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Are Inversions Really That Common?

by Andrew Thostenson, Pesticide Program Specialist, North Dakota State University Extension Service; Jared Goplen, David Nicolai and Ryan Miller, University of Minnesota Extension Educators - Crops

Pesticide applicators have long been obliged and directed by pesticide labeling to understand, identify, and NOT apply during air temperature inversions. This has become acutely important because of the off-target movement of dicamba over the last couple of years. But a similar statement is also found on many other pesticide labels. 

EPA and pesticide manufacturers have made it abundantly clear they do not want pesticides applied during an inversion. But that is easier said than done. Until recently, very few people actually monitored inversions. That is fast changing. North Dakota and NW Minnesota via NDAWN now have 31 stations monitoring this in real time with alerts being posted to smart phones. Missouri has 11 stations. Pesticide manufacturers have also spent a fortune on predictive modeling and distribution of their estimates via mobile apps. Finally, hand held sensors developed by Innoquest are also widely available. Now that we can measure and monitor for inversions, an applicator has to assess this information and make a decision to spray or NOT spray. That is the hard part. 
Over the past couple of months, NDSU and UM extension professionals have been closely observing real time inversion data using fixed stations and the Innoquest hand held device. So far in our work we have some interesting finding we would like to share.

Table 1. Percentage of days with a temperature inversion for at least 10 min per day by inversion intensity level from May 17 to June 12, 2018. Inversion intensity at each location was calculated as the difference in temperature between 1m and 3m heights (ie. Temperature at 3m minus the temperature at 1m). Temperature was logged at 1-minute intervals with the HOBO MX2303 temperature data logger.

  1. Inversions happen in every 24 hour day EXCEPT when it is raining, there is severe weather, or  when the wind speeds are very high and sustained in the night time hours (Table 1) 
  2. Inversions have been observed even when winds have been in the teens and gusting up to 25 mph!
  3. Inversions begin to break apart in the morning hours quickly after sunrise. Temperatures start moving towards neutral (no inversion) within 15 to 30 minutes of sunrise and they are usually neutral within 90 to 120 minutes after sunrise. 
  4. Fog is not always associated with inversions. During an Advection fog you can have neutral or higher temperatures near the ground. Ground/Radiation fog is often associated with an inversion. Telling the difference can be a challenge. You can read more on the types of fogs here:
  5. Inversions start to build in the late afternoon. We have observed a 0.1 °F inversion four to five hours before sunset. However, when an inversion is starting to build that early, it happens very slowly. NDAWN, fixed stations in Minnesota, and hand held devices do not begin to show consistent readings (roughly 0.5°F of inverted temperatures) until about 120 to 180 minutes before sunset. 
From a practical perspective, what is an applicator to do when making a decision to spray or not?
  • In the morning, the sun is your friend. Regardless of cloud cover or wind speeds, an inversion will be quickly neutralized after sunrise by radiant energy striking the earth and heating the air near it. This process is complete in 90 to 120 minutes. 
  • In the afternoon and evening, it is more challenging to determine when to stop spray applications for the day. Especially since we do not know when an inverted temperature begins to seriously impact a pesticide application. At this time, our BEST IDEAS ARE:

a)     When skies are clear, wind speeds are very calm (0 to 3mph), and shifting direction, an applicator should exercise extreme caution and shut down earlier in the afternoon.
 b)    When skies are clear, winds are above 3, to 6mph, and an inversion of 0.5°F or more is observed, the inversion intensity will likely move to 1.0°F or more by the evening hours. We believe this threshold merits curtailing further applications. 
c)     With clear or broken skies, and winds above 6 to 10 mph, there is sufficient mixing in the atmosphere to keep an inversion from forming as quickly or as intensely. In these conditions, one may consider spraying longer into the evening, but would still be wise to finish applications within an hour of sunset.
d)     Finally, local conditions will often influence the timing and intensity of inversions. For example, official sunset times do not take into consideration rolling topography or the shading/wind influences from shelterbelts. Further, the site of application may have varying ground cover or canopy formation relative to a fixed tower site. Thus it is important to not rely exclusively on remote sensing. 

In summary, pesticide applicators, like never before, are under intense pressure to make the right decision every time to avoid off-target pesticide movement. The good news is, we are amassing a massive repository of data to help applicators judge when to spray or not. The bad news is we have much to learn, in academia, industry, and in the regulator community. Until more answers are available, we will do the best we can sharing what we know and WHAT WE DO NOT KNOW about this critical environmental phenomenon.  

For More Information
The basics and importance of Air Temperature Inversions in NDSU’s comprehensive fact sheet, here:
11 minute video clip by Daryl Ritchison, NDSU’s North Dakota Ag Weather Network Director on observations from North Dakota during the 2017 season here:

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