CoCoRaHS Manual Rain Gauge

Raingauge1The CoCoRaHS rain gauge is a good quality manual rain gauge, specially designed for use by amateur observers for making rainfall measurements of good accuracy but utilising a more affordable gauge than the traditional Snowdon design (which is understandably expensive nowadays because of its largely copper or brass construction). To be fully clear, this is a traditional manual rain gauge, which needs to be read and emptied daily in order to maintain an accurate year-round rainfall record. It may nonetheless be useful to customers of our automatic stations who might wish to use a manual check gauge for calibration and reference purposes.

The CoCoRaHS gauge is a more affordable option because it is made from  high-quality plastics rather than copper, but is nonetheless of solid and durable construction using heavy-duty, UV-resistant polycarbonate. The gauge is about 350mm (14″) tall and has a precision-dimensioned 100mm (4″) diameter funnel opening, affording it greater accuracy than cheap narrow gauges. The funnel directs its collected rain into a calibrated inner measuring tube with a capacity of 25mm (1″) rainfall. On the relatively rare (in the UK) occasions when daily rainfall might exceed 25mm, excess rainfall spills over into and is retained by the outer collecting tube, which has an overall capacity of 275mm (11″). Measuring rainfall >25mm is then effected by – carefully – filling the accurate inner measuring tube with the collected rainfall in 25mm batches until the outer reservoir is emptied.

The gauge is also supplied with an easy-to-install quick-connect bracket – this bracket can be screwed to, for example, a vertical wooden post (as shown right) to secure the gauge. The main gauge body is then a slide-fit over the bracket making it quick and easy to remove the gauge for measurement and cleaning. A one-year log-sheet for recording daily rainfall data is also supplied free with each gauge.

This gauge is extensively used in the official US weather community. (The ‘CoCoRaHS’ part of the name is actually an abbreviation for the US-based ‘Community Collaborative Rain, Hail and Snow Network’, which is the US-wide sponsoring network for precipitation measurements, with its own website at www.cocorahs.org/ .) Because this rain gauge is officially approved for use by the network of CoCoRaHS observers it has been thoroughly tested and its readings fully validated. For example, in one year-long comparative study the CoCoRaHS gauge measured an annual rainfall amount within 3% of the total recorded by an official US 8″ reference gauge.

And a new independent UK-based test by Stephen Burt has confirmed excellent performance of the CoCoRaHS gauge against an official reference gauge at a site in SE England. Here are a couple of quotes from the conclusions of the test report:

This analysis has shown that differences over a lengthy comparison period, measured daily, were small – a very acceptable 2 per cent at the comparison site

This gauge is particularly recommended for owners of automatic weather stations whose only precipitation measurements come from the built-in tipping-bucket raingauge within such systems.

To read the complete report please see the PDF on the author’s website.

We now have these gauges in stock and available with either metric or inch calibrations (please specify calibration preference when ordering). The CoCoRaHS gauge is available to buy from our online shop.