A New Rain Gauge Option for CoCoRaHS

A new approved rain gauge, the Tropo rain gauge from Climalytic Instruments, is now available to CoCoRaHS observers. This is a "premium" alternative to the Stratus gauge manufactured by Productive Alternatives. These are the only two gauges approved for CoCoRaHS (along with the NWS 8" standard rain gauge). I've been seeing a lot of comments about the new Tropo gauge, it's cost, and other things the past two weeks. We realize that many observers cannot or do not want to spend the money for the features it offers. The Tropo gauge incorporates many suggestions made by CoCoRaHS observers over the years for improving the overall gauge. One key difference - it IS more accurate than the Stratus gauge, and it was designed that way. We know the Stratus gauge over-measures rainfall by about 2.5%, and the Tropo gauge does not have that error.

Other features of the gauge that differentiate it from the Stratus gauge:

• The Tropo holds about 13" of water vs. 11" for the Stratus. This is something that will be useful heavy rain-prone areas, like those that frequently affected by tropical systems. And although we prefer daily observations, the higher capacity also allows for heavier multi-day accumulations
• The funnel has pre-drilled holes in the rim for mounting the supplied bird deterrent spikes.
• The funnel and inner measuring tube were designed to make capturing the inner tube with the funnel extremely easy.
• The inner measuring tube has a wide base for stability in the outer tube and on your counter if pouring water into it.
• The funnel cap is deeper to minimize splash-out, and locks on to the outer tube to minimize blow-offs.
• The gauges comes with hardware and a much better designed mounting bracket that easily secures to a post or to a pole.
• The gauge comes with a handle that slides into the mounting bracket to aid in pouring into the inner measuring tube for measurement. A small pouring spout is molded into the outer cylinder as well to prevent spilling while causing negligible impact to snow core sampling.

There are a number of other improvements as well, and you can read about those on the Climalytic Instruments web site.


Do I Have to Use the Tropo Gauge?

NO. If you are currently using the Stratus gauge there is no need to change unless you want to. We realize that many observers cannot or do not want to spend the money for the features it offers.

Think of it this way. Many weather enthusiasts have home weather stations. An inexpensive weather station can be purchased at your local big box home store, and that will satisfy the needs of many enthusiasts. On the other hand, other enthusiasts want something more and will spend the money for a better system with more features and benefits.


How Accurate is the Tropo Gauge?

The Stratus gauge has a known error of about 2.5 percent (reads 2.5 percent too high). That is within the stated NWS error range of 4 percent from the 8-inch standard rain gauge. The Tropo gauge is accurate to less than 1 percent error. This accuracy is determined by measurement.

The volume of water of a “disc” one inch high and 4 inches in diameter (the outer cylinder) is 205.926 ml (cm3), and it's mass is 205.926 grams. We know there is an error in the Stratus gauge because the volume of water filled to the one inch mark on the inner measuring tube weighs 200.8 grams, not 205.9 grams. A 10-year comparison study of the 4-inch gauge and the NWS 8-inch rain gauge by the Colorado Climate Center found that the 4-inch gauge was higher than the 8-inch gauge by about 3 percent each year. That is consistent with the known accuracy of the Stratus 4-inch gauge.


I Have Both Gauges and the Measurements Rarely Agree.

Even when identical Stratus or Tropo gauges are mounted side by side there will many times be differences in amounts. If all things were equal in an event, the Tropo gauge would be ~0.01" lower at 0.25" in the Stratus, 0.01 to 0.02" lower at 0.50", and 0.03" lower than the Stratus at one inch. So, a measurement difference of a few hundredths between gauges likely has little to do with calibration - it's just a difference caused by any number of things - wind speed and direction, air turbulence around the gauges, rain intensity, etc.

Although the primary field testing has been completed, we are currently continuing field comparisons to not only Stratus PA Gauge, but to the 8" standard rain gauge as well.


Doesn't Using Different Gauges Affect the Interpretation of Precipitation Measurements?

Both rain gauges fall within the +/-4 percent range of the NWS 8" Standard Rain Gauge. Accurate precipitation measurements are the goal, but the reality is that over over the long term there are many other factors that affect gauge catch and precipitation measurements such as wind, rain intensity, siting and exposure, observer error, etc., that are beyond our control.

As a final thought, consider this old adage (Segal's law), reinterpreted for precipitation measurements:

"A person with a rain gauge knows how much rain fell. A person with two rain gauges is never sure."