The old saying goes, “ If you don’t like the weather in Iowa, wait 15 minutes, it will change.”  And besides the price of corn and nowadays the price of fertilizer, there is nothing Iowa farmers like to talk about more than the weather.   

Weather and climate information can be presented in a lot of ways. In this article, the focus in on temperature and precipitation trends for the “Iowa Average” as reported by the Iowa Environmental Mesonet (https://mesonet.agron.iastate.edu). For the Iowa Average, the Mesonet has monthly temperature and precipitation data from 1893 to the current time. In this article, the data is divided into 2 time periods, each representing about half of the time for which data is available. The “past” time set covers 1893-1957 and the “recent” period covers 1958-2021. 

 

Figure 1. Monthly Average Mean Temperatures During the Growing Season, Iowa Average 

Of most concern for Iowa’s farmers is what is happening to weather during the primary growing season. One measure is the monthly average mean temperature during the growing season. Figure 1 shows the Iowa average mean temperature from mid-April through mid-September for 1893-2021. The blue dots show the data for 1893-1957 and the orange dots from 1958-2021. During the past period of time, the 5-month mean temperature during the growing season was 78.93 degrees. The average temperature was increasing by 0.0095 degree per year. In the recent times (1958-2021) the 5-month mean temperature has been 78.03 degrees, 9/10th of a degree cooler, on average than it was during the 1893-1957 period. The trend during the recent time period has been a decreasing trend with the 5-month mean temperature declining by 0.0249 degree per year.  

Another way to view the growing season weather is to look at combinations of spring and summer temperatures. There can be cool springs and hot summers, cool springs and cool summers, hot springs and hot summers, and hot springs and cool summers. Figure 2 shows the scatter plot of the two 64-year periods. Each dot represents a spring-summer combination of monthly average high temperatures. The vertical blue line depicts the average springtime high for the 1893-1957 period (71.60) and the vertical orange line depicts the average springtime April, May, and June) high for the 1958-2021 period (71.51). The horizontal blue line depicts the summertime (July, August and September)average monthly high temperature (82.93) for 1893-1957 and the horizontal orange line depicts the average summertime monthly high temperature (81.32) for the 1958-2021 period.  The blue circle outlines the area that is 2 standard deviations from the mean for the past time period and the orange circle outlines the area that is 2 standard deviations from the mean for the recent time period.  

 

Figure 2. Spring and Summer Monthly Average Mean Temperature Comparisons, 1893-1957 vs 1958-2021, Iowa Average 

There has not been significant change in the mean of springtime temperatures, although the recent period is a few hundredths of a degree cooler than the past. But there has been a significant decline in the monthly average high temperatures during the recent period compared to the past.  The 3-month average for the 64-year recent period is 1.6 degrees cooler than the 3-month summer average of high temperatures in the 1893-1957 period. There has also been a significant decrease in the deviation of the temperatures from the mean. The standard deviation for the past springtime period was 2.69 and the standard deviation for the more recent springtime period was 2.33.  Similarly, the past summertime standard temperature deviation was 2.55 degrees whereas for the more recent period the standard deviation dropped to 1.82 degrees.  

Iowa has 4 distinct seasons (winter, spring, summer and fall). These can be seen in Figure 3 which shows the 3-month average Mean Monthly Temperatures for the periods of December through February, March through May, June through August, and September through November for the past 100 years. 

 

Figure 3. Iowa State-Average Mean Monthly Temperatures by Season 

Winter (DJF) temperatures exhibit a slight downward trend of 0.0193 degree per year, so in 100 years, the trend is 1.93 degrees cooler. Spring (MAM) temperatures have declined an average of 0.0038 degree per year or 0.38 degrees during the 100-year period. Summer (JJA) temperatures have declined an average of 0.0126 degree per year or 1.26 degrees over 100 years, and fall (SON) temperatures have a decreasing trend of 0.0166 degree per year or a decline of 1.26 degree over a 100-year period. 

Over the past 50 years, some slightly different trends are emerging (Figure 4). Winter (DJF) temperatures are increasing by 0.014 degree per year; Spring (MAM) temperatures are decreasing by 0.289 degree per year; summer (JJA) temperatures are decreasing by 0.009 degree per year and fall (SON) temperatures are increasing by 0.013 degree per year.  

