What type of weather report (SPECI) is made during non-routine occurrences?

The selection of "when certain weather conditions change significantly" as the correct answer highlights the nature of a SPECI report, which is specifically designed to communicate non-routine weather observations. These reports are generated when there are notable changes in weather conditions that could affect operational decisions, such as shifts in visibility, wind speed, or thunderstorms. SPECI reports are issued outside the regular schedule of routine reports (METARs) and focus on significant developments that may impact flight safety and operations. Therefore, the context of a SPECI being related to significant shifts in weather aligns perfectly with the nature of alerting stakeholders to changing conditions that warrant immediate awareness.

In contrast, the other options do not accurately describe the purpose of a SPECI report. Major weather events generally may trigger multiple reports, but SPECI specifically pertains to significant changes rather than the broader classification of major events. Scheduled intervals pertain to routine weather reports and do not capture the essence of non-routine updates. Seasonal calendar events do not directly influence the issuance of SPECI reports, which are driven by substantive changes in atmospheric conditions rather than time-based occurrences.