Hyetometry - PowerPoint PPT Presentation
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Title: Hyetometry
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Hyetometry
- The art or science of precipitation observation
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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
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History
- Aristotle presented topics on rain, snow, clouds,
etc. in his book Meteorologica 340BC - Palestinians may have used rainfall measurements
for agricultural purposes 150AD
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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
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History
- Benedetto Castelli made the first scientific rain
measurement in Europe 1639 - Castelli was a student of _______
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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
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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
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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
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History
- W. Jevons made the observation that elevated rain
gauges collect less rain than gauges at the
surface 1861
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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
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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.
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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.
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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.
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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
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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
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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
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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
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Accumulation Gauges
- Can be automated through the use of a transducer
- Converts gauge output to displacement or voltage
- Rain rate estimate
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Types of Accumulation Gauges
- Pressure
- Siphon
- Tipping Bucket
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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
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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
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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
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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
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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
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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
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Error Sources
- Representativeness
- Wind
- Wetting/evaporation
- Splash out
- Plugging
- Dew Accumulation
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Error Sources Specific to Tipping Buckets
- Loss at low and high rates
- Jams
- Mechanical failure
- Spider webs
- Frogs
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Error Sources Specific to Pressure and Siphon
Gauges
- Temperature sensitivity
- Wind Flow
- Fail to report during emptying
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Truth Measurement
- Since there will always be error in measurements,
the word truth is inappropriate and may lead to
another (bigger) kind of _____ error
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