Definition
A METAR notation indicating that precipitation is occurring at an automated reporting station, but the station's sensors cannot identify what type it is (rain, snow, drizzle, etc.). The code is generated only by automated weather stations equipped with present-weather sensors that detect precipitation but cannot classify it.
Plain English
The weather station can tell that something is falling from the sky, but it can't tell exactly what. So the report just says 'unknown precipitation.'
Context Anchor
Seen in the weather section of an aviation routine weather report, called a METAR, especially from automated reporting stations.
Why Pilots Care
Signals uncertain conditions that may affect visibility, runway friction, or icing risk and warrants checking additional weather sources.
Grounding Statement
The key point is that precipitation is present, but its exact type is not known from that report.
Intuition Check
Do not read UP as meaning the weather is unknown in general or that there is no precipitation. It means precipitation is being detected, but the system cannot identify the kind.
Example Sentence 1
The METAR from the automated field reported UP, so the pilot expected some form of precipitation even though the type wasn't specified.
Example Sentence 2
UP appeared in the observation, prompting the crew to review runway braking action reports before landing.