Hyetometry

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 Crallans 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