Missouri air temperatures are trending warmer over the last couple of decades, which is extending the growing season. Figures 1 and 2 show the statewide averages for spring and autumn minimum air temperatures, respectively, over the last 127 years. The trend line indicates a warming of 1.4°F per century. Autumn air temperatures have trended 0.8°F higher.
Figure 1 Statewide, the average minimum air temperatures in the spring are increasing.
Figure 2 Statewide, the average minimum air temperatures in the autumn are rising.
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 3 findings on Missouri freezing temperatures that stem from comparisons of 20-year averages to 127-year averages:
- The last spring freeze is occurring earlier. Figure 3 shows that the average last spring freeze is occurring 3 to 6 days earlier than historical trends.
- The first autumn freeze is occurring later. Figure 4 shows that 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 7 days (Barton County site) to 20 days (Lewis County site).
Figure 3 Median Date of Last Spring Freeze (≤ 32 Fahrenheit).
Figure 4 Median Date of First Fall Freeze (≤ 32 Fahrenheit).
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.
For more information on frost and freeze dates in Missouri, please check out the MU Frost Freeze Guide, which receives a lot of interest this time of year. The web site includes contour maps that show variations in frost/freeze temperature patterns across Missouri. Extreme dates maps are also included. These maps show the most extreme frost/freeze dates in spring and autumn at 25 weather stations across Missouri.