Of course, Kane County folks already have seen some frost on the windows in the morning, but when will we see the first freeze?

The National Weather Service Chicago says today (Oct. 13, 2016) is about the average date that we get temperatures of 32 degrees or below. Closer to the lake, the average is later in the season (Oct. 23 at Midway Airport) and up in Rockford, it’s earlier (Oct. 8 average).

The earliest fall freeze for Chicago was Sept. 22, 1995, and the latest was Nov. 24, 1931.

The NWS-C says Kane County will see some “patchy frost” in the wee hours of Friday, Oct. 14, with a low of about 35 degrees overnight.

What’s a Frost, and 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.

The maps represent the period of 1981 to 2010, to match the standard normals. The median represents the middle value in the range of dates and is less sensitive than the average to unusually early or late dates. It also fits better with the maps of the earliest, latest, 10th percentile, and 90th percentile, since the median is the 50th percentile.

The actual date varies from year to year. For tender plants, add two weeks to the average date in the spring to protect against the possibility of late season frost. In the fall, subtract two weeks from the average date to be on guard against an early frost.

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