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
Some things that might cause errors in rainfall measurements. - Nearby obstacles. Minimum recommended ratio of height of nearby objects to distance from gauge is ~1:5. - 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) The “gold standard” (but impractical) rain gauge exposure. Pit gauge sits in a hole with its lip at the same level as a wind-baffling grid laid around it. Fig. from Strangeways
Tipping bucket rain gauge Each tip causes a switch to close and re-open (pulse data). Rain amount and rate can be determined.
Standard Tipping Bucket Gauge Percent error Rain rate (mm/h)
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 1.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? 2.How many tips would you expect if the gauge is working properly? 3.What rainfall rate does your test simulate if the calibrator emptied over a period of 12 minutes?
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
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.
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. For electronic recording, a “load cell” positioned under the bucket sends a signal proportional to the weight of precipitation collected. Older models recorded the weight on a rotating chart.
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.