Precipitation Maps of the Prairie

Winter's map for non-drought years
Precipitation minus evaporation in centimeters (Winter, 1989).

Winter's map (above) shows overall precipitation to be equal between the Prairie Peninsula and deciduous forests to the east. However during drought years, (Borchert's map, below) a wedge of dry, westerly air penetrates farther east than usual and blocks the moist Gulf air. The greater severity of drought in the Prairie Peninsula is one of the important factors favoring the persistence of prairie in a climatic regime otherwise ammenable to forests.

Percent of normal rainfall in average July of major drought years (Borchert, 1950).
Illinois State Museum State of Illinois IDNR Search, Last modified October 21st 2003, 02:54AM.