What weather condition must be avoided for an alternate to qualify?

For an alternate airport to qualify under regulations, it must be free from certain adverse weather conditions that could hinder safe operations. Severe thunderstorms are particularly dangerous due to their ability to produce extreme turbulence, lightning, and wind shear, all of which pose significant risks to aircraft. Heavy rain showers, while not as immediately hazardous, can significantly reduce visibility and affect landing and takeoff conditions, making it difficult for pilots to navigate safely. Wind gusts above 30 knots present another serious concern, as they can deteriorate control during critical phases of flight like landing and takeoff, making it challenging for pilots to maintain stable approaches.

Each of these weather conditions—severe thunderstorms, heavy rain showers, and high wind gusts—has the potential to severely impact an aircraft's safe operational capabilities. Thus, avoiding all of these conditions is essential for an alternate airport to qualitatively provide a safe refuge if the primary landing option becomes unavailable. This requirement ensures that pilots have a viable alternative during flight planning and en route, promoting overall aviation safety.