What are the designators used for routine and unscheduled aviation weather observations?

METAR and SPECI are the correct designators for routine and unscheduled aviation weather observations.

METAR is the routine aviation weather report, typically issued at regular intervals and providing current weather conditions such as temperature, wind direction and speed, visibility, and significant weather phenomena. It creates a consistent and reliable set of data that pilots and air traffic controllers can utilize for flight planning and safety.

SPECI is the special weather report that is issued when there are significant changes in weather conditions that could affect aviation operations, such as sudden shifts in wind, visibility, or the occurrence of severe weather phenomena. This report provides timely updates in situations that could arise between routine METAR broadcasts, ensuring that aviation stakeholders have access to the latest critical weather information.

In contrast, TAF (Terminal Aerodrome Forecast) is a weather forecast specifically for a defined area around an airport, focusing on expected weather conditions over a period of time, and AIRMET provides information on significant weather phenomena that may affect aircraft but does not serve as an observation format itself. These distinctions emphasize the specific roles METAR and SPECI play in the data used for immediate and routine aviation weather conditions.