We all probably know keen cloud observers who forecast weather with some degree of accuracy. Perhaps clouds can tell us more than upcoming weather forecasts! Early 2019 corn yield reports indicate yields may be 5% to 15% lower than what farmers planned on this year (Extension Educator Jenny Rees' Blog, Oct. 20, 2019). This may be due in part to the cloud cover in August.

Extension Climatologist Al Dutcher in a CW article noted that the third week of August was exceptionally cloudy in parts of central Nebraska. For example, for the week of August 18-25, 2019, solar radiation levels were 59.7% of normal for Wood River and 64.5% for York. In addition, August temperatures were cooler than normal in most of Nebraska. (See Figure 1 in this Sept. 12, 2019, CW article.)  How do these factors affect corn during the grain-fill period and the eventual yield? Can clouds help us forecast corn yields?

Clouds — Both Sides

First, we usually consider clouds to be positive since they bring rain, but there is another side to clouds. They also reduce solar radiation. Unfortunately, the portion of the radiation spectrum that affects photosynthesis ― photosynthetically active radiation, or PAR ― is also reduced, which can decrease yield. Clouds can also decrease temperatures which in some years like 2004, may extend the growing season and increase corn yield potential.

Based on a summary of previous reports where researchers used shade cloth to simulate clouds, we concluded in an earlier CropWatch story, that reduced solar radiation levels the third week of August in central Nebraska would reduce yield potential to some extent.

Hybrid-Maize Model Simulations

The Hybrid-Maize Model computer program simulates corn growth and yield.  Beyond the agronomic basics of hybrid maturity, planting date, and plant population, the major data used are weather data. It is a simulation model and thus “…represents a simplification of the ‘real-world’ system and as such predictions may differ from actual outcomes.” Nevertheless, we can use tools like this to help us better understand corn.

We wondered if the model could help us understand the impact of the heavy cloud cover central Nebraska experienced the third week of August, August 19 -25, 2019. We chose the automated weather station near Central City, Nebraska and five years of data, 2015-2019. In this simulation, we used a planting date of May 1, 2019, and a fully irrigated, 112-day hybrid with a plant population of 30,000 plants per acre.

2019 Simulation with Actual Data

First, the model forecast a yield of 237 bushels per acre with the crop achieving maturity (R6) on September 20 (Table 1). The average daily solar radiation for that week was 297 langleys per square meter. The average daily high temperature (Tmax) was 78.6^°F and the average minimum temperature (Tmin) was 62.9^°F.

2019 Simulations Substituting Previous Year’s Average Solar Radiation and Temperatures

Next, we substituted the solar radiation and temperature averages for 2015-2018 for the third week in August into the 2019 weather data to see if either solar radiation or temperature (Tmax and Tmin) changes would affect 2019 corn development and productivity. In other words, what would 2019 yields look like if we’d experienced weather similar to the average of the previous four years instead of what we experienced?

Here's the first question we addressed and the model output:

• What if we had “average” solar radiation the third week of August instead of the intense cloud cover? Average daily solar radiation for that week over the previous four years was 434 langleys per square meter, a 46% increase over 2019 (center data column in Table 1). Not surprisingly, forecast corn yields increased 5% to 250 bushels per acre.

Then we asked a second question, knowing that increased clouds also may change temperatures.

• What if we had "average" solar radiation and temperatures for that third week of August 2019 instead of the intense cloud cover? We used the same daily solar radiation of 434 langleys per square meter and in the model substituted the four-year average Tmax of 79.9 ⁰F for the average daily high temperature and an average Tmin of 56.8 ⁰F (right-most data column in Table 1). The forecasted yield of 251 bushels per acre was similar to the projected yield when we changed only the solar radiation values.

The significantly lower radiation in 2019, coupled with significantly higher Tmin during that one week likely led to lower photosynthetic rates and higher respiration during the period, which led to lower yield in the end. We expected that with more clouds in 2019 we could have had lower temperatures, which would slow development, increase the seed fill-period and thus increase yields. But that did not happen. The coincidence of cloudy days with similar Tmax but higher Tmin dampened yield increases that could have accompanied the dense cloud cover the third week of August 2019.  Crop development was not affected; the modeled crop matured on Sept. 20 in all three simulations.

Clouds indeed help some of us forecast the weather. With practice and good crop models, we can better forecast corn yields as well. In addition, crop models can, as Joni Mitchell’s 1967 song suggests, help us look at “…clouds from both sides now.”