Precipitation Measuring Instruments  Hyteology: The science that treats the origin, structure, and various other features of all forms of precipitation.  Hyetometry: The measurement of precipitation and condensation in all forms.  Length (depth) is the standard unit for measuring precipitation  Millimeters is WMO standard l U.S. uses the inch

Non-Recording Gauges  U.S. Standard Rain Gauge l 8 inch diameter at collector. A = in 2 l Inner collection tube cross-section = 1/10 collection area l Rim of collector is 30 inches above ground l Funnel starts 2 inches below rim l Uses measuring stick to determine water amount.

 Volume of collector must equal volume of measuring tube. So:where: R = radius of collector, D = depth of water through collector, r = radius of measuring tube, d = depth of water in measuring tube.  If you want the depth in the measuring tube to be 10 inches when 1 inch passes through the collector, then:  Measuring tube usually 20 inches long.

 British Standard Rain Gauge l 5 inch diameter collector Area = 19.6 in. 2 l Inner collection tube cross-section area = 1/10 collector area l Gauge set into ground so the rim of collector is 12 inches above ground l Funnel starts 4 inches below rim l Uses measuring glass to determine water amount.

 Canadian Standard Rain Gauge l 3.57 inch diameter collector l Collection area 10 square inches l Inner collection tube cross-section area = 1/10 collector area l Gauge is 10 1/2 inches tall. Sets on a post which is 1 1/2 above ground so rim of collector is 12 inches above ground.  Placing the rim close to the ground reduces turbulence about collector. Care must be taken to prevent rain from hitting ground and bouncing into collector.

Recording Instruments  Fergusson Type Weighing Gauge l Bucket rests on platform supported by spring l Pen adjusted to record zero with bucket on platform l Weight of water moves platform down which moves pen arm down causing recording on moving chart

 Advantages: l Removing funnel collector allows collection of solid precipitation Good in cold climates  Disadvantages l Winds over 25 knots cause oscillations of the balance producing errors in the recording l Does not measure or record intensity. l Can make rough estimations of intensity.

 Tipping Bucket Gauge l Water from collector is funneled into one side of the bucket mechanism l Each bucket can usually hold 0.01 inch of water l When full, the bucket tips and the second bucket moves under the funnell l Tipping causes momentary closure of a switch, usually mercury or reed.

 Momentary current drives pen arm to record 0.01 inch of precipitation each time bucket flips.  Advantages: l Continuous remote recording  Disadvantages: l No record for less than 0.01 inch of precipitation Not good for light rain, drizzle l Errors result if rainfall greater than 2 inches per hour - splashing from small bucket. Buckets cannot tip fast enough. l Cannot record rainfall intensity. Can make rough estimations.

Rate-of-Rainfall Recorders  Jardi Recorder l Rate of flow through opening is determined by height of float in a chamber which lifts a tapered spindle which allows more water to flow out of the bottom of the chamber. l Float rises until rate of flow into the chamber equals the rate of flow out. Position of pen on a graph is determined by height of the float in the chamber.

Optical Rain Gauge  Infrared beam transmitted across 0.5 m space through which precipitation falls  Sensors are 24 to 64 photodiodes mounted in an array approximately 200  m apart.  Falling precipitation blocks IR beam from reaching sensor array, creating a shadow on some of the sensors.  The type and rate of fluctuations across the sensor array identify the type and rate of precipitation.

Rain Gauge with Alter Shield

Another type Alter Shield

Turf Wall Rain Gauge

Measurement of Snow  Snowfall - The depth (in. or cm) of new snow (S f )  Snowfall Water Equivalent - Depth (in or cm) of water when snowfall is melted. (S w ) l Conversion: Weighing a core sample Melting a core sample and measuring depth Melting a core sample and weighing Use of a conversion factor. Factor varies for type of snow, where k = factor based on location and temperature.

 Snowcover: Areal extent of snow covered ground expressed as a percent of the total area of a given region.  Snow Density: l The ratio of the water equivalent to the depth of snow.

 Snow Sampling Tube: 8 or 3 inch diameter tube. l Can measure depth and convert using water equivalent conversion factor. l Can weigh snow in tube. One gallon of water weighs 8.33 lb. l Can melt snow and measure depth of water

 Snow Stick: A pre-placed measuring stick.  Snow Board: Placed on top of old snow layer to use as base for measuring new snowfall.

Acoustic Snow Depth Gauge  Acoustic Signal is transmitted downward toward snow surface.  Time for signal to return is dependent on distance of snow surface from transmitter and receiver and speed of acoustic signal l Shorter time - deeper depth of snow  Speed of sound is dependent on pressure and temperature  Speed of sound is first measured over a known distance in the unit prior to snow depth measurement.

Snow Pillows  Envelopes of stainless steel or synthetic rubber about 4 square feet, filled with antifreeze.  Pressure of snow on envelopes forces fluid towards a pressure transducer which converts the pressure change to a change in electric current.  Change in electric current is then a measure of the weight of the snow from which a water equivalent can be determined.

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 Billions of sand-sized meteorites enter the atmosphere daily. As each particle heats and burns in the region miles above the surface, its disintegration creates a trail of ionized gases. These diffuse rapidly disappearing within a second. The process is long enough for SNOTEL communications. The process involves: (1) Master station sending request, (2) Site responds by transmitting data, (3) master station acknowledges receipt and site stops transmitting.

Representativeness  Greatest portion of rainfall over the world is not measured.  U.S. has an average of about one 8-inch gauge per 200 square miles.  % of rainfall is not measured in U.S.

Accuracy of point-source measurements  Loss by wetting receiver: mm  Loss due to evaporation: 0.3 to 0.5 mm  Wind Effects l Higher gauge above ground, less collected due to wind and turbulence l Error may be % for some rain situations.  Closer gauge is to ground, more splash in from drops bouncing off ground into collector.

Instrument Error  Shallow depth of funnel below rim causes loss of some rain due to splashing out.

Non-uniformity of gauges and procedures  Reduces ability to compare measurements.

Recommendations  Use Rain Shields  Use Turf Walls  Use measuring sticks instead of measuring glasses  Use recording gauges rather than non-recording  Locate gauge rim a standard height above ground  Use standard collection area

ASOS Precipitation Accumulation Sensor  Heated Tipping Bucket Rain Gauge l 8 inch collector l Heating elements wrapped about underside of collector funnel and drain tube l Accuracy ± 0.02 inches l Algorithm used to correct for high rain events.

Precipitation Identification Sensor  Optical Rain Gauge.  Movement, frequency, size of fluctuations of “shadow” areas on sensor array provides sufficient information to distinguish between rain, snow, intensity of precipitation.

 Two modulation frequencies are used. l Hz identifies snow l KHz identifies rain  Ratio of intensities in high to low ranges identifies the type.  Presence, type, intensity determined each minute.  Last 10-minutes of the 1-minute values is used to determine type for ASOS observation.

Freezing Rain Sensor  Small cylinder probe which vibrates at a resonance frequency  If ice collects on the probe, the frequency at which resonance occurs will change.  Indicates a yes or no determination for freezing occurrence

 Reporting Precipitation by ASOS with freezing rain

 Obstruction to Vision  Only Fog or Haze can be reported  When visibility drops below 7 miles: l If Dew Point Depression ≤ 4 o F, Fog is reported or appended to the present weather. l If Dew Point Depression > 4 o F, and no present weather is reported by the PI or ZR sensor, then Haze is reported.