The climate of Illinois is defined by observations taken over all time, but the averages for comparing climates are derived from relatively recent data. For precipitation and temperature, the climate normals are defined as the 30-year average for the period 1971-2000. By international agreement, climate normals are recalculated every ten years. Normals are generated from data sets in which all missing observations have been estimated so that a complete record is available for each site. In addition, adjustments are made to temperatures when stations move or instruments change so that the record is comparable over the full 30 years. The 30-year normals are a suitably long period to obtain a stable estimate of the average climate conditions. However, extreme conditions such as severe droughts and floods are infrequent. A longer period is needed to adequately estimate the frequency of such extremes. In fact, the approach here is to use all available data for this purpose.
Some additional climate variables that are important to water planning, such as soil moisture and evaporation estimates, are not available for the standard 1971-2000 period. However, in these cases the available data are adequate to approximately define the spatial patterns and seasonal cycles and provide a useful depiction of their relationships to the balance of water inputs and outputs:
For more information on the current climate of Illinois, please visit the Office of the State Climatologist on the web.
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Illinois State Water Survey, Contact Information University of Illinois at Urbana-Champaign |
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