Definition
In a METAR weather report, PR is a descriptor used in combination with the obscuration codes FG (fog) or DU/SA (dust/sand) to indicate that the phenomenon covers part of the airport rather than the whole field. It appears as a two-letter qualifier attached to the obscuration code, most commonly as PRFG, meaning partial fog -- fog covering a substantial portion of the airport but with the rest still clear.
Plain English
PR means the fog or dust is only covering part of the airport, not all of it. You'll see patches of it across the field instead of a solid blanket.
Context Anchor
Seen in METAR weather reports, most commonly as PRFG, meaning partial fog.
Derivation
From the Latin 'pars' meaning 'a part.' In METAR coding, PR signals that the phenomenon affects a portion of the airport rather than its entirety -- the everyday meaning of 'partial' carries straight through.
Why Pilots Care
Tells the pilot whether reduced visibility or other conditions will affect the entire airport or just selected areas during takeoff, landing, or taxi.
Intuition Check
Partial does not mean the fog is weak or unimportant here. It means the fog covers only part of the observed airport area.
Example Sentence 1
The METAR included PRFG, so the pilot expected fog over part of the airport even though the tower reported good visibility from their position.
Example Sentence 2
PR indicated partial obscuration, allowing the crew to expect clear areas on the opposite side of the field.