The change of characteristics of an air mass as it moves away from its region of origin. For example, maritime air at midlatitudes that originally has high humidity and cool temperatures can be modified as it blows onshore over coastal mountains, where orographic precipitation causes the air to become drier and warmer by the time it reaches the lee side of the mountains. The rate of airmass modification depends on the differences between its original characteristics and those of the new surface over which it flows.