Plain English Meaning
/RM marks the start of the extra notes at the end of a weather report. Anything written after it is added detail that the main part of the report did not cover.
Full Definition
In a METAR or TAF, /RM is a separator code that introduces the Remarks section. Everything after /RM contains supplementary information not included in the main body of the report, such as observed weather phenomena, sea-level pressure, hourly precipitation amounts, and station-specific notes.
Context Anchor
Seen in the coded format of PIREPs in aviation weather reports.
Derivation
RM is short for 'Remarks,' from the Latin re- (again) and marcare (to note or mark). A remark is something noted in addition. The slash separates this added section from the standard report body.
Why Pilots Care
Gives pilots a place to report important extra details that can affect safety or route decisions for others.
Intuition Check
Do not read /RM as a runway, radio, or distance code. In a PIREP, /RM simply marks the remarks section.
Example Sentence 1
After decoding the main body of the METAR, the pilot read past /RM and noted that lightning had been observed northwest of the field.
Example Sentence 2
After /RM the pilot added that visibility dropped rapidly near the ridge.