What weather conditions mandate a pilot to file an IFR flight plan when departing from a controlled airport?

Filing an IFR flight plan is required when weather conditions at the departure airport fall below specific visual flight rules (VFR) minimums. When visibility is less than 3 statute miles or the ceiling is less than 1,000 feet, the conditions are considered inadequate for VFR flight, which requires a pilot to transition to instrument flight rules (IFR). This ensures that the pilot can operate safely in the lower cloud bases and reduced visibility that do not allow for visual navigation.

In contrast, clear skies with unlimited visibility, heavy storms with strong winds, or cloud cover at 5,000 feet with good visibility do not automatically necessitate IFR flight planning. Clear skies suggest good VFR conditions, while weather with heavy storms can vary widely and might depend on specific visibility and ceiling readings, and levels of cloud cover alone do not determine the need for an IFR flight plan if visibility remains adequate. Thus, the situation that requires IFR filing is specifically tied to the visibility and ceiling metrics outlined.