Kan - Precipitation

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• Precipitation
• Manual gauges
• Tipping bucket gauges
• Weighing gauges
• Calibration of rain gauges
• Measuring snow

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Features of a good rain gauge

• Receiving funnel with small hole to reduce
  evaporation.
• Receiving funnel has larger area than collection
  container, or collection container narrows toward
  the bottom. Gives amplification for ease of
  reading rain amounts.
• Collection container is shaded to reduce
  evaporation.
• What score would you give to the gauge in the
  picture ? / 10

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Manual rain gauge Met Service of Canada
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Some things that might cause errors in rainfall
measurements.

• Nearby obstacles. Minimum recommended ratio of
  height of nearby objects to distance
  from gauge is 15.- wind flow around gauge may
  deflect smaller droplets (under-catch)- wetting
  and evaporation losses (under-catch)- splash-out
  from funnel in heavy rain (under-catch)-
  plugging, then evaporation from funnel
  (under-catch)

Fig. from Strangeways
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Tipping bucket rain gauge
Each tip causes a switch to close and re-open
(pulse data). Rain amount and rate can be
determined.
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Standard Tipping Bucket Gauge
Percent error
Rain rate (mm/h)
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Tipping Bucket with siphon
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Tipping bucket gauge with siphon
Rain rate (mm/h)
Percent error
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• Calibration of a Tipping Bucket Rain Gauge
• You have a TBRG with a 20 cm diameter
  funnel. It tips and sends a pulse for every 0.2
  mm of rainfall
• You have a calibration device that holds 650 ml
  of water, and then slowly releases this water
  into the gauge. What total rainfall will have
  been simulated when the calibrator is empty?
• How many tips would you expect if the gauge is
  working properly?
• What rainfall rate does your test simulate if the
  calibrator emptied over a period of 12 minutes?

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• Nipher shielded snow gauge
• central cylinder can be lifted out
• measure depth of snow in cylinder
• melt snow in cylinder to get water content
• turbulence generated by unshielded gauge
  deflects falling snow
• unshielded gauge catches only 60 of snowfall

Wind
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Single double Alter wind shields around
weighing gauges
Wind is deflected down by the blades, to create
calmer air at the mouth of the gauge and reduce
snow under-catch.
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A weighing gauge for precipitation measurements
Funnel is removed in the version currently used
by Met Service, so that both snow and rain are
measured. Anti-freeze in bucket in winter melts
snow, so gauge measures rain plus snow water
content.
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Ultra-sonic snow depth sensor Time for sound
pulse to bounce back to sensor is a measure of
the snow depth. Meteorological Service uses a
standard gravel surface under the sensor. GPS for
sensing snow depth Look at difference between
signal received directly from GPS satellite and
signal that bounces off snow. Difference related
to distance between GPS receiver and snow
surface. Averages over a larger area than the
ultra-sonic sensor.
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