We often hear that the more things change, the more they stay the same. A quick check on Google shows that that statement has been applied to virtually all-human endeavors at one time or another. Although day-to-day weather often seems to "average out" with time, the longer-term ice ages and warm/cold and wet/dry episodes of the past rather clearly demonstrate that climate exhibits dramatic changes over decades, centuries, and longer.
Twelve thousand years ago Lake County was covered by the southern extremities of the Laurentide Ice Sheet, a glacier that covered all of Canada and parts of north-central and northeastern United States. That presence certainly reflected a dramatically different atmospheric environment than those areas experience today. The large glacier made its own climate and that of the peripheral area. The period between roughly 8,000 and 5,000 years ago is an example of the other extreme, when world-wide temperatures were several degrees warmer than today, demonstrated by less permanent ice on land masses, higher sea level, and a redistribution of natural vegetation.
In more recent times, recall the following shorter-term episodes. The snow-plagued Illinois winters of 1976-77, 1977-78 and 1982-83, the spring and summer drought and attendant crop failures in 1988, the persistent and heavy rainfall and attendant flooding during the summer of 1993, and the disastrous hurricane season of 2005. These were substantial extremes, but prompts the question as to whether they represent a change.
Illinois weather during recent centuries behaved as erratically as in the long-ago and recent few years. Weather information of various types has been observed and recorded for several hundred years; however, early material (prior to systematic observations by trained observers) is generally restricted to non-quantitative, discontinuous or sporadic commentaries collected on an ad hoc basis and recorded in personal diaries, letters, and so on. In Illinois, systematic daily observations under the direction of the Smithsonian Institution, the Army Signal Corps and/or the U.S. Weather Bureau (USWB), now the National Weather Service (NWS), are only extant from seven sites prior to 1870, with another ten beginning during the 1870s, 97 more in the 1880s and an additional 70 in the 1890s.2 These ca. 200 NWS Cooperative sites today record maximum and minimum daily temperatures, daily precipitation, snowfall and snow on the ground.
Twenty-four hour continuous weather observations are currently taken by the NWS in Illinois at the Springfield, Moline, Peoria, Rockford, Chicago O'Hare and Midway airports. In addition, the Federal Aviation Authority records hourly weather observations (generally 12-16 hours per day) at several additional airports about the state. Hourly data include cloud type, amount and height above the surface, visibility, obstructions to visibility type and amount of precipitation, atmospheric pressure, temperature, dew point, and wind direction and speed. Prior to these systematic observations, little is known and certainly less freely available.
Two examples of early, discontinuous "snapshots" of weather at specific locations include journal comments from the Lewis and Clark Expedition while in Illinois:
12 November 1803, on the Ohio River adjacent Ft. Massac: Rain shortly after sunrise, continuing until mid -afternoon. River very low making canoe traffic difficult. Used horses to pull boats over exposed gravel.
15 November 1803, near confluence of Ohio and Mississippi Rivers: Overcast skies prevented latitude measurements. Ohio and Mississippi Rivers
28 November 1803, on the Mississippi opposite mouth of the Kaskaskia: Morning is smokey (misty).
5 December 1803, adjacent present-day Jefferson Co. MO: Sufficient water volume to power a mill.
7 December 1803, adjacent present-day St Louis Co. MO: Dark, rainy morning. Rained all night. Hard northeasterly wind-against us all day. …