Which element is NOT mandatory in a METAR observation?

In METAR observations, the mandatory elements are those required for the report to be complete and to function correctly in aviation weather reporting. Temperature, while important for understanding atmospheric conditions, is not a mandatory component. This means that while you will often see temperature included in METAR reports as part of the descriptive data, it is not required for the report's validity.

On the contrary, elements such as wind speed, visibilities, and dew point are mandatory. Wind speed is crucial for pilots to assess the conditions they may encounter during takeoff and landing. Visibility is essential for safety, directly affecting a pilot's ability to see the runway and other critical landmarks. Dew point provides information about humidity and potential for cloud formation, integrating with other weather factors that influence flight operations.

Understanding that temperature is not a mandatory observation yet is often provided helps clarify the purpose of the METAR format and the emphasis placed on critical elements that directly affect flight safety and decision-making.