Computing the “normal” temperatures

Dear Tom,
It seems to me that previously the normal high for the middle of January was 29 and the low 15 degrees. Now it’s 31 and 16. When was there a change? What is the city’s average number of days with highs below 32 and subzero lows.
— George Heyman Mount Prospect

Dear George,
By international convention, normal temperatures are computed every 10 years covering the most recent 30-year period, so the city’s current normals are based on the climatological data from 1981-2010. The previous set of normals was actually lower than you remember, with the city’s annual lowest temperatures based on the 1971-2000 data — a high of 28 and a low of 12 on Jan. 15-16. The next set of normals should be released in 2021 and will cover the period from 1990-2020. Currently, the city logs an average of 40 days with subfreezing highs and seven subzero days annually.