When Are Freeze Warnings and Frost Advisories Issued?
It is commonly mistaken that a freeze warning or frost advisory is issued solely on temperatures, and this is not the case. Here are when freeze warnings and frost advisories are issued.
A frost advisory and freeze warning are issued in the autumn until the end of the growing season (marked by the occurrence of the first widespread freeze). The normal end of the growing season is mid to late October in the state of Iowa. However, during anomalously warm autumns, the growing season may be extended past the normal end of the growing season.
In the spring, a frost advisory and freeze warning are issued at the start of the growing season (when it is late enough to cause damage to new plants and crops). This is determined by the number of growing days that have occurred before the widespread freeze that is expected.
So while the overnight temperature in Des Moines is 30 with a freeze warning, and the overnight temperature in Cedar Rapids is 28 with no freeze warning, this is due to the number of growing days that have occurred in those locations prior to the freeze. As shown in this map, you can see the areas in the state that have received enough growing days for a frost advisory or freeze warning to be issued.
What is a frost advisory?
A frost advisory is issued when temperatures, winds, and sky cover are favorable for frost development. This is most likely when temperatures are less than or equal to 36 degrees. Coverage of frost in these cases should be more than patchy. If a frost is sufficiently severe to end the growing season, it is commonly referred to as a ‘killing frost.’
What is a freeze warning?
A freeze warning is issued when low temperatures are expected to be 29-32 degrees.
What is a freeze watch?
A Freeze Watch may be issued a few days ahead of time if the potential exists for temperatures to fall into these thresholds.