Spring 2010

How Does Your Rain Gauge Work?

When it rains, your gauge measures the amount of precipitation that falls through the area at the top of the gauge.  When you read the gauge, you measure the depth of water that has fallen through the area and accumulated in the bottom, that is the depth of water.

How, you may ask, can this gauge work properly if the inner tube is ten inches long but only records one inch depth of water?  The reason has to do with accuracy. The National Weather Service (NWS) has adopted the criteria that the gauge should be able to measure to an accuracy of 0.01".  The problem with a gauge that's 4" (CoCoRaHS) or 8" (NWS standard) in diameter, is that it's nearly impossible to read the depth to an accuracy of 0.01".  That's where the funnel and inner tube come in.

The funnel of the CoCoRaHS gauge squeezes the water into the area of the inner tube, which is 1/10th of the area of outer cylinder (The NWS gauge has a similar funnel and inner tube).  By reducing the area that the water falls into, the depth can be stretched by the same factor of ten.  In this way, the total volume of water (area times depth) that fell through the top of the gauge and the total volume in the inner tube are the same.  This stretching allows us to read the depth of water an accuracy of 0.01".

Incidentally, the Fort Collins, Colorado weather station has a CoCoRaHS and NWS gauge side-by-side and has been keeping track of their measurements for a number of years.  The results show that both gauges record very similar amounts of precipitation.

November 2009

What are the criteria for Heavy Rain and when should I submit a live "Intense Precipitation Report"?

Great question!

We allow flexibility here as the impacts from heavy rain vary from place to place around the country.  People can report whenever they think it is significant.  For example,  it may only have been raining .20"/hour but if it keeps up for 10 hours or more, that's more than 2" and that can be significant.  For general guidance, we recommend the traditional National Weather Service definition of heavy rain which is 0.30"/ hour or greater.

To make an "Intense Precipitation Report" look at the panel on the left under 'Enter New Reports'. The reports should be made in real time.  This is an extra report and you should make your normal 24 hours daily precip report as usually the next morning.

September 2009

How Long Have People Been Tracking Precipitation?

In his book Meteorologica, Aristotle (340BC) mentioned topics such as clouds, mist, rain, snow, etc, but not the measurement of precipitation.  Measuring rain and keeping records of it was apparently still far off in the future.

The earliest quantitative device for measuring rainfall seems to be credited to a king in Korea called King Sejong who lived from 1397 to 1450.  One of his goals as king was to make his people literate, so not only did he invent a rain gauge, but more importantly, he invented a language and movable type for that language.

He decided that instead of digging into the soil to check for moisture, it would be better to have a standardized container about 30cm in depth and 14cm in diameter that stood on a pillar to measure the rainfall.   These containers were to help villagers determine their potential harvest and to give King Sejong a better idea of how much the farmers should be taxed! So, these standard containers were distributed to each village.  The rain gauge was invented in the fourth month of 1441, according to records.

The tipping bucket rain gauge was invented by Christopher Wren in Europe around 1661 and used the standard of weight, or sometimes volume, of the liquid precipitation.  This tipping bucket idea is still used in many of the automated electronic gauges today.

In 1887, Mr. Cleveland Abbe wrote a manual on "Meteorological Apparatus and Methods" for the U.S. Army Signal Corps (agency responsible for U.S. weather observations at the time).    In this booklet, Mr. Abbe described the standards for the weather gauges to be used by the U.S. Army Signal Corps.  This standard 8 inch diameter gauge is still in use by many National Weather Service offices and cooperative weather observers across the United States and abroad.

January 2008

New Snowfall Depth . . . Report to the tenth of an inch!

An observer in Oklahoma writes: "I have some confusion on snow measurement despite reading through the procedure. I tried to enter a snow amount of 1/4 inch or 0.25 but it wouldn't accept it.  What am I doing wrong?"

