Frost Climatology in Illinois

frost2It is that time of year to discuss the nemesis of every gardener out there – the late spring frost or freeze. We have a full suite of maps showing the earliest, 1 in 10 years, median, 9 in 10 years, and latest frost (32 degrees) and freeze (28 degrees).

One of the things you notice about this map, and all the others in the series, is that even though the date for the last spring frost/freeze gets later the farther north you go, you can get big differences between nearby locations. The reason for that is that most of these last frosts occur during calm, clear, cold nights. As a result, subtle local features can make a big difference. For example, cold air can settle in low-lying areas, causing frost pockets. Or locations out in the countryside may be just a few degrees cooler than in town, due to warm buildings, that may make them vulnerable to a later frost. Also, keep in mind that we measure the air temperature at 5 feet above the ground while the ground and some objects may cool to lower temperatures on calm, clear nights. Therefore, you may see frost even if the temperature is reported as 34 or 35 degrees.

Personally, I use these maps as a general guide. However, I watch the latest weather forecasts closely for a few weeks after I have new plants in the ground for any surprises. The NWS does put out hard freeze warnings (temperatures at or below 28 degrees with or without frost), freeze warning (temperatures at or below 32 degrees with or without frost), and frost advisories for widespread frost.

spring-freeze-32-median
Click to enlarge.

Frost in Illinois

frost2For much of Illinois April is typically the last month that we see freezing temperatures until Fall (at least we hope). Below are the maps showing the median dates when we see 28 and 32 degrees for the last time in Spring. The median represents the middle value in the range of dates and is less sensitive than the average to unusually early or late dates.
We have more discussion and maps, including the earliest and latest dates of freezes during the 1981-2010 period on our frost webpage.
In addition you can track the status of this spring (2013) in terms of how things stand on hitting 28 and 32 degrees from the Midwestern Regional Climate Center.

Click to enlarge.
Click to enlarge.

Click to enlarge.
Click to enlarge.

Frost on the Pumpkin in Illinois

Here are the median dates of the first time we hit 32 degrees in the fall. Since we don’t have reports of when frost occurs, we use 32 degrees as a proxy. It is no surprise that the frost date occurs earlier in northern Illinois than southern Illinois.
If you look closely, you may see differences of several days between neighboring sites. Nighttime temperatures, especially on calm, clear nights in the fall, can be very sensitive to the local conditions. Sites in town tend to be a little warmer than those in the countryside, and result in delaying the arrival of colder temperatures.
Chilly temperatures have already visited the northern part of the state this weekend. Mt. Carroll reported a low of 29 on September 23 and Elizabeth reported a low of 28 on September 24.
You can read more about frost and see many more maps on the state climatologist’s frost webpage.

Median date of first fall frost. Click to enlarge.

Frost Damage to Corn

There is an excellent article in the integrated pest management Bulletin at the University of Illinois by Emerson Nafziger about the damage of frost to corn this April and it’s impact on yield. You can read the full article at http://bulletin.ipm.illinois.edu/article.php?id=1619 

Here are the low temperatures reported last week, sorted from coldest to not so coldest. 

