement cylinder to determine the true rainfall amount to the nearest 0.01". This would require a collection vessel with a cross sectional area five times that of the measurement cylinder, with the collected water funneled into the narrower measurement cylinder.[/FONT]
[FONT=Arial, sans-serif]This brings us to the mathematics of cross sectional areas.[/FONT]
[FONT=Arial, sans-serif]The most commonly available vessels that might be used as either the collection vessel or the measurement vessel are indeed cylindrical. And the formula for the area of a circular cross section is:[/FONT]
[FONT=Arial, sans-serif]π * r²[/FONT]
[FONT=Arial, sans-serif]Where (r) is the radius (half of the diameter) of the cylinder. So the cross sectional area of a cylinder with an internal diameter (ID) of, say 3 inches is:[/FONT]
[FONT=Arial, sans-serif]π * (3/2)²[/FONT]
[FONT=Arial, sans-serif]or π * (1.5)² = 7.0685...[/FONT]
[FONT=Arial, sans-serif]or π * (1.5)² = 7.0685...[/FONT]
[FONT=Arial, sans-serif]or approximately 7.07 square inches. For a multiplier of five, you'll need a collection vessel with a cross sectional area of 5 * 7.07 = 35.35 square inches, and the diameter (d) of a circle with an area (A) of 35.35 square inches is:[/FONT]
[FONT=Arial, sans-serif]d = 2 * √(A/ π)[/FONT]
[FONT=Arial, sans-serif]So the diameter of the collection funnel must be:[/FONT]
[FONT=Arial, sans-serif]d = 2 * √(35.35/π)[/FONT]
[FONT=Arial, sans-serif]or 2 * √(11.25225...)[/FONT]
[FONT=Arial, sans-serif]= 2 * 3.354438...[/FONT]
[FONT=Arial, sans-serif]or 2 * √(11.25225...)[/FONT]
[FONT=Arial, sans-serif]= 2 * 3.354438...[/FONT]
[FONT=Arial, sans-serif]or approximately 6.71 inches.[/FONT]
[FONT=Arial, sans-serif]The slope of the sides of the funnel is not relevant, just the cross sectional area of the top of the funnel and also the cross sectional area and height of the measurement cylinder that it is fitted to.[/FONT]
[FONT=Arial, sans-serif]So all you need for this example is a collection funnel with a top diameter of 6.71 inches fitted to a measurement cylinder with a diameter of 3 inches. A measurement cylinder with a height of 30 inches would accommodate a total rainfall of 6 inches.
It would probably also be best if the collection funnel had a vertical "fence" fitted around its outer edge and extending up perhaps an inch or two to more clearly delimit the edge of the collector surface. That, and a means of scribing the measurement cylinder in 0.10 inch intervals, each of which will represent 0.02” of rainfall..[/FONT]
[FONT=Arial, sans-serif]Do you want to fabricate such a contraption? It might be a whole bunch easier to just buy one![/FONT]
[FONT=Arial, sans-serif](Note: The "tipping bucket" rain gauge does a much better job of solving the fine measurement problem while also accommodating large rainfall amounts. The collection "bucket" has a very small capacity, and each time it fills, it quickly tips, dumping its contents, then immediately resetting, while also tallying the number of "tips".)[/FONT]
<hr size="2" width="75%"> [FONT=Arial, sans-serif]Any wizards out there feel free to critique this presentation, and although I am a degreed "mathemagician", my 'rithmetic is often more like that of the Jethro Bodine School o' Cypherin', so feel free to make any corrections to my cypherin' as well.
[/FONT]
Last edited:
Now if you'd like to buy a rain gauge, here are some that might be of interest.
Click → AmbientWeather.com: Rain Gauges.
(Although my personal preference wold be for either one of the complete <style type="text/css"> <!-- @page { margin: 0.79in } P { margin-bottom: 0.08in } --></style>[FONT=Arial, sans-serif]Davis Instruments Vantage Pro2 Weather Stations[/FONT] or perhaps the considerably more economical <style type="text/css"> <!-- @page { margin: 0.79in } P { margin-bottom: 0.08in } --></style>[FONT=Arial, sans-serif]Davis Instruments 6250 Vantage Vue Wireless Weather Station.)[/FONT]
Click → AmbientWeather.com: Rain Gauges.
