What does the METAR RVR code "D" represent?

The METAR RVR code "D" signifies a downward trend in visibility. In aviation weather reporting, the Runway Visual Range (RVR) is crucial for informing pilots about the visibility conditions for takeoff and landing. Each letter in the RVR code indicates a specific trend, with "D" explicitly denoting that visibility is decreasing, which is particularly important for pilots to consider as it can affect their operational decisions.

Understanding this code helps pilots assess whether they can safely operate in the current weather conditions, especially during critical phases of flight. By recognizing a downward trend, pilots may choose to delay their approach or reconsider their landing plans based on safety protocols and operational criteria.

The choice indicating upward trend, no change, or data not available pertains to different aspects of visibility reporting and do not reflect the specific meaning of "D." Thus, "D" appropriately signifies a worsening visibility condition that requires immediate attention.