Good question!  Here's our answer: New snowfall depth is measured to the "tenth" of an inch, not "hundredth" as rainfall is. So the system would not take 0.25", but it will take 0.3". So when reporting snowfall depth remember only one decimal place. 

If you are using a regular ruler here is a review of the conversions to the nearest tenth-inch increments: 1/16 = 0.1, 1/8 = 0.1, 3/16 = 0.2, 1/4 = 0.3, 5/16 = 0.3, 3/8 = 0.4, 7/16 = 0.4, 1/2 = 0.5, 9/16 = 0.6, 5/8 = 0.6, 11/16 = 0.7, 3/4 = 0.8, 13/16 = 0.8, 7/8 = 0.9, 15/16 = 0.9

We do have snow rulers marked off in tenths available from if you are interested in purchasing one.

December 2008

The four inch gauge -- how is it calibrated?

An observer from Alabama writes:
"I've always tried to figure out how rain gauges are calibrated and I read somewhere that the principle is how much water falls on one square inch of ground. In that case, a gauge with a one inch square opening would need to be six inches tall to measure six inches of rain. Trying to reconcile this knowledge with my new gauge, I figured that a 4" opening represents 12.56 square inches (A=pi*r*r), and that the tube should be 12.5" tall to measure an inch of water. However, from the bottom of the tube to the inch mark it isn't that tall. Could you help me understand the principle?"

Great Question!  Here is our answer:
Rainfall is a DEPTH measurement and not a "volume" measurement.  In other words, it's not "an inch per square inch" but it's an inch for any area in your immediate vicinity that the rain happens to land on.  In the case of your new rain gauge, the "inch" of rain is falling into a cylinder that has an inside diameter of slightly less than 4" is then being funneled into a calibrated cylinder of a much smaller diameter (just greater than 1.2 inside diameter).  The area of the opening of the inner cylinder is exactly 1/10th the area of the funnel and outer cylinder.  This means, the inner tube will magnify the depth of rain by a factor of exactly 10.   What this means is that 1.00" of rain will fill that inner cylinder to a depth of 10.0".  It is then scaled accordingly.

December 2008

Snow reporting -- A Change!

We are making a small change to the "Daily Precipitation" entry form.

Effective December 15, 2008 the default value for daily snowfall will be changed from 0.0" (i.e. assumes no snow) to NA (i.e. no assumption).

The reason for this change is that some observers who measure precipitation but who don't measure snow have been leaving the 0.0" snow amount. This fills our snowfall maps with incorrect information.

If you receive snowfall in the past 24 hours, please enter that amount in the "Depth of New Snow" category by replacing the NA with the appropriate value in inches and tenths. If you do not receive any snow, be sure to enter 0.0 in this field. If you enter 0.00" for your daily precipitation amount, it will automically put in a zero for snowfall.

October 2008

Was that really 2.00" or actually 1.96"?  Please avoid rounding off your precipitation measurements

As we quality control the CoCoRaHS Observations each day, we occasionally come across measurements which look like they have been rounded off, usually to the nearest tenth, quarter or whole inch.  The likelihood of actually measuring these exact amounts is usually pretty rare,  but they do occasionally occur (if they actually do please make a note that it's the actual amount in the comments field).  As an observer in the CoCoRaHS network, we ask everyone to measure and report to the actual hundredth of an inch, as precisely as possible.  Since the data are being used for scientific purposes it is important that we have the most accurate measurements possible.  With the 4" diameter CoCoRaHS rain gauge, which we require everyone to use, measurements to the hundredth of an inch are very easy to observe.

For further review please take a look at our CoCoRaHS training slide show on the right hand side of our home page:

So if on occasion you are tempted to round off, please take an extra ten seconds to measure right to the actual amount. We as well as the nation greatly appreciate it.

August 2008

"Monthly Zeros" Report Page

The Monthly Zeros Report page . . . what's that you might ask?  This is a neat feature where an observer can quickly enter a zero amount if there was no precipitation in the gauge that day and also back-fill a day or two where it did not rain and they forgot to report a zero amount.

