Did You Know...(How we Weigh Rain)?

The National Weather Service has several ways to “measure” rain. The method used to
measure rainfall depends on the type of weather station being used. This quick fact sheet will
discuss three types of weather stations and how each measures, or in one case weighs,
accumulated rainfall. The first type of weather “station” is the Automated Surface Observation
System, or ASOS, in use at larger airports controlled by the Federal Aviation Administration.
An ASOS measures precipitation by a mechanism known as a tipping bucket. The second type
of weather station is the National Weather Service’s Cooperative Observer Network station. Our
coop observers use standard, 4-inch (diameter) rain gages to measure 24-hour liquid precipitation
amounts. Select coop stations also “weigh” the amount of rainfall that fell during the calendar
month. This type of measurement is taken by a Fisher Porter rain gage.

The National Weather Service owns and maintains ASOS units throughout the United States. ASOS sites in south-central and southeastern Wisconsin include the main airports at Milwaukee, Madison, Racine, Kenosha, Sheboygan, Fond Du Lac, and Lone Rock. In a sense, an ASOS measures rainfall by weighing it. The tipping bucket, one of the ASOS’s instruments, is basically a small cup capable of holding one-hundredth (0.01) of an inch of water. When it is raining at an ASOS site, the water falls into the cup until it is full. The weight of the water forces the water to “tip” and dump the water out. The ASOS’s computer system counts the number of tips that occurs in a one-hour period. Each tip equates to one-hundredth (0.01) of an inch of rainfall, so if a rain event causes the tipping bucket to “tip” ten times in an hour, one-tenth (0.10) of an inch of rain fell during that hour. One-hour, six-hour, and 24-hour precipitation values are sent out to the world in the form of surface observations. Illustration 1 shows an ASOS tower.
Illustration 1: ASOS Instrument Tower.

Our coop observers take daily measurements of the maximum and minimum temperature over a 24-hour period. Some of these observers also measure the amount of precipitation that occurred over the same 24-hour period. These coop observers use a standard, 4-inch rain gage to measure the rainfall. This rain gage works with a specially designed measuring stick to measure the depth of the water in the rain gage. The depth of water that falls into a 4-inch diameter rain gage equals the amount of rainfall that fell at the station. The measuring stick measures rainfall to the nearest tenth of an inch. Since this rain gage can only hold two inches of water, the entire gage is placed into a metal “overflow can”. The overflow can (Illustration 2) is an 8-inch diameter, metal tube designed to collect water that overflows the 4-inch rain gage. If more than two inches of rain occur, the coop observer dumps all of the water out of the 4-inch gage (2 inches) and then pours the water from the overflow can back into the 4-inch rain gage. The observer then adds two inches to the amount measured in the rain gage.
Illustration 2: 8-Inch Overflow Can. The 4-Inch Rain gage and connecting funnel rest inside this can.

 The Fisher Porter, or FP, is another type of rain gage that collects and measures rainfall
over a longer period of time, typically around one month. The FP consists of a metal bucket that
rests on a scale inside of the FP unit. This bucket collects more rain each time it rains, and is
normally only drained when the bucket is nearly full. Now, how does a rain gage “weigh” rain?
As stated earlier, the bucket sits on top of a weighing scale located within the FP unit. This scale
is similar to the scale you weigh yourself with each morning. The scale is linked into a
mechanical system composed of gears, a “punch plate”, and a paper tape. The punch plate is
composed of different combinations of pins that punch a hole in the paper tape. Each
combination of pins depends on the weight imposed upon the scale. As more rain falls into the
bucket, and the weight of the bucket increases, the “punch plate” turns on a dial. Every 15
minutes, the dial locks into place. This also locks into place the correct pin combination for the
amount of weight on the scale. A motor within the FP unit then forces the pins to “punch” the
paper tape. The paper tape feeds forward so a new hole can be punched every 15-minutes. The
paper tape is removed at the end of each month. National Weather Service personnel specially
trained to read the tapes determine the amount of rain weighed over the course of the month.
They can also see, in 15-minute increments, when the rainfall occurred.

The data from all three types of rain measurements discussed are then sent to the National
Climatic Data Center, or NCDC, in Asheville, North Carolina. NCDC personnel incorporate the
data into the nation’s climate database. The data can then be utilized by researchers for studies
on long term climate studies such as El Niño and global warming. We thank all of our
cooperative observers for their continuous, volunteer efforts towards America’s climate program.

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