What atmospheric condition requires an Instrument Rating when departing on a commercial flight?

Cumulus clouds, fog, rain, or any form of visibility-reducing weather can create Instrument Meteorological Conditions (IMC). When operating under IMC, pilots must rely on instruments for navigation and control of the aircraft, as visual references outside the cockpit may be severely limited or entirely obscured. This is why an Instrument Rating is required; it signifies a pilot's ability to fly solely by reference to instruments and to safely navigate in conditions where visual flight is not practical.

In contrast, other conditions listed do not necessitate an Instrument Rating. Clear weather does not pose restrictions for visual flight, and VFR (Visual Flight Rules) conditions during the daytime allow pilots to operate without the need for instrument reliance. Night flying can require an Instrument Rating if the weather is not clear and visibility is compromised, but simply flying at night does not automatically invoke the need for instruments. Therefore, IMC conditions are specifically the atmospheric scenario that mandates the use of an Instrument Rating.