Hyetometry

The art or science of precipitation observation

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

History

• Aristotle presented topics on rain, snow, clouds, etc. in his book ‘Meteorologica’ – 340BC

• Palestinians may have used rainfall measurements for agricultural purposes ~ 150AD

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

History

• Benedetto Castelli made the first scientific rain measurement in Europe – 1639

• Castelli was a student of _______

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

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

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

History

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

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

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.

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.

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.

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

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

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

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

Accumulation Gauges

• Can be automated through the use of a transducer

• Converts gauge output to displacement or voltage

• Rain rate estimate

Types of Accumulation Gauges

• Pressure • Siphon• Tipping Bucket

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

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

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

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

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

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

Error Sources

• Representativeness• Wind• Wetting/evaporation• Splash out• Plugging• Dew Accumulation

Error Sources Specific to Tipping Buckets

• Loss at low and high rates• Jams– Mechanical failure– Spider webs– Frogs

Error Sources Specific to Pressure and Siphon Gauges

• Temperature sensitivity• Wind Flow• Fail to report during emptying

Truth Measurement

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