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On GFAs, TAFs, METARs we see PTCHY LGT TURB, MOD TURB, -RA, +RA, etc. I was wondering where could I find the technical definition of these terms? For example the percentage coverage of these weather phenomena or the sky coverage in terms of oktas?

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  • $\begingroup$ Check NOAA's FMH-1 and see if that answers your question; if not I'll try to put together an answer later. $\endgroup$
    – randomhead
    Commented Jul 26, 2022 at 12:27
  • $\begingroup$ Please be sure to mark the below answer as „accepted“, if you feel it has answered your question. $\endgroup$ Commented Aug 17, 2022 at 7:50

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For the METAR codes you mentioned (compare United States of America Aeronautical Contractions, Section 1: Decode):

abbrev. term
PTCHY patchy
MOD moderate
TURB turbulence
RA rain
LGT light

The conditions for Light Rain are defined in the Federal Meteorological Handbook-1.

The conditions for Light/Moderate Turbulence is defined by the ICAO (I could only find the SkyBrary definition, not the original source in the ICAO documents).

The conditions for Patchy ("covers 25% or less") are defined by the FAA, eg. in 7930.2 Notices to Air Missions (NOTAM).

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