Like most sites in Illinois, Chicago has experienced large temperature swings in October. The month started with below-normal temperatures on October 2-5. The departures were as large as 8 degrees below normal on October 3 and 4.
This cold period was followed by a period of much above normal temperatures on October 6-12. The departures were as large as 16 degrees above normal on October 9 and 10. In fact, the high of 86 degrees reported at O’Hare on October 9 tied the record set in 1962.
Slightly above-normal temperatures prevailed on October 13-18. The early cold period and later warm periods resulted in an average of 58.7 degrees for the first 18 days in Chicago, 3.9 degrees above normal.
Parts of southern and eastern Illinois have struggled with dry conditions since July/August. Those conditions have expanded in recent weeks across more of Illinois. The latest U.S. Drought Monitor has slightly more than half the state in at least the “abnormally dry” category and 11 percent in “moderate drought”. Most of the impacts so far have been in agriculture and horticulture and not water supplies.
There is a strong La Niña occurring in the Pacific. Based on past events, Illinois is typically warmer and drier than normal during fall (September-November). By winter (December-February), the wetter than normal conditions typically appear in Illinois.
Here are the average dates in fall when the 4-inch soil temperature falls below 50 degrees (left panel) and below 60 degrees (right panel). Consider these as a “rule of thumb” for planning purposes. Consult the current soil temperatures, as discussed in the earlier post, for decisions on applying nitrogen in the fall.
The Illinois State Water Survey operates a network of 19 sites that report daily soil temperatures at 4 and 8 inches.
Soil temperatures in the fall are critical for the application of nitrogen (N) fertilizer. According to the Agronomy Handbook (University of Illinois), “fall N applications should be done when daily maximum bare soil temperature at 4 inches is below 50 degrees.”
The daily maximum and minimum soil temperatures are measured over grass. In addition, the values are adjusted using regression to estimate the bare soil temperatures, to better represent the soil temperatures in a cultivated field. Grass tends to insulate the soil so the daily temperature swing is a little smaller than over bare ground. In addition, soil temperatures within a particular field may vary due to soil color, soil moisture, and crop residue on the surface.
Frost is defined as ice crystals that form on a freezing surface as moist air comes in contact with it. Farmers, landscapers, and gardeners are interested in frost in both spring and fall. However, frost is usually not measured directly at weather stations. Instead, we choose dates when the air temperature crosses the threshold of 32°F.
The Midwestern Regional Climate Center keeps tabs on what places have hit 32 degrees so far this fall. Click to enlarge.
The average date ranges from October 7 in far northern Illinois to October 21 or later in far southern Illinois (see map below). The actual date varies from year to year. For tender plants in the fall, subtract two weeks from the average date to protect against an early frost.
Although 32°F is the temperature traditionally used to identify frost, visible frost can be seen on the ground and objects when temperatures are slightly above 32°F. This occurs 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.
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 protect tender plants by providing this same type of protection if they cover their plants when a frost is expected. Plants near heated buildings sometimes are spared too. Those living in the country tend to see frost earlier in the fall than those who live in town, because of the many warm buildings and trees in town may ward off frost in some cases.