The Weather Guys

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.

Less than two weeks ago, on Feb. 17, Madison reached its annual climatological amount of snowfall — 51 inches — with a 5.2-inch fall. This is yet another wrinkle in what has been a very unusual winter.

Almost everyone remembers the early snowfalls of late October and early November that delivered a total of 16 inches of seasonal snowfall by Nov. 11. Perhaps forgotten, however, is the seven-week snow drought that followed, with the next 1-inch snowfall — 1.5 inches to be exact — finally arriving on Dec. 30.

An unusually snowy period followed, beginning on Jan. 10. From that day until the end of February, 35 inches of snow fell — nearly a foot more than the normal 24 inches for that interval each winter.

The Jekyll and Hyde nature of this past winter is also reflected in the temperatures that started out 5.2 degrees below average in November and then soared to 7.0 and 7.3 degrees above normal for December and January, respectively. The recently completed February was almost exactly normal.

Hemispherically, the areal extent of the 23-degree air about 1 mile above the ground registered its third-lowest value seasonal average in the 72-year record. So, the relatively mild winter we experienced here in Madison was enjoyed by a large number of other locations around the Northern Hemisphere this year.

Looking forward, March averages 8 inches of snowfall, while the daily high temperatures rise from an average of about 38 degrees at the start of the month to near or just above 50 degrees by its end. While there is no guarantee that March will treat us equally kindly, even if we have a cold March it will have been a relatively easy winter when it finally does end.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.