Definition
In a METAR or surface aviation weather observation, a modifier is a code element that qualifies the report itself or a weather group within it — indicating, for example, that the report is automated (AUTO), corrected (COR), or that a weather phenomenon is in the vicinity (VC), light (-), or heavy (+).
Plain English
A short code in a weather report that changes or qualifies the meaning of what follows — telling you things like 'this report came from an automated station,' 'this report is a correction,' or 'this weather is light, heavy, or nearby rather than at the field.'
Context Anchor
Seen in coded aviation weather reports such as METARs, near the start of the report after the time of observation.
Derivation
From Latin modificare, 'to limit or qualify.' A modifier doesn't stand alone — it adjusts the meaning of something else. In a weather report, it tells you how to read the surrounding information.
Why Pilots Care
Tells the pilot whether the data source is fully automated without human input or whether an earlier report contained an error that was later fixed.
Intuition Check
Do not read modifier as a vague “extra word.” In this weather-report context, it is a specific code that tells you something about the status or source of the report.
Example Sentence 1
The METAR began with 'METAR KXYZ 121753Z AUTO,' so the pilot noted the AUTO modifier and remembered the report came from an automated station with no human backup.
Example Sentence 2
The observer reissued the observation with COR as the modifier after correcting the visibility value.