Home › Concepts › Lines & Boundaries
The 100th Meridian
The historic line between the humid East and arid West
The 100th meridian west is a line of longitude running down the middle of the country - through the Dakotas, Nebraska, Kansas, Oklahoma, and Texas - that has long served as a rough boundary between the humid East and the arid West. The explorer and scientist John Wesley Powell argued in the 1870s that east of this line there was generally enough rain to farm in the usual way, while west of it agriculture would need irrigation.
The pattern is real: average rainfall drops off sharply near the 100th meridian, shaping where ordinary farming gives way to ranching, irrigation, and the short-grass plains. Recent research suggests that, in a warming climate, this dividing line has been creeping eastward toward the 98th meridian. The marker here sits on the 100th meridian in the central Great Plains, along one of the most meaningful invisible lines in American geography.