Another great feature of this page is that it allows the observer to view their data on a month by month basis.  It also helps you do see if you have skipped any days.

For more information on this topic visit:

October 2007


The first major snow in the West last weekend and the big rains in the Tennessee Valley and Mid Atlantic states have more people than usual looking at our CoCoRaHS maps and reports. Even the Weather Channel showed CoCoRaHS data this week.

With so many people using our data, we need to be accurate. Yet, with so many new observers we will always make some mistakes.

YOU are our BEST defense against data errors. When you enter your report each day, always check it closely. If you find an error, even after entering it, go back and correct it by using the edit tools.

The best tool for checking data are our CoCoRaHS precipitation maps because then you can compare your report to others in your area. Your data will appear on the CoCoRaHS data reports immediately and on the maps within 15 minutes of when you submit your data.

In a future message, we will list the most common errors that we should avoid.

October 2007


Remember that we have a special data entry form for reporting accumulated precipitation that falls over a period of days when you are not home to measure the daily amounts. If you are gone for a few days and come back to find rain in your gauge, please use the "Multi-day Accumulation" entry form from the "Enter My New Reports" menu on the left side of your data entry page

Also, remember that if you are having heavy rain or snow, use the "Intense Precipitation" entry form to send in special reports at any time during the day or night. These reports are specifically to alert weather forecasters of active weather in your area. You will still need to fill out the "Daily Precipitation" report the next morning for your 24-hour precipitation amount.

October 2007


We get lots of questions about what we mean by "a trace of precipitation".

A trace may be just a few drops on the funnel of your gauge, or a few flakes of snow in the in the air, or a few sprinkle splashes in a pond or puddle or on your arm or face.  A trace may also be when your sidewalk is almost wet and when there is a bit of moisture in the bottom of your gauge, but not enough to get to the first mark (0.01 -- the first measurable increment). Trace amounts aren't terribly significant, but it does mean that moisture did fall from the clouds.  Sometimes that is important to know.

For snowfall, a trace is reported whenever you observe snow falling through the air, but it never accumulates on the ground to a depth of 0.1 (one-tenth) inch.

How do I report it when I get a trace?
That's easy -- Just type in T

What if I'm not at home to see it?
Only report a trace if you actually observe it or if you find a bit of moisture in your gauge (or your neighbor or family says "There was a trace!".  Also, remember that if the moisture came from dew or frost, then don't report that as a trace.

Thanks for your reports!

September 2007


Thanks for participating in CoCoRaHS and for entering your precipitation report today.

We realize that it takes time to enter your data each day.  It may be more interesting to measure and report precipitation when there is something in your gauge. But keep in mind that it is just as important to know when and where it DIDN'T rain or snow (Yes, that season is coming soon) than to know were it DID.  We even care about trace (T) amounts.  Your reports of 0.00" for dry days are greatly appreciated.

If you don't have time to enter each day, we have a feature entry report that makes it easy to enter zeros.  Feel free to use the "Monthly Zeros" report.  This is also a quick way to view your precipitation reports one
month at a time.

July 2007


Hundreds of scientists, water managers, news media and other professionals are utilizing our rain, hail and snow data on a regular basis.  The most common question we get is "How do you know that your volunteers are providing accurate data?"

Most of the reports we receive are GREAT, but all of us can make a mistake now and then  ( such as 25" when you meant to type 0.25" , or maybe the wrong date )

We have a fantastic CoCoRaHS volunteer who checks all the reports each day for accuracy.  If they see something that looks inconsistent, they may send you an e-mail to verify your report.  We try not to change any of thedata we receive without your permission, however, it is imperative that suspect reports are verified and corrected as soon as possible so that our users have the highest level of confidence in the CoCoRaHS data that they receive. If you get an e-mail from:,  please reply as soon as you can and please make sure your SPAM filter accepts messages from this address.  This helps CoCoRaHS VERY MUCH!