Climate Change in Illinois - Temperature and Precipitation

These two figures (click to enlarge) are a best estimate of annual temperatures and precipitation in the last 170 years. For temperature, the 1800s were considerably cooler with a remarkable warming trend through about 1940, followed by a cooling trend through the early 1980s. While temperatures have warmed in recent years, they were not as warm as the 1930s.

For precipitation, the mid 1800s were considerably wetter than today. This was followed by a trend towards drier conditions that ended around 1900. Precipitation remained low until a remarkable increase occurred in the 1960s. This period of wetter conditions remains today.

Illinois Annual Temperature

Illinois Annual Precipitation

Caveats

Unfortunately, the number of stations available in the 19th century was considerably less than today. For example, during the period from 1875 to 1900, between 10-30 stations were available. The number of stations drops off rapidly before 1875. In most cases, the early, continuous records are only available at major cities such as Chicago and St. Louis.

Besides fewer stations, the observation practices and the location of the instruments were different than today. For example, early thermometers and rain gages were located on the roof tops in the downtown portion of the city while thermometers were located on the north porch of the observer's house in rural areas. As a result, the level of uncertainty in the earlier numbers is much higher. Still, these graphs can still provide some guidance on the conditions of the 19th century and how they compare to the 20th century.

An 11-year, moving average was included to smooth out some of the year-to-year variability in the records.