What can be seen to indicate you may descend below 100' AGL?

Threshold lights are an essential visual reference for pilots, especially during the final approach phase of a landing. They are located at the beginning of the runway and are designed to indicate the runway's threshold. Seeing these lights suggests that the aircraft is near the landing zone and approaching the runway, which typically occurs when descending below 100 feet above ground level (AGL).

This is a critical decision height for pilots, as it is the point where they must have visual contact with the runway or a recognized reference point for a successful landing. If the threshold lights are visible, it signifies that the pilot is in a position to land and can safely continue the descent.

Other options like visual obstacles, aircraft ascent limits, and weather patterns do play a role in flight safety but do not serve as direct indicators for descending below 100' AGL in the context of landing procedures. Visual obstacles may necessitate a higher approach, and weather conditions could impact the ability to see the airport, while ascent limits relate to the aircraft's performance capabilities, not specifically to the landing phase.