rain gauge

A meteorological instrument used to measure the amount of precipitation fallen at a place over a given period of time. The one used in weather observatories is approved by the World Meteorological Organization (WMO) and is of the Hellmann type, named in honor of the German meteorologist Gustav J. G. Hellmann (1854-1939), to whom we owe its invention (in the year 1886). The first containers built ad hoc to capture rainwater and keep a record of it were used in Ancient Greece and date back some 2500 years. A marked vessel used for the same purpose is also documented in India around the 4th century BC. The planning of agricultural tasks was what drove the invention of these early rain gauges, as was the one that appeared in Korea, in 1441, in the time of King Sejong the Great (1418-1450). Known as cheugugi, it consisted of an iron container, of standardized dimensions, of which exact replicas were made and distributed to each provincial governor, which allowed rainfall records to be kept throughout that country. In 1639, the Italian physicist Benedetto Castelli (1578-1643) - a disciple of Galileo - made a simple graduated glass vessel, used to capture rainwater, which also served the functions of a rain recorder. Although the invention of the first rain gauge in history (as a properly scientific instrument) is usually attributed to the English architect Christopher Wren (1632-1723), that honor belongs to Castelli, since what Wren invented - in 1662 - was the first tipping-bucket rain recorder. It was a rain gauge with tipping buckets that incorporated a mechanical system, which allowed a pen with ink to trace, on graduated paper, a continuous record of the rain that had fallen. It was not until the end of the 19th century that the problem of reliably measuring precipitation was resolved, since the various rain gauges designed up to that point failed to overcome all the difficulties involved in measuring that variable; mainly the loss, through evaporation, of part of the precipitated water. The rain gauge invented by Hellmann successfully passed the quality control tests. Its functionality is matched by its resistance to inclement weather. Externally, the instrument is a galvanized sheet-metal cylinder formed by two pieces that fit together, one on top of the other. The upper one is open and constitutes the mouth of the rain gauge. In those approved by the WMO, its surface area is 200 cm². From that opening a funnel runs downward, through which the water of the rain, snow, or hail passes, ending in a container housed inside the lower piece. To carry out the observation, the two cylindrical vessels are separated, the aforementioned container is removed from the lower one and - in the event there was precipitation during the time elapsed since the previous observation - the collected water is poured into an appropriately graduated measuring cylinder. Its reading directly provides the millimeters of height of the water film that the precipitation left, assuming it is distributed over an impermeable surface and that there is no evaporation. Those millimeters are equivalent to liters per square meter.