A more detailed explanation can be found here.
[Used in the order: Qualifier + Descriptor + Phenomenon ... thus for heavy thunderstorm with rain=+TSRA; light freezing drizzle=-FZDZ]
(some* use PE)
(used by auto-METAR)
ice crystals (or diamond dust)
small hail/snow pellets
* although PE was the original two-letter abbreviation recommended for the METAR/TAF weather code, since the change whereby two (or more) weather groups can be used came into force, some users objected to the possible combination of rain (RA) and pellets (PE).
well developed dust/sand whirls
NB: all phenomena are considered to be 'at the station' unless prefixed by VC (= vicinity); thus VCFG is fog in the area, but not affecting the airfield. Vicinity is defined as within 8 km of the airfield.
(see note below)
(i.e. supercooled and depositing rime/clear ice - but see note below)
NB: When used with FG, the qualifier 'PR' is used for fog banks, i.e. an extensive area of fog impinges upon an airfield, reducing visibility over part of same to less than 1 km; 'BC' [ patches ] would be used when a discrete, small-scale area of fog drifts/forms over the airfield, again reducing visibility below 1 km but not in all directions. In practice, it is not easy to tell the two apart!.
NB: When use with FG, the qualifier 'FZ' is now used to mean BOTH fog depositing rime AND fog occurring with an air-temperature below zero deg.C; this latter may or may not be depositing rime ice.