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Unknown precipitation type (UP) in a surface observation

The precipitation sensor from an Automated Surface Observing System (ASOS) is really good at detecting raindrops, freezing rain and snowflakes. But it doesn’t do so well with other precipitation types such as drizzle, freezing drizzle, ice pellets, snow grains, hail and when there’s a mixture of precipitation types.

As a raindrop or snowflake passes through the precipitation sensor the particle creates a shadow that modulates the light, which then passes through the sensor’s horizontal slit aperture beam. When many particles fall through the beam, a scintillation pattern is created and this is analyzed to reveal how much energy or power is contained in the various frequency bands. (Scintillation, is a generic term for variations in apparent brightness, color, or position of a luminous object viewed through a medium.) For example, a predominance of power in low frequencies from 75 to 250 Hz indicates snow. When energy is predominantly in a band from 1000 to 4000 Hz, the precipitant is almost certainly rain. However, when there’s a mixture of both rain and snow, the smearing of the power makes it difficult to differentiate from snow or rain and the ASOS reports this as UP or unknown precipitation type.

You will generally see UP in the present weather field of a METAR for automated observations (AUTO) only. If there’s a human observer present, they will most certainly edit the automated observation and report RASN (or SNRA) which implies a mixture of both rain and snow. But, to err is human - there are times where human observers fail to augment the observation.

Most pilots are weatherwise, but some are otherwise

Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

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