Which condition typically requires an amendment to a METAR report?

A METAR report, which provides routine weather observations, typically requires an amendment when there is a significant change in weather conditions that could impact aviation operations. The requirement for an amendment is driven by the need for timely and accurate information for flight planning and safety.

New precipitation starting is a notable event that warrants an amendment because it can directly affect visibility, runway conditions, and overall safety for pilots and aircraft operations. The introduction of precipitation can signify changes in weather phenomena that may alter conditions significantly, making it crucial for such an event to be promptly reported.

Other events, such as a thunderstorm dissipating or a change in wind direction, are important as well but may not necessitate an immediate amendment to a METAR unless they reach certain thresholds of impact or duration. While improvements in visibility are noteworthy, they do not necessarily trigger an amendment because METARs focus on current weather conditions and significant changes rather than positive improvements per se. Therefore, the start of new precipitation stands out as a condition that commonly prompts a mandatory update to ensure that pilots have the latest information regarding potentially impactful weather.