GOSHEN, Ind. — Mark Twain was once quoted, “Facts are stubborn things, but statistics are pliable.” With that phrase clearly in mind, I will take a chance at writing about the odds of frost in upcoming weeks, based on data collected from 1981 to 2018.
First, I should clarify what a frost is. We all learned in grade school that water freezes at 32o F, the temperature that generally defines the first frost of the fall or the last frost of spring. Tender annual plants will easily die after a few minutes at that temperature. Some tougher plants, such as corn, need hours of exposure at 32o before succumbing.
It turns out, many horticulturalists and meteorologists consider a killing freeze as 28o or lower because few annual plants can survive temperatures at that level for more than a few minutes.
I have provided a table with the information for frosts for the South Bend area over the past 38 years. The dates below each percentile indicate when the date when temperature was typically observed. For example, the “average” 32o frost occurred before or on October 16 over the 38 years of the study in South Bend. Another way to say that is 50% of the 32o frosts occurred at or after October 16.
As another example, 10% of the time, the first 28o temperature of the season occurred before or on October 21 in South Bend, with the average (50 percentile) being November 1.
You really cannot look at a table like this and say when a frost will be. Combining this sort of information with current weather patterns and observations is highly more accurate than looking at a table or reading an almanac. However, what we can glean from this is that patterns exist and trends can change.
Looking at the past to predict the future is not an exact science. We know from similar observations made over the past few decades that our average 32o last frost of spring has migrated to the last week of April, compared to May 10 when I moved here 35 years ago. In addition, our average first 32o frost of the fall was October 10 in the 1980s, but has moved to October 16 over time. In other words, we have gained on average about 2.5 to 3 weeks on our growing season.
— Jeff Burbrink, Extension Educator, Purdue Extension Elkhart County
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