frost

September 13, 2011

Possible Frost Wednesday Night in Northwest Illinois

The National Weather Service office in Davenport has issued this statement of a possible frost on Wednesday night:

Canadian high pressure will bring the threat of an early season frost by late Wednesday night. Areas along and north of Interstate 80 may dip down into the middle to lower 30s after midnight. These kind of temperatures will produce patchy to areas of frost by early Thursday morning. People in these areas should plan ahead and be ready to take the necessary precautions to prevent damage to cold sensitive vegetation. A Frost Advisory may eventually be issued for portions of the area. North winds maintaining 5 to 10 mph will help keep temperatures in the upper 30s south of I-80 and thus limit the frost potential in those areas.Normal 1st 32 degree temperatures range occur around early October in Northeast Iowa and Northwest Illinois, to mid October in West Central Illinois and Northeast Missouri.

You can check freezing temperatures from around the Midwest at the Midwestern Regional Climate Center…

Frost maps — Frost and Freeze map for the Midwest, using temperature thresholds of 32 and 28 degrees. Courtesy of Midwestern Regional Climate Center.

April 14, 2011

Spring Frost in Illinois

Frost in spring is a concern to farmers, landscapers, and gardeners. Frost in Illinois is usually not measured directly at weather stations. Instead, it is inferred from the air temperature – when the air temperature crosses the threshold of 32°F.
The average date of the last spring occurrence of 32°F ranges from April 7 in far southwestern Illinois to April 28 in northern Illinois (see map below). The actual date can vary from year to year. The spring dates are getting earlier by about 5-10 days over the last few decades.
The actual date varies from year to year. For tender plants, add two weeks to the average date in the spring to protect against late season frost.
Although 32°F is the temperature traditionally used to show frost, visible frost can be seen on the ground and objects when temperatures are slightly above 32°F 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. Because of the abundance of warm buildings and trees in towns, they tend to experience frost less often than those living in the country.

0407_springfrost-1 — Average dates of last spring frost in Illinois.

November 1, 2010

October Was Warm and Dry in Illinois

Precipitation

For Illinois, the statewide average rainfall for October was 1.4 inches, 1.5 inches below normal or 48 percent of normal. This ranks as the 20th driest October on record. The largest monthly rainfall total was reported at Belvidere with 3.94 inches. See map below for rainfall departures across the state.
While northern Illinois was close to normal on rainfall in October, parts of southern and eastern Illinois remained dry. The U.S. Drought Monitor lists those areas as “abnormally dry” and southeastern Illinois as”moderate drought”. At this time of year, the main impacts on agriculture would be on pasture conditions and winter wheat.

Soil Moisture

With the vegetation preparing for a long winter’s nap and lower temperatures, the demands on soil moisture are close to zero. So soil moisture should start to recover in the next few months even if precipitation remains below normal. The Illinois State Water Survey posts their latest soil moisture survey a few days after the end of the month here.

Temperature

The statewide average temperature for October was 56.2 degrees, 1.6 degrees above normal. The highest temperature for the month was reported at Fairfield with 93 degrees on October 10. The lowest temperature for the month was reported at Minonk with 22 degrees on October 29 and Sidell with 22 degrees on October 30.

During October, nearly all of Illinois has experienced temperatures down to 32 degrees and many areas have reached 28 degrees or less. See map below.

Figures

October rainfall Illinois — October rainfall departures from normal.

freeze Illinois — Occurrence of Frost (32F) and Freeze (28F) in Illinois at end of October 2010.

All data are preliminary.

October 6, 2010

Frost in Illinois

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.

Frost Climatology

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.

Fall Frost — Average date when temperature first drops below 32 degrees in Fall.