 

Figure 4. Iowa State-Average Mean Monthly Temperature Past 50 years, Seasonals 

When monthly data is considered, it becomes clearer why fall and winter temperatures are increasing. The 50-year trend for December temperatures is increasing by 0.0828 degree per year which would be 4.14 degree over a 50-year period. January temperatures are increasing as are June, September and November temperatures. February, April, May, July, August and October monthly mean temperatures have been declining for Iowa, on average.  

 

Precipitation in Iowa 

Annual precipitation is increasing for Iowa (state average) and has been for the past 128 years. From 1893 to 1957, annual precipitation increased at the rate of 0.0002 inches per year. This means that trendline precipitation in 1893 was 30.96 inches per year and by 1957 it was 30.97 inches per year. The more recent period has seen annual precipitation increasing by 0.0768 inches per year meaning that by 2021, trendline precipitation had increased to 36.4 inches per year (Figure 5). This is an increase in expected annual precipitation of 5.43 inches when comparing today with the mid 1950s. 

 

Figure 5. Iowa State Average Annual Precipitation 1893-2021 

Figure 6 shows how that 5.4 inches of increased annual precipitation is distributed across the year. January has become a bit drier. Expected precipitation in April is now 0.8 inch greater than it was 64 years ago. Expected precipitation in May is 1.46 inches greater than it used to be. July, August and September together are expected to see 1.37 inches more precipitation than previously occurred, and October has become wetter by more than an inch of precipitation. 

  

Figure 6. Monthly Average Precipitation Comparison, Trendline 1958 vs 2022 

Figure 7 shows the percentage change in monthly expected (trendline) precipitation that has occurred since 1958. IF it feels like springtime is wetter, that is because it is – 25% more precipitation in April and 33% more precipitation in May. Summer rainfall is up modestly, but harvesttime precipitation (October) is up 40% from what it used to be.  The annual average expected precipitation is now 16% greater than it was in 1958.  

 

Figure 7. Percentage Change in Monthly Expected (Trendline) Precipitation 1958 vs 2021 

From an overall crop growing perspective, the rising trend in precipitation during the primary growing season has been a positive. Expected precipitation during the growing season is 12% greater than it was in the 1950s and despite the dryness in northwestern Iowa and southcentral Iowa in 2022, the upward trend suggests that 10 years from now, Iowa can expect to get 0.42 more rain during the growing season that it currently receives.  

 

Figure 8. Iowa State Average Monthly Precipitation 1893-2021, Growing Season 

The dominant factor in trend yields for Iowa’s corn and soybeans is improvements in genetics.  Nearly all of the 2+ bushels per acre in corn yield and the nearly 0.5 bushels per acre in soybean trend yields is due to improvements in genetics, fertilization and crop management.  But yield trend can be affected by changes in long-term precipitation and temperature trends. 

 

Figure 9. Iowa Corn and Soybean Yields 

The old saying is, “Rain makes grain” and part of the reason for increasing corn yields is the increase in precipitation during July (Figure 10). For Iowa corn, the additional  0.66 inches of rain during July (comparing 1958 with 2021) contributes 2.7 additional bushels to trendline yields.  

Corn also likes sufficient, but not excessive temperatures, and the cooler (-2.05 degrees) temperatures during July contribute 5.0 bushels per acre toward Iowa’s trendline corn yield. 

 

Figure 10. July  Precipitation and Iowa Corn Yield (1958-2021) 

 

Figure 11. July Average High Temperature and Iowa Corn Yield (1958-2021) 

For Iowa soybeans, increased precipitation (0.48 inch) during August is adding 0.3 bushels per acres to trendline soybean yields and cooler August high temperatures (-1.97 degrees) are adding 0.8 bushels per acre to trendline soybean yields. 

 

Figure 12. August Precipitation and Iowa Trend Adjusted Soybean Yield (1958-2021) 

 

Figure 13. August  Average High Temperature and Iowa Trend Adjusted Soybean Yield 

The data suggests that if Iowa becomes hotter and drier as the climate models indicate is likely to happen in the future, then both of those changes would be expected to create drags on Iowa’s trendline yields instead of being a boost to trendline yields as is currently happening. 


Economic analysis provided by David Miller, Consulting Chief Economist, Decision Innovation Solutions on behalf of Iowa Farm Bureau.