Station Name Low Temp(°F) Month/Day
PAXTON 20 11-Apr
CHAMPAIGN 9 SW 21 11-Apr
PAW PAW  23 11-Apr
JERSEYVILLE 2 SW 23 17-Apr
MT CARROLL 24 11-Apr
SIDELL 5 NW  24 11-Apr
WATSEKA 2 NW 24 11-Apr
LISLE-MORTON ARBORETUM 25 11-Apr
SHABBONA 5 NNE 25 10-Apr
ALTONA 25 11-Apr
MONMOUTH 4 NW  25 11-Apr
NORMAL 4NE 25 11-Apr
CONGERVILLE 2 NW 25 11-Apr
NORMAL 4NE 25 11-Apr
PERRY 6 NW 25 12-Apr
GALENA 26 11-Apr
STOCKTON 3 NNE 26 11-Apr
MARSEILLES LOCK  26 11-Apr
LA HARPE 26 11-Apr
PRINCEVILLE 2W 26 11-Apr
DWIGHT 26 11-Apr
HOOPESTON 1 NE 26 11-Apr
NEOGA 4NW  26 11-Apr
SULLIVAN 26 11-Apr
ELIZABETH  27 11-Apr
KEWANEE 1 E  27 12-Apr
ROCKFORD GTR ROCKFORD AP 27 11-Apr
DE KALB  27 11-Apr
MCHENRY STRATTON L&D 27 11-Apr
MUNDELEIN 4 WSW  27 11-Apr
BENTLEY  27 12-Apr
GALESBURG  27 11-Apr
PRAIRIE CITY 2S  27 11-Apr
CHENOA 27 11-Apr
DANVILLE 27 11-Apr
EFFINGHAM  27 11-Apr
NEWTON 27 11-Apr
PARIS WTR WKS  27 11-Apr
TUSCOLA  27 11-Apr
WINDSOR  27 12-Apr
FREEPORT WWP 28 11-Apr
HENNEPIN 28 12-Apr
MOLINE QUAD CITY INTL AP 28 12-Apr
ROCHELLE 28 11-Apr
ROCHELLE 28 11-Apr
BARRINGTON 3 SW  28 11-Apr
ELGIN  28 11-Apr
MARENGO  28 11-Apr
MORRIS 28 11-Apr
KNOXVILLE  28 11-Apr
BLOOMINGTON 5W 28 11-Apr
DECATUR  28 11-Apr
MINONK 28 11-Apr
OGDEN  28 12-Apr
PONTIAC  28 11-Apr
CARLINVILLE  28 11-Apr
MORRISONVILLE  28 11-Apr
SPRINGFIELD CAPITAL AP 28 11-Apr
CHARLESTON 28 11-Apr
PALESTINE 2W 28 11-Apr
RAMSEY 28 11-Apr
ROBINSON 28 11-Apr
SHELBYVILLE DAM  28 11-Apr
ALEDO  29 11-Apr
CHICAGO BOTANICAL GARDEN 29 11-Apr
OTTAWA 29 11-Apr
ROMEOVILLE LEWIS UNIV AP 29 11-Apr
QUINCY RGNL AP 29 11-Apr
RUSHVILLE  29 11-Apr
URBANA 29 11-Apr
GRIGGSVILLE  29 13-Apr
PITTSFIELD #2  29 12-Apr
SPRINGFIELD #2 29 11-Apr
WHITE HALL 1 E 29 12-Apr
WINCHESTER 29 11-Apr
HIDALGO 3SW  29 11-Apr
PANA 29 11-Apr
MT VERNON 3 NE 29 12-A
pr
GENESEO  30 11-Apr
NEW BOSTON DAM 17  30 13-Apr
AURORA 30 11-Apr
JOLIET BRANDON RD DM 30 11-Apr
LITTLE RED SCHOOL HSE  30 11-Apr
PARK FOREST  30 11-Apr
PEORIA GTR PEORIA AP 30 11-Apr
FISHER 30 11-Apr
RANTOUL  30 11-Apr
JACKSONVILLE 2 E 30 12-Apr
OLNEY 2S 30 11-Apr
TRIMBLE 1E 30 11-Apr
VANDALIA 30 11-Apr
CARBONDALE SEWAGE PLT  30 12-Apr
SPARTA 30 11-Apr
FAIRFIELD RADIO WFIW 30 12-Apr
ILLINOIS CITY DAM 16 31 12-Apr
CHANNAHON DRESDEN ISLAND 31 12-Apr
CHICAGO MIDWAY AP  31 11-Apr
CHICAGO MIDWAY AP 3 SW 31 11-Apr
KANKAKEE METRO WWTP  31 11-Apr
FLORA  31 12-Apr
IUKA 31 11-Apr
BELLEVILLE SIU RSRCH 31 12-Apr
DU QUOIN 4 SE  31 12-Apr
ROCK ISLAND L&D 15 32 11-Apr
ROCK ISLAND L&D 15 32 11-Apr
CHICAGO OHARE INTL AP  32 11-Apr
QUINCY DAM 21  32 12-Apr
STREATOR 32 11-Apr
JACKSONVILLE 2 32 11-Apr
CARLYLE RSVR 32 11-Apr
KASKASKIA RIV NAV LO 32 12-Apr
LEBANON  32 12-Apr
SMITHLAND L&D  32 12-Apr
MORRISON 33 17-Apr
GLADSTONE DAM 18 33 11-Apr
BIRDS 2E 33 11-Apr
LAWRENCEVILLE  33 11-Apr
SALEM  33 11-Apr
NASHVILLE 1 E  33 12-Apr
CAIRO 3 N  34 12-Apr
GRAND CHAIN DAM 53 34 12-Apr
BROOKPORT DAM 52 34 12-Apr
ALTON MELVIN PRICE 41 10-Apr
GRAYVILLE  44 10-Apr

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

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.

Average dates of last spring frost in Illinois.

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.

Frost

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.

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.
Locations that have experienced a freeze so far.  Image courtesy of the Midwestern Regional Climate Center.

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.