The process works similar to that of growing degree days, which farmers use to chart plant development progress. Enz's results were verified with National Agricultural Statistics Service data, which are collected from surveys. The original purpose of the study on planting days was to help the Extension Service determine a farmer's available working days during planting. Farmers were getting a lot bigger, buying more land, so this was to offer help on how much machinery they'd need to get the planting done in the time available in a year. Enz says that when he worked at the University of Minnesota in St. Paul in the 1970s, scientists there used a similar method to predict the ice-out dates of its lakes.
Ice is a lot simpler, from a microclimatic aspect.
Grand Forks Herald.