It’s that time of year when we start seeing frost on the ground in Illinois. Frost is caused by the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. If a frost period is severe enough to end the growing season or delay its beginning, it is referred to as a “killing frost”.
Although 32° is used to identify frost, visible frost can be seen on the ground and objects when the reported temperatures are slightly above 32°. On calm, clear nights, the cold, dense air collects near the ground. Under these conditions, the temperature near the ground can be a few degrees cooler than at the 5-foot height of the official National Weather Service temperature sensors.
This map shows the median date of the first time in fall that we observe 32°. The median is the middle value in the range of dates of the first fall frost from 1981 to 2010. While there is an overall pattern of earlier dates in northern Illinois and later dates as you move southward, sometimes the local conditions (such as topography) are just as important.
More maps and information on frost can be found on our frost web page. You can also find historical frost dates and track the situation this year across the central US at the Midwestern Regional Climate Center Climate Watch page.