Which scenario requires an alternate airport for a flight?

The scenario that requires an alternate airport for a flight is when the weather at the destination is below VFR (Visual Flight Rules) minimums. According to aviation regulations, a flight must have an alternate airport available if the weather conditions at the intended destination are forecasted to be below the minimums necessary for landing. This ensures that, should the flight be unable to land at the primary destination due to low visibility or other adverse conditions, there is a safe place for the aircraft to divert to.

Having good visibility and clear skies, high altitude turbulence, or direct routing with no weather issues do not necessitate an alternate airport, as these conditions generally do not pose a significant risk to the safety of landing at the intended destination. Instead, they would typically support a safe arrival. Thus, the requirement for an alternate airport is primarily based on the weather conditions prevailing or expected at the destination.