It has now been more than two weeks since our last below-zero morning here in Madison — Feb. 3 was minus 8 degrees. Thus, a natural question might be this: Are we done with bitter cold for the winter?
If we use an overnight low temperature at or below zero as the metric, then 45 of the 83 winters since 1939-40 have seen a cold spell after Feb. 20 — a surprising 54.2% of the time. In fact, in 1982, the last zero degree (or colder) night was not until April 7, the latest date in the last 83 years.
On the other hand, 12 of the past 83 years have seen the last below-zero morning of the year before Feb. 3, with the earliest ever such date being Jan. 8 in 2017.
None of this really has any predictive value for the rest of this year. In fact, it should be noted that in the past 20 years, the latest below-zero morning occurred on March 10, in 2003.
Recent research at UW-Madison has documented a systematic warming of the lower troposphere during Northern Hemisphere winter over the past 74 seasons. Elements of that analysis have suggested that the hemisphere begins its warmup much earlier than the spring equinox, which occurs around March 22. In fact, the hemispheric warmup really accelerates in the first few days of March.
Though it is true that this hemispheric signal does not always translate to a similar warmup at any given location, it is comforting to know that the entire hemisphere is trying to shake off winter as early as the next couple of weeks. So, climatologically, we really do not have that much further to go.
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 firstname.lastname@example.org or email@example.com.