Climate is not merely average weather, although averages of weather observations are often used as a basis of comparison over space and time. Instead, climate is defined by the full set of atmospheric conditions and processes that can occur at a place over time, including variations from normal and extreme events. The climate of Illinois is a key factor in water supply planning. Precipitation over the state and surrounding region replenishes our water supplies, while temperature largely governs the loss of water from the surface.
Climate interacts with water supply over many time scales: some surface and groundwater reservoirs may reflect precipitation that fell months, years, or even centuries and millennia ago. Climate inputs to the Illinois water supply have varied in the past, and are also expected to change in the future. Therefore, we need to look at today’s climate, yesterday’s climate, and tomorrow’s climate to get a better picture of the climate controls on our Illinois water supply.
The location of Illinois on the south and east side of the center of a vast continent is quite fortuitous. Cold and dry continental winters give way to abundant heat and precipitation during most warm seasons. Because of its substantial north-south extent, the climate in northern Illinois is somewhat different from southern Illinois, exposed as these regions are to different amounts of solar radiation and different frequencies of weather systems and air masses.
There are also more localized factors that affect climate, including topography, land use, and proximity to Lake Michigan. However, broad scale mechanisms are the keys to the establishment of the normal climate patterns of today, and that control past variations and future changes.
In general, the data presented are for the whole state. For the northeast and east-central Illinois planning areas, this presentation shows how their climate conditions relate to regional features.
Winstanley, Derek. and Wayne M. Wendland, 2007. Climate Change and Associated Changes to the Water Budget, in "Climate Change and Variations: A Primer for Teachers (Vol. 1)", National Council for Geographic Education Special Publications, in press. (pdf ~1.5 MB)