Weather during the 2004 growing season in the Midwest, along with improved plant genetics and farming practices, produced exceptionally high yields of all major crops with resulting record yields 10 to 25 percent above prior records, an exceptional increase. Record corn and soybean yields in Illinois and five other states had profound effects on crop prices, given the large foreign demand and the decreasing dollar value, resulting in a huge income increase for Midwestern farmers, $13- $14 billion. Crop experts, regional farmers, and crop-weather models failed to detect and predict the enormous magnitude of the final yields. This inability to assess the magnitude of the 2004 crop yields likely resulted from a lack of information about the presence and effect of the frequent days with clear skies in 2004. The number of clear days, 50 percent to 105 percent above average across the entire Midwest, also were accompanied by much below average temperatures and normal rainfall. Examination of Illinois climate conditions over the past 117 years reveals that when many clear days occurred, most summers were quite hot and dry. Only one prior summer (1927) had comparable conditions to those in 2004. Summer 2004 weather conditions also were unusual in other respects, including having characteristics that were beneficial for all crops. For example, prior record high yields occurred in 2003 for corn, in 1994 for soybeans, in 2001 for sorghum, and in 1990 for alfalfa. Seldom does the entire Midwest experience near uniform summer weather conditions, reflecting another climatologically-unique aspect of summer 2004 weather. Canadian high-pressure centers, resulting from the intrusion of 20 strong cold fronts, frequently dominated the atmospheric circulation across the central United States during the summer, limiting the movement of warm, moist air into the region and creating the season's high frequency of clear days.