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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