First frost? Don't hold your breath
CHAMPAIGN — The median date of the first 32-degree freeze in Champaign-Urbana is Oct. 17, but there isn't even an inkling in the forecast of weather that cold.
The coldest night of the upcoming week is forecast for Thursday, when the low will be in the low 40s, according to the National Weather Service.
The actual date of the first frost varies from year to year, according to the Illinois State Water Survey. But farmers, landscapers and gardeners anticipate the first fall frost in October, a certain sign that the growing season has ended.
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, according to state climatologist Jim Angel of the Illinois State Water Survey. If a frost period is severe enough to end the growing season or delay its beginning, it is referred to as a "killing frost."
The median date of the first 28-degree freeze in Champaign-Urbana is Nov. 1.
Although 32 degrees is used to identify frost, visible frost can be seen on the ground and on objects when the reported temperatures are slightly above 32 degrees. 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.
Open, grassy areas are usually the first to experience frost, while areas under trees are more protected. Homeowners can protect plants by covering them when a frost is expected. Plants near heated buildings sometimes are spared too.
Last year, the first 32-degree reading in Champaign-Urbana was on Oct. 6. The first 28-degree freeze was on Oct. 8.
The last 32-degree reading in Champaign-Urbana this year was on April 21. The six-month anniversary of that date is 10 days from today.
The state climatologist office's web page has a map of the first fall frost by location at http://www.isws.illinois.edu/atmos/statecli/Frost/frost.htm.