Rain Gauge Accuracy Experiments: TROPO vs Stratus

Is The TROPO Accurate?

Some people have become concerned about the fact that their TROPO gauge frequently reports less rain than their Stratus gauge. To help remedy these concerns, we encourage you to perform one of the following accuracy calibrations on your own TROPO.

Calibration by Volume

1. Measure 206 mL (the volume of 1-inch of water given a 4-inch diameter catchment opening) of water using a graduated cylinder or measuring cup
2. Pour the 206 mL of water into your TROPO inner cylinder
3. Check that the TROPO reports 1-inch of water
4. Repeat the steps on other rain gauges to compare accuracies

Calibration by Weight

1. Tare the TROPO inner cylinder on a digital scale
2. Fill the TROPO inner cylinder with water to the 1-inch mark
3. The scale should report 206 g (the mass of 1-inch of water given a 4-inch diameter catchment opening)
4. Repeat the steps on other rain gauges to compare accuracies

We found that while it initially seems like the TROPO has a low bias of ~3%, it’s actually the Stratus that has a high bias of ~3%.

A recent CoCoRaHS blog summarized the TROPO by saying, “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.”

Does this mean that you have to stop using your trusty Stratus? No, but if you’re looking for the most accurate precipitation measurements, and want to use an instrument that offers nearly 20 user-friendly features, the TROPO is the superior option!

Long-term Comparisons

In mid-May 2023, the CoCoRaHS team installed the TROPO gauge among several other precipitation gauges at the official Fort Collins weather station on the campus of Colorado State University (CSU). The photo below is of the weather station (NOAA ID USC00053005, CoCoRaHS CO-LR-255) at CSU, with various weather instruments and precipitation gauges, including the TROPO. Alongside the other gauges, the weather observers at this station are recording measurements from the TROPO. This data will eventually serve as the basis for a long-term comparison study, much like the 10-year study comparing the Stratus against the NWS Standard Rain Gauge, by Nolan Doesken, founder of CoCoRaHS. We will share any preliminary results as soon as we can.

Above: The TROPO rain gauge installed at the CSU weather station.