How to read your rain gauge.
It is important to be as accurate as possible when reading
your gauge. Scientists, engineers, and other professionals
may use your data.
The rain gauge is composed of three parts: a funnel, a measuring
tube, and a 4-inch diameter overflow tube, and a mounting
bracket. The funnel directs the precipitation into the measuring
tube and magnifies it by a factor of 10. This allows observers
to report rainfall to the nearest 0.01" (one hundredth of
an inch). The measuring tube, when full, will hold "one inch"
of rainfall. When it rains more than one inch, the excess
water collects in the overflow tube.
Reading the rain gauge is a very easy thing to do, but mistakes
can occur. Here are several situations that you may encounter
when reading your rain gauge and how you should deal with
1. The surface of the water in the gauge looks curved. How
do I know where to read?
As water fills up the measuring tube, a curved surface is
formed called a meniscus. This meniscus is formed by
the surface tension of a liquid in contact with the sides
of the tube. Always read the base of the meniscus when taking
measurements. Here is an illustration to help you understand
2. What do the numbers mean on the side of the measuring tube
and how do I read them?
The measuring tube is designed to magnify the rainwater. This allows
more precise readings. Remember that the measuring tube when full
holds one inch of precipitation.
The measuring tube is divided, marked and labeled in smaller
parts (increments) in tenths and hundredths. Imagine that the
gauge is equivalent to a dollar, but instead of having one paper
bill you have ten dimes or one hundred pennies. The smallest
line on the measuring tube would equal one penny. We would write
in our checkbook that we have 0.01 dollars, and when recording
rainfall you would write 0.01 inches. This is also called one
hundredth of an inch.
The longer lines on the measuring tube would be like a dime.
We record a dime in our checkbook as 0.10 or if you have two
dimes 0.20. When recording rainfall that has reached these lines
we write them in the same way 0.20
If rain has reached the line .30, you would say that
you had three tenths of an inch of rain or 30 one-hundredths.
Here is an illustration of the calibrated measurement
tube of the rain gauge that contains 30 one-hundredths
of an inch (0.30 inches).
3. What if I have a measurement on the middle line between 0.30
This would be saying that you have three dimes and five
pennies. You would record this as 0.35 inches and report
that you have 35 hundredths of an inch of rain. There
are nine smaller lines (hundredth marks) between each
larger lines (tenth marks). This means that you will never
have a measurement like 0.0599, for this measurement would
Always remember that the greatest number of decimal places
you will have is two. Please do not use your ruler when taking
measurements. A ruler is not calibrated to the measuring tube
and will affect the data.
The most common error observers make is confusing tenths
and hundredths. Please be careful, and don't make this mistake.
4. I know some rain fell, but there is nothing (or just a tiny bit but
less than 0.01 inch) in my gauge. What should I report?
We call this a "trace" of rain. This should
be reported as "T" on your reports. Even if
there were just a few drops that don't even dampen the
gauge, you should still report a trace.
5. What if there is no precipitation to report?
Just record zero (0.00). It is very important to report 0.00,
especially on days when there are scattered storms in the area.
It is just as important to know where it didn't rain,
as it is to know how much it rained where it did.
If you really donít want to enter a 0.00 every day, we have provided a
"Multiple Zero Readings" form that makes it very easy to go back in later
and enter all the 0.00 readings you skipped.
6. What do I do if the inside measuring tube is full?
We can determine that more than one inch of rain has fallen
by simply observing that the inside measuring tube is full and
the additional water has spilled out into the larger container.
Remember that the measuring tube will only hold one inch of
water. Follow these steps when measuring rainfall more than
Step 1. You will first need to read the precipitation
in the measuring tube, record it, and empty the water in the
measuring tube. Do not empty out the water that has collected
in the Overflow Tube.
Step 2. You will need to carefully pour the remaining
water, from the overflow tube, into the measuring tube, record
it, and add up the totals. The easiest way of doing this is
by pouring the water into the measuring tube a little bit at
a time. You do not have to fill up the tube to the one inch
mark every time, rather fill up the tube half or three quarters
of the way, record it, and add up the totals. Be careful
not to spill any precipitation. When in doubt, place a bucket
or other large container under the gauge as you pour, so that
you can collect the water if you do happen to spill. If more
than 2.00 inches of rain fell, you will have to repeat the process
several times. Make sure you write down each rainfall quantity
and add up the total. If you don't write it down immediately,
you could forget.
Why canít you just assume that if I donít record, it should be 0.00?
If we assume all blanks are 0.0 and see that you have a blank but another recorder in your area has 0.50Ē, it would mean we have to assume 0.00 for you. BUT, if you just happened to be on vacation and hadnít gotten around to entering your actual 0.89Ē, that would be incorrect. Consequently, if there is no entry, we have to ignore it.
7. Do I report morning dew that has collected in my rain gauge?
On damp, dewy spring and summer mornings a few drops of moisture
may collect in your rain gauge. Do not to report this as rain.
If the only moisture in your gauge came from dew (rarely will dew
collect to more than 0.01 inch), report "0.00".
8. Should I keep a written record of my precipitation data?
Yes, it is important to maintain a written record of your data.
This gives you a permanent record and gives us a backup in case
we have computer or phone problems. This is also important if we
find that your data have been entered erroneously into the NeRAIN
computer system. It might happen that a NeRAIN student intern
spots an suspicious or erroneous report for your station. If so,
they may contact you and discuss it with you. Keep your written
records handy. Please do not be offended, we are simply trying
to collect the best data as possible. You can print out our NeRAIN
Precipitation and Snow Measurement Form to record your
9. What If I leave for a weekend or for a vacation and find precipitation
in the gauge when I get back?
If you are gone for a few days and return to find precipitation
in your gauge, please report the date as "multiple".
Specify, in the comments section, over what period of days
the moisture accumulated. This is very helpful information to us.
If you think you know approximately when it fell, make note of that.