Written collaboratively by Laura Edwards and Nathan Edwards.
Pesticide application season is in full swing. Breezy and windy conditions are always a challenge for pesticide applicators in South Dakota, however, temperature inversions are just as important of a challenge. Inversions can trap droplets and volatile pesticides, which can then move horizontally off target. Sometimes this pesticide drift can occur 12 or more hours after application, for example when a strong overnight inversion sets in.
Temperature inversions are simply defined by warmer air over cooler air. The difference in the temperature between higher and lower heights determine how strong an inversion is. For pesticide application, spraying during an inversion is at best to be avoided, and at worst illegal on some labels.
With the help of weather data from the Mesonet at SD State (mesonet.sdstate.edu), it is possible to look at how often inversions occur and at what times of day. The graph below shows data for eastern South Dakota for the months of May, June and July in 2016.
The number of days of each month are on the vertical (y-axis), and the hour of the day, from midnight to midnight, are on the horizontal (x-axis). The gray shaded areas are nighttime hours, from sunset to sunrise. The yellow shaded areas are the first hour after sunrise, and the last hour of the day before sunset.
There are two key messages:
- Nighttime inversions are very common during these three months in eastern South Dakota. At any particular time at night, there is a 70 to 80 percent chance of having an inversion.
- Inversions are just as frequent in the first and last hour of the day as they are in the overnight hours. They generally build during the last one to two hours of the day before sunset, and dissipate one to two hours after sunrise.
As a result, our advice is to be particularly cautious when applying pesticides from one hour before sunset until one hour after sunrise. We also suggest utilizing the Mesonet Spray Tool to help determine real-time conditions in the field.
This spring, the Mesonet at SD State added two additional thermometers at every weather station. Now, all 27 sites have three thermometers that directly measure temperature inversions across the state. This information is displayed on the Spray Tool website and updated every five to ten minutes.
Figure 1. Frequency of inversions, by the hour of the day, for the months of May, June and July for eastern South Dakota.