Employing data from the last 72 winter seasons — December, January and February — we have been examining the size of the area one mile above sea level over the Northern Hemisphere that was colder than 23 degrees.
After recording the warmest December ever, since the cold area was the smallest found using this methodology, this year’s winter season ended up as the fifth-warmest of the last 72 — fairly impressive. In fact, perhaps not surprisingly, 14 of the 20 warmest winters by this measure have occurred since 2001-02, with the warmest being 2014-15.
Using similar data, but including March as well, we have recently been attempting to develop a reasonable measure of the duration of winter over the Great Lakes region. We divided the 72-year data set into two 30-year periods, 1948-49 to 1977-78 and 1980-81 to 2009-10. We then determined, for each 30-year period, the calendar days on which there was, on average, a 90% or 10% chance that the coldest day of the year was still yet to come.
For instance, no winter in the record has recorded its coldest day as early as Dec. 1. Thus, on that day, the chance that a colder day is still to come is 100%. Conversely, no winter has ever had its coldest day as late as March 31. That calendar day therefore has a 0% chance that a colder day is still coming.
In the older data set, the 90% day is Dec. 16, while the 10% day is March 3. Thus, one could make an argument that, on average, our winter extended from Dec. 16 to March 3 in that period. In the more recent data set, the 90% to 10% interval extends from Dec. 30 to Feb. 18 — nearly four weeks shorter than the prior period.
Thus, as the Earth continues to warm, not only is winter less extreme, but it may also be substantially shorter.