At what elevation change within a distance of 10 nautical miles does mountainous terrain begin?

Mountainous terrain is typically defined by a significant elevation change over a relatively short horizontal distance. In aviation and land navigation, an elevation change of about 3000 feet within a distance of 10 nautical miles is generally used as a benchmark to classify terrain as mountainous. This standard helps pilots and navigators recognize areas that may present challenges due to their ruggedness or inability to provide suitable landing or takeoff opportunities.

When a terrain exhibits a change of 3000 feet or more within a 10 nautical mile span, it indicates steep gradients and abrupt changes in topography. This can affect weather patterns, visibility, and flight safety, which is why this particular threshold is used in the context of mountainous terrain.