This presentation will provide complete knowledge on forms of precipitation, recording and non recording type rain gauges.

1. Measurement of Rainfall
Vijitha Vikneshwaran
Lecturer(Temporary)
Faculty of Technology
University of Jaffna

2. Introduction
• Rainfall is an important natural resource which provides
water for different usages such as domestic, industry,
agriculture and public requirement.
• Accurate measurement of rainfall helps in conserving
and managing water resources efficiently.
• Measuring the rain is the most fundamental part of the
water budget.
• Harvesting, conservation and utilization of rainwater are
important for undertaking agriculture and other allied
activities.
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3. Hydrological cycle
Figure 01: Hydrological cycle 3

4. Precipitation
• Precipitation is defined as the liquid or solid products of the
condensation of water vapor falling from clouds or deposited
from air onto the ground.
• It includes,
Rain
Hail
Snow
Dew
Rime
Hoar frost
Fog precipitation
• The total amount of precipitation which reaches the ground in
a stated period is expressed in terms of the vertical depth of
water. 4

5. Precipitation,cont….
Rain
• Most common type of precipitation
• Liquid droplets
• Two different form
• Showers : Heavy, Large drops, Last for a period of time
• Drizzles : small droplets, Last longer
Figure 02: Rain
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6. Snow
• Second most common type of precipitation
• Forms when water vapor turns directly into ice without
ever passing through liquid sate
• Average density : 0.1
Figure 03: Snow
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Precipitation,cont….

7. Hail
• Created when moisture and wind are together
• Ice crystals form inside the cumulonimbus clouds
• Shapes : Spherical, Conical and Irregular
• Average density : 0.1
Figure 04: Hail
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Precipitation,cont….

8. Dew
• Small drops of water that appears on cool surfaces(grass)
in the morning
• Forms due to condensation of atmospheric vapor in the
colder night air
Figure 05: Dew
8
Precipitation,cont….

9. Rime
• White opaque deposit of ice granule
• Forms by rapid freezing of super cooled water drops
impinging on exposed surface.
Figure 06: Rime
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Precipitation,cont….

10. Sleet
• Consists of transparent, globular, solid grains of ice
• Forms by the freezing of raindrops or largely melted ice
crystals falling through a layer of sub freezing air near the
earth surface.
Figure 07: Sleet 10
Precipitation,cont….

11. Fog
• Same as cloud
• This is a cloud forms near the earth surface
• There are 4 types:
• Radiation fog
• Advection fog
• Upslope fog
• Evaporation fog
Figure 08: Fog
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Precipitation,cont….

12. Mist
• Bunch of small water droplets in the air
• Forms when the cold air occurred above a warm surface
• Very similar to fog and only differ in their visibility
Figure 09: Mist
12
Precipitation,cont….

13. Glaze
• Clear and smooth ice coating
• Forms on exposed surfaces by the freezing of super
cooled water deposited by rain or drizzle.
Figure 10: Glaze
13
Precipitation,cont….

14. Measurement of Precipitation
• Precipitation measuring devices and techniques are
listed below.
• Rain gauges
• Snow gauges
• Radars
• Satellites
• Scratching of snow packs
• Water equivalent in snow packs
• Rain gauges are most commonly used for the
measurement of precipitation both in terms of rainfall
and snow.
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15. Consideration for location selection
• Airflow around the rain gauges should be horizontal.
• Avoid concave, elevated or tilted sites.
• The site should be far from mountain ridges, where local
winds are strongly distorted.
• Site should be away from trees, building and other
instruments.
• The rain gauge should be installed at a distance from any
objects equivalent to at least 2-4 times of their height.
• The ground surface around the rain gauge should be flat and
covered with short grass.
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16. Types of rain gauges
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Rain gauges
Non recording type Recording type
Tipping bucket Weighing type Floating type

17. Non recording rain gauge
• It does not provide the distribution of amount of precipitation
in a day.
• It gives the amount of accumulated rainfall after 24 hrs.
• It consists of a funnel, body, receiver and base.
Figure 11: Non-recording rain gauge
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18. Procedure:
• Remove the funnel
• Take out the receiver
• Pour the rainwater in to the measuring cylinder
• Measure the rainfall
• Reset the funnel and receiver
If snow or other solid precipitation accumulates in the
funnel,
• Pour in a known amount of warm water to melt it
• Measure the total amount of water
• Subtract the amount of added water from the total 18
Nonrecordingraingauge,cont….

