What is the only report modifier used in a manual weather observation?

The modifier used in a manual weather observation is important because it provides context regarding the nature of the observation being reported. The correct choice, which indicates a correction to a previously reported observation, is particularly essential in ensuring that the information provided is accurate and reliable.

In weather reporting, a correction modifier signals that there was an error in the earlier report, and the new observation rectifies that mistake. This helps prevent confusion among users of the data and ensures that the most accurate weather information is transmitted. The use of "COR" helps differentiate between real-time observations and those that have been amended following an error.

Understanding the function of your answer demonstrates a grasp of the importance of accurate reporting in meteorological practices, especially in instances where previous information may affect flight safety, forecasting, and various weather-related operations.