Precipitation Accumulation
The original precipitation accumulation measuring device used for automated airport weather stations was the heated tipping bucket. The upper portion of this device consists of a 1-foot (0.30 m) diameter collector with an open top. The collector, which is heated to melt any frozen precipitation such as snow or hail, funnels water into a two-chamber, pivoting container called a bucket. Precipitation flows through the funnel into one compartment of the bucket until 0.01-inch (0.25 mm) of water (18.5 grams) is accumulated. That amount of weight causes the bucket to tip on its pivots, dumping the collected water and moving the other chamber under the funnel. The tipping motion activates a switch (either a reed switch or a mercury switch), which sends one electrical pulse for each 0.01-inch (0.25 mm) of precipitation collected.
Because of problems the heated tipping bucket has with properly measuring frozen precipitation (particularly snow), the All Weather Precipitation Accumulation Gauge (AWPAG) was developed. This sensor is essentially a weighing gauge where precipitation continuously accumulates within the collector, and as the weight increases, precipitation is recorded. Only select NWS ASOS units have been equipped with the AWPAG.
Read more about this topic: Automated Airport Weather Station, Observing Equipment
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