What is a requirement for an alternate airport shown in green on an Enroute High Altitude US Chart?

The requirement for an alternate airport depicted in green on an Enroute High Altitude US Chart is a minimum of 5,000 feet of hard-surfaced runway. This specification ensures that the alternate airports can accommodate a variety of aircraft types, especially during situations where a diversion is necessary due to weather conditions, mechanical issues, or other operational factors.

Having a runway that is at least 5,000 feet long helps to provide adequate safety margins for landing and takeoff, particularly for larger aircraft that may require more distance to safely operate. The green color coding indicates that these airports meet the specific criteria set by the regulatory authorities, making them suitable options as alternates. This ensures that pilots can rely on these facilities knowing they meet the minimum safety requirements for operations.