Increasing Growing-season Length in Illinois during the 20th Century

Using daily minimum air-temperature (Tmin) data from the state of Illinois, the dates of spring and fall freezes – and the resulting growing-season length – are examined for trends during the period 1906–1997. Of the stations in the Daily Historical Climate Network, most show trends toward earlier spring freezes; however, trends in fall freezes are not consistent over the station network. Although the time series are highly variable (noisy), results suggest that the growing-season length in Illinois became roughly one week longer during the 20th century. To examine how changing freezedate statistics relate to changing air-temperature probability distributions, percentiles of Tmin for moving 10-year periods were analyzed for trends during the typical times for spring and fall freezes in Illinois (i.e., the months of April and October). The lower portion of the April probability distribution shows substantially larger warming (0.5–0.7 ◦C/100 yrs) than the upper portion of the distribution (0.2–0.3 ◦C/100 yrs), suggesting that although cold events are warming during April, warm events are not warming as fast. Conversely, the lower portion of the October probability distribution shows modest cooling in Tmin (–0.2 ◦C/100 yrs for the 10th percentile), while middle and upper portions of the distribution show very large rates of cooling (up to –1.5 ◦C/100 yrs for the 40th–70th percentiles). Analysis of the entire probability distribution provides a more-comprehensive perspective on climatic change than does the traditional focus on central tendency.