Spherical gauges for improving the accuracy of rainfall measurements

Rain-gauge catch efficiencies are affected by wind. Wind makes raindrops fa ll at an angle of inclination and the effective diameter of the rain gauge orifice smaller than if raindrops fall into the gauge vertically. Two spher ical and two semi-spherical orifices were designed to modify standard gauge s and others in use today. The two spherical orifices catch rain with an ef fective diameter always equal to the actual diameter regardless of wind spe ed and direction. The semi-spherical orifices, used side-by-side with a sta ndard gauge, correct 50% of catch deficiencies made by the standard gauge. Tests based on 115 storms show that the four new gauges caught more rainfal l than the standard gauge, with an average catch increase ranging from 8% t o 16%. Compared with the pit gauge, average deficiency in catch ranged from -1% (spherical rain gauge orifice with cylinders) to 4%, whereas the defic iency for the standard gauge was -10%. Percentage deficiencies of the new g auges were positively affected by wind speed, raindrop inclination and rain fall intensity. Although the new gauges tended to underestimate the standar d gauge in small storms (< 0.25 cm) and overestimated the pit gauge under s trong winds, their deviations are small. Underestimates for small storms co uld be improved by using gauge materials that reduce surface temperature, e vaporation and water retention. The gauges are simple in design, easy to op erate and inexpensive. In order to maintain a historically consistent set o f rainfall data, a dual-gauge (standard gauge + spherical gauge) is recomme nded for existing rainfall stations. The new rain gauge orifices are suitab le for large-scale applications. Copyright <(c)> 2001 John Wiley & Sons, Lt d.