1. Hyetometry The art or science of precipitation observation

2. History …. The Long Version History is difficult to trace Mayan Indians most likely took measurements Measurements were probably taken by early scientists in Greek laboratories

3. History Aristotle presented topics on rain, snow, clouds, etc. in his book ‘Meteorologica’ – 340BC Palestinians may have used rainfall measurements for agricultural purposes ~ 150AD

4. History Earliest quantitative record of rain gauge use credited to Korean King Sejong (1397 – 1450) – 1441 Gauge was ~30cm deep and ~14cm in diameter and stood on a pillar Standard is not known

5. History Benedetto Castelli made the first scientific rain measurement in Europe – 1639 Castelli was a student of _______

6. History Christopher Wren invented the tipping bucket rain gauge – 1662 Used the standard of weight, or sometimes volume of the liquid precipitation Still in use today

7. History Difficult to find who was making observations until Benjamin Franklin Mr. Franklin was famous for many inventions, and precise records His records cover a little over six decades of weather observations W. Jevons made the observation that elevated rain gauges collect less rain than gauges at the surface – 1861

8. History G.J. Symons addressed the problem of standardization of gauges to be used in a network of observers – 1860 His group of observers organized and determined a standardized gauge to be used in England

9. History W. Jevons made the observation that elevated rain gauges collect less rain than gauges at the surface – 1861

10. History Reverend TE Crallan began observing rainfall catches with gauges of uniform openings, but composed of different materials – 1866 They were also spread over different areas and elevations to see how the effect of elevation and wind direction changed the readings

11. History Results of Crallan’s observations Materials: The material of the gauge is important. It must be a smooth surface that is durable in all weather conditions. Ebonite was recommended, but copper was found to be much less expensive with very little change in results.

12. History Size of opening: Different openings were carefully examined and experiments conducted using gauges with various sized openings all being the same height above the ground. It was found that the gauges between 4 and 24 inches were very close in readings, so the five inch gauge was most practical.

13. History ALTITUDE: It was found that the higher the gauge above ground, the less moisture was captured. Wind was the variable that caused this discrepancy. Mr. Symons published his findings that wind had a dramatic affect on the amount of rain collected at various heights above the ground.

14. Precipitation Measurement One of the most difficult things to measure in meteorology Different methods are required depending on type of precipitation Point measurement versus area measurement

15. Precipitation Rate Defined as the mass flow across a horizontal plane per unit time Divide mass flow by the density of water (or ice) Typical units mm/hr, in/hr Depth to which a flat horizontal surface would have been covered per unit time if no water were lost by run-off, evaporation or percolation

16. Methods of Measurement Point measurements – Gauges which can directly measure precipitation Area measurements – Radar which can estimate precipitation over large areas – Ground-based and space-based

17. Accumulation gauges Collect precipitation and hold it (typically in fluid form) until it is manually or automatically emptied Can be recording or non-recording Typical orifice opening size in U.S. is _ inches Minimum measurement for ASOS gauges is ____ inches

18. Accumulation Gauges Can be automated through the use of a transducer Converts gauge output to displacement or voltage Rain rate estimate

19. Types of Accumulation Gauges Pressure Siphon Tipping Bucket

20. Pressure Gauge Water depth is measured with a gauge-type pressure sensor Sensor is connected via tubing to the bottom of the gauge Aneroid sensors typically used to measure pressure

21. Siphon Gauge Water depth in a cylinder is measured with a capacitive sensor Cylinder is emptied using a siphon effect (~30 seconds) Depth is detected using a capacitive transducer Capacitance Equation No moving parts Must be heated

22. Tipping Bucket Gauge Twin bucket rests on a knife-edge support mounted under a collection funnel Water falling into the bucket causes it to get heavy and ‘tip’, bringing the other bucket into the collection position Both 8 inch and __ inch gauges are used in the U.S. Easily to automate, can be heated Subject to under-reporting errors

23. Optical Rain Gauge Detects passage of rain (and snow) through a beam of light Source is an infrared LED When a drop falls through the beam, the intensity of light detected fluctuates slightly Amplitude and frequency of the fluctuation is a function of drop size, fall speed and count Rate can be determined using above information

24. Calibration of Rain Gauges Simplest method is to pour a measured amount slowly into the gauge Water can be siphoned out of a flask and into a gauge at a controlled rate – Can be used to test the gauge over a range of rates

25. Exposure Requires an area free of obstructions Light winds are ideal Gauges are typically installed a few feet above ground Should never be placed around or on buildings

26. Error Sources Representativeness Wind Wetting/evaporation Splash out Plugging Dew Accumulation

27. Error Sources Specific to Tipping Buckets Loss at low and high rates Jams – Mechanical failure – Spider webs – Frogs

28. Error Sources Specific to Pressure and Siphon Gauges Temperature sensitivity Wind Flow Fail to report during emptying

29. Truth Measurement Since there will always be error in measurements, the word ‘truth’ is inappropriate and may lead to another (bigger) kind of _____ error