Growing degree days (GDD), also known as heat units, are calculated by taking the average of the high and low temperature in a day, minus the base temperature of 50°F. The optimal number of growing degree days varies depending on the crop and maturity. As you can see in the graph below, Des Moines, Red Oak, and Fairfield have accumulated more growing degree days so far this year than the northern locations and the Iowa average. The bottom graph compares 2016 to previous years. Des Moines is accumulating growing degree days faster than in 2013-2015, but the state average is fairly similar to 2015.
As of July 17th, Iowa crop conditions are looking good with the 81% of the corn crop and 80% of the soybeans rated Good/Excellent. The timely rains last week helped to boost these ratings.