In METAR, what does "GR" denote?

In METAR reports, "GR" specifically denotes hail. Hail is a form of precipitation that consists of balls or irregular lumps of ice, and it is important for pilots and meteorologists to be aware of such weather phenomena as they can significantly affect flying conditions. Recognizing the abbreviation "GR" allows flight crews to take the necessary precautions if hail is present or anticipated in their flight path. This understanding impacts safety and decision-making in aviation operations.

The other options do not align with standard METAR reporting codes; thus, they serve to distract from the accurate identification of meteorological phenomena significant to aviation.