19. Causes of errors
• Some water is used to wet the surface of instrument.
• The rain recorded may be less than the actual rainfall due
to the direction of the rainfall as affected by wind.
• Dents in the instrument may also cause error.
• Losses due to evaporation
• The volume of stick of funnel replaces some water.
Note:
The accuracy of precipitation measurement significantly
depends on the environment and wind at the installation
site rather than the performance of the instrument itself.
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20. • Ensure that rain gauge is leak proof
• Check the funnel for deformation
• Measuring cylinder should be kept clean
• Plant leaves and other foreign particles should be
removed from the receiver
• The instrument should be kept 30-40 cm above the
natural surface to prevents splashing of water into the
funnel.
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Precautions

21. Recording type rain gauges
• There are 3 kind of recording type rain gauges.
• Tipping bucket
• Weighing type
• Floating type
• These are named according to their principle of
operation.
• This type of gauges continuously record rainfall and thus
rainfall depth can be obtained for any period.
• It enables to determine rainfall characteristics such as,
• Intensity
• Frequency
• Duration
21

22. Floating type rain gauge
• It enables automatic, continuous measurement and
recording of precipitation.
• The collected rainwater level is measured by the position
of a float resting on the surface of the water.
• The recording pen is connected to a float.
• As the rainwater gathered in the cylinder, the float rises
along with the pen which records the rainfall on a clock-
driven drum with chart.
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23. • When the rainwater in the storage tank reaches a level equivalent to
a fixed amount, the entire water gets drained by the siphon.
• So that the recording pen drops to the zero position.
• When the rainfall stops, the pen traces a horizontal line.
Figure 12: Floating type rain gauge 23
Floatingtyperaingauge,cont……

24. Figure 13: Parts of Floating type rain gauge
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Floatingtyperaingauge,cont……

25. Figure 14: Principle of siphon out
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Floatingtyperaingauge,cont……

26. • Number of rainfall events, number of siphon outs, total
rainfall, maximum and minimum intensity and the
duration of rainfall can be estimated by using this chart
Figure 15: Rainfall chart 26
Floatingtyperaingauge,cont….

27. Tipping bucket rain gauges
• It generates an electrical signal for each unit of
precipitation collected.
• It allows automatic observation with a recorder.
• It consists of a receptacle that collects and channels the
collected rainwater through the funnel into a tipping
bucket.
• Tipping buckets are 2 triangular vessels attached to the
left and right of the rotation shaft.
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28. • After a pre-set amount of precipitation falls, the lever
tips and dumps the collected water in to the drain
cylinder.
• This process causes generation of an electrical signal.
• The reed switch is connected to the tipping bucket, to
generate the signal at each time the buckets tip.
• The subsequent rainwater is filled into the other bucket.
• Tipping bucket rain gauge recorder is responsible to
count and record the signals.
28
Tippingbucketraingauges,cont…

29. • Eddy type cam drives the recording pen and mark a trace
on the recording paper of a clock driven drum.
• When the pen tip reaches the maximum position on the
recording paper, the cam will move the pen tip to the
zero position.
Figure 16: Tipping bucket rain gauge
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Tippingbucketraingauges,cont…

30. Figure 17: Parts of tipping bucket rain gauge
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Tippingbucketraingauges,cont…

31. • It is the most common self-recording rain gauge.
• It consists of a receiver bucket supported by a spring or
lever balance or some other weighing mechanism.
• As the rainwater enters into the bucket, the weight will
increase and the bucket will move down.
• The movement of bucket due to the raise in weight is
transmitted to a pen which traces record on a clock
driven chart.
• It gives a plot of the accumulated rainfall values against
the elapsed time.
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Weighing type rain gauge

32. Figure 18: Weighing type rain gauge
32
Weighingtyperaingauge,cont…..

33. • Instrument errors
• Errors in scale reading
• Dents in the collector
• Moistening of inner surface of the funnel and the tube
• Splashing of rain drops from the collector
• Residual moisture in the storage bottle and receiver
• Inclination of the gauge.
• Heavy wind
• Evaporation
• Absorption of rainwater by dirt, dust or by other inerts 33
Causes of errors

34. Precautions in recording type rain gauges
Floating type rain gauge
• Check the siphon drains water when it reaches the
predetermined amount.
• Check the clock driven drum for deformation
Tipping bucket
• Verify the tipping buckets tips smoothly
• Check the contact points operates properly upon tipping
Weighing type rain gauge
• Check the spring for deformation
• Ensure the proper function of the pen arm 34

35. THANK YOU
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