Based on new 1971-2000 averages, State Climatologist Jim Angel of the Illinois State Water Survey (http://www.sws.uiuc.edu), a division of the Illinois Department of Natural Resources says the first fall frost usually occurs between October 7 (northern Illinois) and October 21 (southern Illinois), with an average date of October 14 in central Illinois (see map).

Average first fall frost dates for Illinois.

Angel is quick to point out that the actual frost date varies quite a bit from year to year. "For example, during the same 1971-2000 period, the earliest fall frost date ranged from September 17 (north) to September 25 (south), and September 22 (central Illinois). The latest fall frost date ranged from October 31 (north) to November 14 (south) and November 7 (central Illinois). There were reports of frost and snow in July and August in the 1800s, but these predate the Water Survey's digital record," says Angel.

Although 32 degrees Fahrenheit is the temperature traditionally used to define the first frost, visible frost can be seen on the ground and objects when temperatures are slightly above 32 degrees 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. Frost events can be classified as either radiation frosts (when skies are clear and winds are light) or as advection frosts (when winds are moderate to strong).

Radiation frosts are more typically the first frosts. They occur when a cool high pressure area moves in from Canada. At the center of the high, winds are light and skies are clear. Whether or not frost occurs can vary considerably at different locations on calm, cloudless nights when heat is radiated back into space. Open, grassy areas are usually the first to experience frost while areas under trees are more protected because the trees help prevent the heat from escaping. Homeowners can extend the fall season and provide this same type of protection if they cover their plants when a frost is expected. Plants near heated buildings sometimes are spared too. However, we've all noticed how car windows tend to frost up early and often because cars are very efficient at radiating their heat back into the atmosphere, causing them to cool off rapidly when car engines are shut off. Urban areas have more trees and heated buildings than outlying areas so temperatures are slightly warmer, which delays frost.

Advection frosts result from deep cold air masses entering the region from Canada. This was certainly the case on September 22, 1995, when there was frost over the entire state. Such events can be quite dramatic and rapidly plunge temperatures into the 20s. In many cases, advection frost leads to hard freezes that kill most nonhardy vegetation. Fortunately, the first fall frost comes less frequently from this type of event.

"Despite concerns about global warming, the average first fall frost date has fluctuated over time and has actually been occurring a little earlier at most locations since the early 1970s, suggesting somewhat cooler conditions in fall," says Angel. Data from 13 long-term cooperative observer sites throughout Illinois, chosen for their lack of urban influences, reveal that the date is about 16 days earlier in McLeansboro; 14 days earlier in Anna and Champaign-Urbana; 9 days earlier in Hoopeston and Windsor; 7 days earlier in Sparta; 5 days earlier in Marengo, Walnut, and Minonk; and unchanged in Mt. Carroll and Aledo. This appears to be the prevalent pattern across Illinois, except for a slight trend toward frost 3 days later in Carlinville (west-central) and 1 day later in Rushville (northeast).

"There's still a good chance of more mild weather if Indian Summer occurs after the first fall frost this year," concludes Angel.