Average Last Occurence of 32 F in Spring. Print Version (pdf)

"The last spring frost usually occurs between April 7 (southern Illinois) and April 28 (northern Illinois), and April 14-21 (central Illinois) based on 1971-2000 averages," says State Climatologist Jim Angel of the Illinois State Water Survey (http://www.sws.uiuc.edu), a division of the Illinois Department of Natural Resources.

"The actual frost date varies quite a bit from year to year," says Angel. He suggests adding two weeks to the average frost date for your area before planting tender plants to protect them against the possibility of a late season frost. "By doing so, the odds are only 1 in 10 that frost will occur later in spring" continues Angel.

Although 32 degrees Fahrenheit is the temperature traditionally used to identify frost, visible frost can be seen on the ground and on objects at slightly warmer temperatures on calm, clear nights that allow cold, dense air to collect near the ground. Under these conditions, the temperature near the ground actually can be a few degrees cooler than at the 5-foot height of the official National Weather Service thermometer.

Open, grassy areas usually experience frost first while areas under trees are more protected because the trees help prevent the heat from escaping. Covering tender plants when a frost is expected can provide this same type of protection. Plants near heated buildings sometimes are spared too. An abundance of warm buildings and trees in town means that urban areas tend to have frost less often than rural areas.

According to Angel, average high temperatures in spring (March-May) range from the upper 50s (north), to the low 60s (central), to the upper 60s (south), while the average low temperatures range from the upper 30s (north), to the lower 40s (central), to the upper 40s (south). Data from the Water Survey WARM Network also indicate that soil moisture across the state is near to above average for this time of year.