Illinois heat waves 1856–1999

In a digitization project at the Midwestern Climate Center, daily data from the 1800s for several Illinois stations have been keyed. This data includes maximum and minimum temperatures from the 1880s on, as well as 3-times-a-day temperatures for the 1850s-1880s. These daily data can be used to extend the record of heat waves back to 1856. For the U.S., both annual averages and extremes of heat waves deaths are greater than those associated with any other weather condition. Heat waves also generate peak demand for energy use. As an extreme weather event, heat waves may be affected by climate change, through changes in several factors that influence the severity of heat waves, including daily maximum temperature, daily minimum temperature, and humidity. The newly available digitized daily data includes a combined 30 years of overlapping maximum/minimum and 3-times-a-day temperature observations at 6 Illinois stations. Analysis of these data suggests that the 2pm temperature is a good proxy for the maximum daily temperature. The 2pm temperature may underestimate the maximum somewhat, especially on days with precipitation in the early afternoon. This effect is relatively rare on days with extreme temperatures. Changes in observing practices, including thermometer exposure, over the last 150 years may also affect the observation of extremes, as well as heat island effects in urban stations. In general, in the 1850s-1880s Illinois experienced moderate numbers of heat waves, similar to the 1910s, 1920s, and 1940s-1950s. This is in contrast to the high numbers of heat waves in the 1930s and the relatively low numbers in the 1960s and 1990s.