Any empty container will do (although some are better than others.)
First, I will give the necessary technical details; and then I will give you specific details for a particular common container.
Your container can be a jar, a rectangular or round pan, a funnel on top of a glass, or any open top container. Just select a container and sit it outdoors in an open area away from trees or buildings. Then wait for rain.
After the rain has stopped, empty your "rain gauge" into a measuring cup or pitcher that is graduated in ounces. Determine how many ounces you have collected (you may want to estimate decimal fractions of an ounce also.)
Now comes the tricky part. You need to make some calculations. Some of these you can do once before you put out your "rain gauge." Measure the very top opening of your "rain gauge." If it is rectangular, measure the width (W) and the length (L). Then calculate the total area of the opening with the formula:
AREA = W times L
If the opening is a circle, measure the diameter (D) (that's the longest measurement across the circle.) Then calculate the total area of the opening with the formula:
AREA = .79 times D times D
If the opening of your container is not a circle or a rectangle (Note: a square is a special form of a rectangle where W = L) then you must find the area by some other means. Once you know the area, you can use the following formula to determine the amount of rainfall in inches:
INCHES = 1.8 times OUNCES divided by AREA
There, isn't that easy?
If you don't think so, I will give you a specific example.
Many people save empty MIRACLE WHIP (R) salad dressing jars. Such a jar looks something like this:
Current jars have an opening whose inside diameter is approximately 2.42 inches. This yields an area of 4.63 square inches. Your rainfall formula becomes:
INCHES = 1.8 times OUNCES divided by 4.63
This can be simplified by dividing 1.8 by 4.63. The simplified formula is:
INCHES = .39 times OUNCES
(Note: If you have an old jar, you may find the diameter is 3 inches. This gives an area of 7.11 square inches. Then, the formula will be:
INCHES = .25 times OUNCES)
Take a piece of masking tape and attach it vertically to the outside of your jar. Measure out 2.56 liquid ounces of water. (You may find this difficult and you may need a laboratory graduate to measure this amount accurately. If your graduate is calibrated in milliliters, measure 75.7 milliliters.)
Now pour the measured water into your "rain gauge."
Using permanent ink draw a horizontal line on the masking tape at the level of the water. Mark it "one inch."
Add another 2.56 ounces of water and mark the level, "two inches."
Continue adding the same amount of water and marking succeeding levels as 3, 4, 5, etc. inches.
Now your gauge is complete.
(Note: If you have the old style MIRACLE WHIP jar, It is easier. You add exactly 4 ounces of water for each inch to be marked.)
INCHES = .21 times OUNCES.
After collecting rain, you can read off the water volume by using the fact that each one-quarter cup is two ounces. You can estimate to the nearest tenth of an ounce and apply your formula.
Project by Bob LaFara
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http://www.auntannie.com/castoaks
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