What factor would necessitate the need for an alternate airport?

The need for an alternate airport is primarily driven by factors that can affect the safety and feasibility of landing at the intended destination. When crosswinds exceed operational limits, it means that the aircraft may not be able to land safely due to the intensity and direction of the wind relative to the runway. This directly impacts the ability of the flight crew to control the aircraft during landing, increasing the risk of an accident. Thus, when faced with such conditions, having an alternate airport designated becomes necessary to ensure the safety of the flight.

In contrast, air turbulence alone, while it may pose discomfort or complicate flight operations, does not directly necessitate an alternate airport. Alternatively, if the forecasted weather at the estimated time of arrival is good, it would likely not require a diversion to an alternate airport. Lastly, while flight crew fatigue levels are important to monitor, they do not immediately dictate the need for an alternate airport; instead, they would influence operational decisions made prior to the flight or during long flights.