Dry Soils and Low Humidity Will Impact Air Temperature Inversion Timing, Duration, and Intensity (05/20/21)
North Dakota’s drought situation continues to grip most of the state. Because soils are drier than normal, they do not absorb as much solar radiation during daytime hours. Further, the atmospheric relative humidity is lower than what we typically experience. That is why we have recently observed relatively cold nighttime lows with repeated frosts. Indeed, Daryl Ritchison, NDSU NDAWN Director, has been warning for several weeks that we may expect more intense frosts and later frosts than normal well into May and beyond.
For pesticide applicators, it means inversion timing, duration, and intensity will be significantly influenced by the dry weather.
Timing and Duration. In general, inversions develop sometime after the day-time high temperature has been reached. In a typical year, inverted temperatures can be detected three to four hours before sunset. However, in a dry year, an inversion will begin earlier than normal, perhaps one to two hours earlier. On the flip side, in the morning, an inversion will dissipate faster. Instead of one to two hours to restore neutral conditions after sunrise, it may only take 30 to 90 minutes. In drier conditions, applicators are going to encounter a shorter spray window because inverted temperatures will likely exist well into the afternoon and throughout the evening hours.
Intensity. With low relative humidity in the atmosphere and our dry soils, the intensity of inversions will be greater. Three to five-degree F spreads between temperature probes set at three and ten feet above the ground are common, especially on calm clear nights in a normal year. However, under dry conditions the spread could easily go to ten degrees F or more. (Indeed, the NDAWN Logan Center Station in Grand Forks County recorded a ten-degree F inversion on May 10.) When you have that level of intensity it will have a major impact on fine spray droplets. Spray drops will be firmly suspended in the relatively dense cool air near the ground and then easily be moved downrange in light winds.
You can read more details on inversions by downloading: Air Temperature Inversions Causes, Characteristics and Potential Effects on Pesticide Spray Drift (AE1705, Revised Oct. 2019).
Finding acceptable weather to spray any pesticide is challenging, unfortunately the dry weather we are experiencing is going to add another complication from stronger and longer air temperature inversions. The good news is, through NDAWN, we have excellent tools to monitor them and we also can be alerted via cellphone when they exist. Simply type “NDAWN Inversion” into your favorite app store search and you can download, install, and setup real-time alerts for a station near you.
Andrew A. Thostenson
Pesticide Program Specialist