Because spatial temperature patterns are determined mainly by latitude, southern Minnesota experiences more evapotranspiration than in the north. Semi-humid climates are areas where average annual precipitation exceeds average annual evapotranspiration, leading to a net surplus of water. In semi-arid areas, evapotranspiration exceeds precipitation on average, creating a water deficit. In Minnesota, the boundary between the climate regimes cuts the State roughly into east-west halves. Climate Variability over Time.
Seasonal variability occurs, as different air masses are dominant.
Nearly two thirds of Minnesota's annual precipitation falls during the growing season of May through September, a period during which Gulf of Mexico moisture is often available. Dry spells occur when this moisture source is obstructed or when atmospheric patterns divert storm systems around Minnesota. When hot, dry air prevails, increased evapotranspiration combines with deficient rainfall to create drought conditions. Drought can occur in all areas of Minnesota, however it is more likely in western and northwestern areas more distant from Gulf of Mexico moisture. When Gulf moisture is abundant and numerous storms move through Minnesota, unusually heavy precipitation falls.