The One Minute Geographer: The Great Plains — the 20 inch Rainfall Line

Jim Fonseca
2 min readOct 12, 2021
The 100th meridian in yellow; rainfall — red; elevation — blue; and grasslands — green. Base map from; lines drawn by the author.

The 100th Meridian (yellow on the map), is a fascinating line because it happens to closely coincide with other important geographical features of the Great Plains.

Today we’ll take a quick look at the 20-inch rainfall line in red, above. I drew it to roughly correspond with the 20-inch line on the map of annual precipitation below.

Map from

The red line on the top map roughly follows the 20-inch rainfall line on the map immediately above. The 20-inch line separates the tan area to the west and the light green area to the east.

The 20-inch rainfall line is significant because it generally divides arid climate and arid lands to the west from humid climates and land to the east. This line, a zone really, separates rich farmland without need for irrigation to the east from land that needs irrigation on the west. To the east lies the corn and soybeans of the Midwest. To the west starts the Great Plains with cattle grazing, wheat lands and the irrigated crop circles you saw on your last flyover.

Map from National Drought Mitigation Center, Line drawn by the author

As shown on the map above, the 100th meridian (in red) also more-or-less follows the transition zone between semiarid and subhumid climates. In the semiarid zone, multiple-year droughts can, and have, occurred.

Remember the 100th meridian is easy to find on a map. It’s the north-south line that separates the Texas Panhandle from Oklahoma.

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Jim Fonseca

Geography professor (retired) writes The One Minute Geographer featuring This Fragile Earth. Top writer in Transportation and, in past months, Travel.