100th meridian west

In the United States, this meridian roughly marks the boundary between the semi-arid climate in the west and the humid continental and humid subtropical climates in the east and is used as shorthand to refer to that arid-humid boundary.

Starting at the North Pole and heading south to the South Pole, the 100th meridian west passes through: In the United States the meridian 100° west of Greenwich forms the eastern border of the Texas panhandle with Oklahoma (which traces its origin to the Adams-Onís Treaty in 1819 which settled the border between New Spain and the United States between the Red River and Arkansas River).

In the central Great Plains, it roughly marks the western boundary of the normal reach of moist air from the Gulf of Mexico, and the approximate boundary (although some areas do push the boundary slightly farther east) between the semi-arid climate to the west and the humid continental (north of about 37°N) and humid subtropical (south of about 37°N) climates to the east.

West of the meridian, raising livestock is much more economically important than east of it, and what agriculture does exist relies heavily on irrigation.

White settlement, spreading westward after the American Civil War, settled the area around this meridian during the 1870s.