Increasing growing-season length in Illinois during the 20th century

Using daily minimum air-temperature (T-min) data from the state of Illinois , the dates of spring and fall freezes - and the resulting growing-season l ength - are examined for trends during the period 1906-1997. Of the station s 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), res ults suggest that the growing-season length in Illinois became roughly one week longer during the 20th century. To examine how changing freeze-date st atistics relate to changing air-temperature probability distributions, perc entiles of T-min for moving 10-year periods were analyzed for trends during the typical times for spring and fall freezes in Illinois (i.e., the month s of April and October). The lower portion of the April probability distrib ution shows substantially larger warming (0.5-0.7 degrees C/100 yrs) than t he upper portion of the distribution (0.2-0.3 degrees C/100 yrs), suggestin g that although cold events are warming during April, warm events are not w arming as fast. Conversely, the lower portion of the October probability di stribution shows modest cooling in T-min (-0.2 degrees C/100 yrs for the 10 th percentile), while middle and upper portions of the distribution show ve ry large rates of cooling (up to -1.5 degrees C/100 yrs for the 40th-70th p ercentiles). Analysis of the entire probability distribution provides a mor e-comprehensive perspective on climatic change than does the traditional fo cus on central tendency.