When spring or autumn comes around, the National Weather Service issues public bulletins with terms like frost advisory, freeze warning and killing freeze. But what do these terms all mean?
The important difference between them is the air temperature near and at ground level and its impact on various crops during the growing season.
One crop still growing on farms that may not be mature yet could be double-crop soybeans. Because cooler temperatures typically occur in the northern half of Indiana first in the fall, the growing season from killing freeze to killing freeze is shorter, and doublecropping is a much higher risk as you move farther north.
Terms associated with frosts and freezes
If the air temperature 5 feet above ground is expected to remain above freezing but frost will likely form on the ground, then a frost advisory is issued.
If the air temperature at the 5-foot height is forecast to hold above 28° but at 32° or less, then a freeze warning is issued.
The air temperature closer to the ground will likely be even colder, of course. This condition is sometimes called a "light freeze" and can cause damage to garden crops such as lettuce, tomatoes, peppers, and vine crops, for example.
A killing freeze or "hard freeze" occurs when the 5-foot air temperature falls to 28° or less. Temperatures at ground level can be colder and can kill garden and most field crops such as corn and soybeans. The extent of damage depends on the type of plant and how many hours the air temperature remains below the critical level.
Indiana maps of average first autumn and last spring freeze dates at 5-foot height air temperatures of 36 degrees F, 32 degrees and 28 degrees can be found online. Weather experts have also prepared tables that show the probability of various frost or freeze events at specific locations within Indiana.