Vince_Condella's Blog

The feast days of the three chilly saints have passed and that means the threat for frost has ended.  It's safe to plant in the garden.  I thought about that as I gazed at some patchy frost on my lawn early this morning.  According to nature, frost is still a possibility in the second half of May, but according to old European tradition, the Ice Saints say frost is now a memory.

The Ice Saints are Mamartus, Pancras, and Servatius and their feast days in the olden days of the European church are May 11, 12, and 13.  A date called the "cold Sophie" is celebrated on May 15.  Once those days have passed, tradition claims it was safe to plant your garden.  But we all know that frost is still possible in late May.  Could the Ice Saints tradition have been a faulty one?

Back in 1582, Pope Gregory VIII reformed the Julian calendar.  The old calendar's May 15 date for the old Sophie now corresponds to our modern calendar date of May 22.  So we may need to shift our thinking about frost-free planting back about one week.

Looking at the statistics for Milwaukee, the average date for the last frost at Mitchell Int'l. Airport is April 26.  But remember that is simply the average date.  Some years we have experienced frost much later.  And frost on the ground can occur when the actual air temperature drops to 35 or 36 degrees because the air temperature is measured in a shelter approximately five feet above the ground.

On nights of radiational cooling, with clear sky, low dewpoint (dry air) and light wind, like we had last night, the air directly in contact with the ground cools off more than the air five feet above the ground.  So surface temperature may drop to 32 degrees when the actual measured temperature is 35 or 36 degrees.

Traditional instrument shelter that houses maximum/minimum thermometers.

Also,  Mitchell Int'l. Airport is closer to the lake and in a more urban environment, both sources of warmth during a cold spring night.  Outlying areas away from the city and lakeshore will get colder later into the spring season.

(left) Clouds block the escape of rising warmth away from the Earth's surface at night.  However, (right) a clear night with light wind and dry air allows warmth to radiate away from the Earth's surface and rapidly cool the ground.

The old Europeans were serious about observing the dates of the three Ice Saints.  As the old saying goes, "He who shears his sheep before Servatius day loves his wool more than his sheep."