This shipping season, the Lake Superior region was expected to reach its “500 Freezing Degree Days” mark by Dec. 31, 2017, fully two weeks ahead of the usual Jan. 8 date. That formula used to determine when the Soo Locks would close, which was seven days after that date.
Jim Sharrow of the Duluth Seaway Port Authority explains the formula thus:
"Freezing Degree Days" refers to how much below freezing the average temperature is on each day, accumulated since freezing weather first occurs.
For instance, if the simple average of the high and low is 20°F, or 12°F below the freezing point of 32°F, that adds “12 Freezing Degree Days” for the day. When the average is 0°F, the Freezing Degree Days are 32 for the day. Each day with below 32°F temperatures adds toward that 500 Freezing Degree Days total. Before Congress legislated a Jan. 15 closure of the Soo Locks, the plan in place during the 1980s was to close the locks seven days after the “Freezing Degree Days” total reached 500.
One year before the Congress mandated the Jan. 15 date, the 500 mark was reached in December and the locks closed around Jan. 1, leaving several steel mills short of ore for the winter. The steel companies asked Congress to determine a firm fixed closure date, which is how we got to the present plan. In an average year, the 500 total is expected to occur on Jan. 8, which means that the locks would close on Jan. 15, the date Congress mandated.
The closure can be adjusted to a few days later IF customers petition the U.S. Army Corps of Engineers during the fall for an extended closure (weather permitting). The Corps has the right to deny the request if their maintenance plans require the full winter shutdown period. No extension was asked for this season.
Photo of the James R. Barker in the Poe Lock courtesy U.S. Army Corps of Engineers Detroit District Facebook page; link here.