(Although my personal preference wold be for either one of the complete <style type="text/css"> <!-- @page { margin: 0.79in } P { margin-bottom: 0.08in } --></style>[FONT=Arial, sans-serif]Davis Instruments Vantage Pro2 Weather Stations[/FONT] or perhaps the considerably more economical <style type="text/css"> <!-- @page { margin: 0.79in } P { margin-bottom: 0.08in } --></style>[FONT=Arial, sans-serif]Davis Instruments 6250 Vantage Vue Wireless Weather Station.)[/FONT]
A way around having to worry too much about overflow is to have the smaller diameter tub sit inside a larger diameter tube that is also covered by the top of the funner (basically the outer edges of the funnel snap over the outer edges of the lager diameter tube. When the inner diameter tube is full, it just overflows into the larger diameter tube.
This is how my collection device works. The inner tube can collect 1" and the outer tube 10", so the system as a whole can collect 11" in one event. Never had it over flow on me yet!
-John
This is how my collection device works. The inner tube can collect 1" and the outer tube 10", so the system as a whole can collect 11" in one event. Never had it over flow on me yet!
-John
Eureka! Ya found it!
"Wold" you believe I throw a few of those in from time to time to see if y'all are awake?
If ya believe that, I have a Tempe (AZ) Town Lake I can sell ya! Ooops, too late. … Can I interest you in a Tempe Town Rotting Swamp and mosquito/West Nile virus breeding ground instead?
P.S.: From the Tempe Town Lake article:
… near the University of Arizona (in Tucson)? Yeah, right!
Last edited:
booondocker
New member
Okay, what about the effects of evaporation on that there equipment?
Where is the formula for rain that is "compacted" by the wind? "Sheet" rain??
Concern for freezing, and temperature adjustments???
Hummm....
Let's see how yah build one of deem suckers....??
For me...I jist use the dog bowl...
Hey...it sure is hard be'n the top of the pile when your do'n dah math and englisheesh.
Everbody is out to catch yah make a mestake.....but evrry good tacher should thro some out there to make the students feel goooood once in awhile....sort of makes a person feel human...even dough...we NO, yur not!
Human that is....
Where is the formula for rain that is "compacted" by the wind? "Sheet" rain??
Concern for freezing, and temperature adjustments???
Hummm....
Let's see how yah build one of deem suckers....??
For me...I jist use the dog bowl...
Hey...it sure is hard be'n the top of the pile when your do'n dah math and englisheesh.
Everbody is out to catch yah make a mestake.....but evrry good tacher should thro some out there to make the students feel goooood once in awhile....sort of makes a person feel human...even dough...we NO, yur not!
Human that is....
I also use the dog bowl method, but remember to account for the dog realted variables.
Therefore:
w= water in bowl
pw= previous water in bowl
fc= food chunks in bowl
sd= size of dog
td= thirst of dog, measured as a product of
lt=length of tongue
td=time drinking
and of course tb= tennis ball dropped in water dish
x=amount of rain, therefore to solve for x
x=if(tb=o,((w-pw)-fc)-(lt*td)),(((w-pw-tb)-fc)-(lt*td))
Therefore:
w= water in bowl
pw= previous water in bowl
fc= food chunks in bowl
sd= size of dog
td= thirst of dog, measured as a product of
lt=length of tongue
td=time drinking
and of course tb= tennis ball dropped in water dish
x=amount of rain, therefore to solve for x
x=if(tb=o,((w-pw)-fc)-(lt*td)),(((w-pw-tb)-fc)-(lt*td))
Last edited:
booondocker
New member
Trick is to be the FIRST to the dog bowl...this then negates the variables above....not always possible if someone gave the dog a big ole bite of their pizza the night before...but, hey...who said weather was an exact science anyways???