In a SPECI observation, which letter is entered into column 1 of the MF1M-10C?

In a SPECI observation, the letter "S" is entered into column 1 of the MF1M-10C. This designation specifically denotes that the report is a special weather observation, which represents significant changes in weather conditions that can impact aviation. Such observations are typically issued when there are sudden and important changes in factors like visibility, wind, or significant weather phenomena occurring at the reporting station.

The use of "S" helps differentiate these special observations from routine reports (METAR), which are typically designated with a different letter, ensuring that pilots and other personnel are alert to the more critical and timely information being conveyed in a SPECI report. Understanding the specific designations and their implications is crucial for pilots and aviation meteorologists in order to stay informed about rapidly changing weather conditions.