What does 'GR' refer to in weather reporting?

In weather reporting, 'GR' specifically refers to hail or graupel. Graupel is a type of precipitation that forms when supercooled water droplets freeze onto a snowflake, leading to a soft, white ball of ice that can appear similar to hail but is generally smaller in size. The designation 'GR' is used consistently in METAR reports to inform pilots and meteorologists of any occurrence of hail or graupel, which can have significant implications for flight safety and operating conditions.

The other options relate to different types of precipitation but are indicated by different codes in METAR reports. For example, freezing fog is indicated by 'FZFG', light freezing rain would be reported as 'FZRA', and sleet or ice pellets would be designated by 'PL'. Understanding these codes is crucial for accurate weather interpretation and operational decision-making in aviation.