A common Midwestern question: Where have all our 90°F days gone?

A regional study examined the frequency of summer 90°F days in the Midwest and found a fairly uniform decrease in frequency since the end of World War II. Summer daily temperature data from NWS cooperative stations were examined at rural and urban locations. A similar long-term decrease in summer daily diurnal ranges was apparent at most stations, especially late in the growing season. These changes in daily temperature characteristics appear to be related to two interrelated regional factors: 1) changing agricultural practices that have increased the amount of water vapor transpired to the lower atmosphere from maturing corn and soybean fields and 2) increased summer Midwestern precipitation amounts. Understanding these changes in the regional climate of the Midwest will assist a wide array of decision makers and forecasters as they monitor summer climate issues.