Editor’s note: The following was written by Patrick E. Guinan, University of Missouri Extension climatologist, for the Integrated Pest Management website March 3.
Missouri air temperatures are trending warmer over the last couple of decades, which is extending the growing season. The trendline for statewide averages for spring minimum air temperatures over the last 127 years indicates a warming of 1.4°F per century. Autumn air temperatures have trended 0.8°F higher.
Dates of last or earliest frost have also shifted. It seems good to focus in on those shifts as we approach the 2022 planting season. Here are three findings on Missouri freezing temperatures that stem from comparisons of 20-year averages to 127-year averages:
- The last spring freeze is occurring earlier. The average last spring freeze is occurring 3 to 6 days earlier than historical trends.
- The first autumn freeze is occurring later. The average earliest autumn freeze is occurring almost one week later than historical trends.
- The average increase in growing season days across Missouri ranged from seven days (Barton County site) to 20 days (Lewis County site).
These are anticipated trends in a warming world, but Missouri weather is still variable, and seemingly random freeze events can and will occur late in spring or earlier in the fall.
The 11 weather stations used in this analysis have a long, reliable track record. They are part of a network called the National Weather Service Cooperative Observer Program, which was established in 1890 when President Benjamin Harrison signed the Organic Act. A major premise of the program was to define the United States climate.