Definition
In aviation weather reports (METAR/TAF), UP is the code used to indicate precipitation of an unknown type. It is reported by automated weather observing systems when the sensor detects falling precipitation but cannot determine whether it is rain, snow, drizzle, or another form.
Plain English
It means the airport's automatic weather station can tell something is falling from the sky, but it cannot work out what kind of precipitation it is.
Context Anchor
Seen in coded airport weather reports, especially reports made by automated weather equipment.
Derivation
UP comes from the first letters of 'Unknown Precipitation.' The code follows the standard METAR convention of using two-letter abbreviations for weather phenomena (RA for rain, SN for snow, DZ for drizzle, and so on).
Why Pilots Care
Seeing UP in a report tells the pilot that precipitation is occurring but the type has not been identified. This is a flag to gather more information before flight, since the actual conditions could range from light drizzle to freezing rain, each with very different operational implications.
Intuition Check
UP does not mean upward here. In this weather-report context, it means precipitation is present but its type is unknown.
Example Sentence 1
The METAR included UP in the body of the report, so the pilot called Flight Service to find out what was actually falling at the field.
Example Sentence 2
Automated weather stations insert UP in the report when visibility drops but the sensor cannot classify the precipitation.