The National Weather Service Chicago says the first real freeze of the fall season could be here as soon as tonight (Thursday, Oct. 24, 2019) or Friday — and a cold Halloween could be coming next week.

For the Kane County area, the first freeze has come as early as Sept. 22 and as late as Nov. 24. The median date in our neck of the woods is Oct. 10.

Coldest Air of Season Next Week?

The Climate Prediction Center’s six to 10 day outlook is showing high probabilities for below average temperatures next week.

Normals for the last week of October are highs in the mid to upper 50s and lows in the mid to upper 30s. It is likely that temperatures next week will average colder than those normals, possibly much colder.

That might or might not mean a colder-than usual Halloween on Thursday, Oct. 31, 2019. Most of the time, high temperatures on Halloween get into the 50s and the lows into the 40s, but there have been extremes.

On Halloween in 1950, the temperature in Chicago got up to 84 degrees. On Halloween in 1873, it got down to 23 degrees.

What’s a Frost, What’s a Freeze?

According to the U of I Extension, frost is 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”.

Frost in both spring and fall can be a concern to farmers, landscapers and gardeners. However, the NWS-C usually does not directly measure frost at weather stations in Illinois. Sometimes observers may note the presence of frost in their comments on the forms.

To get around the lack of direct observations, the NWS-C uses a temperature threshold of 32 degrees for frost and 28 degrees for a hard freeze.

Although 32 degrees is used to identify frost, visible frost can be seen on the ground and 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. Because of the abundance of warm buildings and trees, we see frost arrive in town a little later than in the countryside.

Check with University of Illinois Extension for suggestions on dealing with frost.

SOURCES: National Weather Service Chicago, University of Illinois Extension