Importance of sky conditions on the record 2004 Midwestern crop yields

Weather during the 2004 growing season in the Midwest produced exceptionally high yields of all crops with resulting record yields that were 10% to 25% above prior records, an exceptional increase. Crop experts and crop-weather models failed to predict the enormous magnitude of the final yields. This inability to assess the magnitude of the 2004 crop yields resulted from a lack of information regarding the presence and effect of the numerous days with sunny skies in 2004. Clear days ranged from 43% to 110% above average across the entire Midwest, and the oddity is that these sunny days came with much-below-average temperatures and normal rainfall. Examination of climate conditions in the past 120 years reveals that when many clear skies days occurred, most summers were quite hot and dry. Only one prior summer (1927) had conditions similar to those in 2004. The summer 2004 weather conditions were unusual in other respects including the great areal extent of the favorable weather. Thus, the conditions produced record yields of all crops for the first time in history. The record yields had profound effects on crop prices, given a large foreign demand and decreasing dollar value, resulting in a huge income increase for Midwestern farmers of $14 billion. Seldom does the entire Midwest experience near uniform summer weather conditions, reflecting another unique aspect of 2004. Canadian high pressure resulting from the intrusion of 20 strong cold fronts dominated the atmospheric circulation in the central United States during the summer, limiting the movement of warm, moist air into the region and creating the high frequency of clear days. Results suggest that in the future, sky conditions measured during the growing season need to be incorporated in assessments of potential final crop yields.