It 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.
For 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.
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.
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.
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.
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.
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.
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.
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.
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.