What does "RA" denote in a METAR report?

In a METAR report, "RA" specifically denotes rain. This is a standardized abbreviation used universally in aviation meteorology to indicate that rain is present at the reporting station. When a METAR includes "RA," it means that rain is currently falling or has recently fallen and is relevant for pilots and meteorologists who need to assess weather conditions for flight operations.

Other options presented in the question are not applicable in this context. Relative humidity is expressed differently in METAR reports and does not have a specific abbreviation like "RA." A term for a rainless period would not be indicated in a METAR report since that is not typically relevant information for pilots. Radiation fog refers to a specific weather phenomenon resulting from radiative cooling, and it is also identified differently in METAR reports if applicable. Therefore, "RA" distinctly and specifically refers to the condition of rain.