Dicamba Movement: Predicting Temperature Inversions Proves Challenging

Missouri researchers are zeroing in on what makes temperature inversions tick – primarily when and how they’re most likely to form.

The research is driven mostly by ground applications of dicamba herbicide and the rising number of complaints about off-target movement. Previous research on inversions and drift focused on aerial applications.

The research began in 2015 at three locations in Missouri and has recently expanded to other Midsouth and Midwest states, including Kentucky, Illinois, Indiana, Louisiana, Mississippi, Tennessee and Arkansas.

“We found that temperature inversions are pretty common during evenings during the growing season,” said Mandy Bish, University of Missouri weed specialist, one of the authors of a paper on the research. “On the new dicamba labels, you have to quit spraying two hours before sunset, which is the window of when inversions are starting to occur.”

But that’s not always the case, Bish adds.

“What we found through this analysis the last few years is that cutoff is good for some locations, but there are other locations in the Missouri Bootheel, west Tennessee and northern Arkansas where inversions set in a little bit earlier.

“We’ve seen some start at 4 p.m. and earlier,” said Missouri weed scientist Kevin Bradley, another participant in the study. “We’re starting to see a trend that inversions might form earlier the farther south you are.”

Other Factors And Variables

Topography and wind direction can also be significant factors for when temperature inversions form, according to Bish. Recent research helped confirm this after a study of two sites around Martin, Tenn. The two sites are about a mile apart, and both have a prevailing wind from the south.

The studies found that temperature inversions typically formed much earlier at Site 2 because:

It’s lower than Site 1. Cool air tends to settle in low areas. In a temperature inversion, warmer air is on top of cooler air.

Site 2 has a tree line to the south that serves as a wind obstruction. Less wind allows inversions to form earlier.

Three Years Of Research, So Far

Here’s a closer look at inversions at three locations in a Missouri study which took place from 2015 to 2017:

Albany – July of 2015 and 2016 each had 6 inversions, the fewest number of evenings in which inversion formed. However, inversions formed on 15 evenings in July of 2017. April of 2016 had the most inversions occurring at night with 22.

Columbia – June of 2015 and July of 2016 had the fewest number of inversions formed with 9 each. The most inversions occurred in May 2017 with 22 evenings.

Hayward – June 2015 had the least number of inversions occurring with 9. May of 2017 had 26 evenings in which inversions occurred, which was the most observed across all sites and years.

Researchers noted that fewer inversions in 2015 were likely related to increased cloudy weather that year. Cloud cover can interfere with the formation of inversions by trapping radiation in the atmosphere.

Online Tool For Inversion Predictions (Within Limits)

Another result of the Missouri study is an online tool for predicting the occurrence of temperature inversions – the Missouri Real Time Monitoring Map. Similar systems are in place in other states, including Kansas, North Dakota and Oklahoma.

The Missouri online tool includes real-time data (updated every five minutes) at weather stations at 13 locations in Missouri. Temperature probes at each station measure air temperature at 18 inches above ground level and 10 feet above ground level.

The relationship of these temperatures to each other can help predict when inversions can occur, or if one has already formed.

For example, when the temperature at 10 feet above ground level is warmer than the temperature at 18 inches above ground level, the potential for temperature inversion exists. A temperature inversion is unlikely when temperatures at 10 feet are cooler than temperatures at 18 inches.

Other data available on the online site include wind speed and relative humidity, which are also crucial for decision-making about applying chemicals.

Bish said that applicators can use the online tool as a guide.

“With it, you can see what’s happening regionally,” she said. “One of the things we’ve noticed is if you monitor it over time, you can see that inversions are typically going to start on the east side of the state, then move west. So, you can get an indicator of when the inversion is going to form in your area.”

Another advantage of real-time data is that it will help applicators document their spraying decisions in case something does go wrong later, Bradley noted.

And things can definitely go wrong later.

“Applicators are in a very tough spot,” Bradley says. “There are not a lot of time frames that line up perfectly for it to be feasible to spray. I don’t know if I’d want to be in their shoes sometimes, with all the requirements.”

For example, “Data shows that dicamba can escape as a gas as much as 72 hours after an application,” Bradley said. “That’s troubling because how in the world can you control what’s happening 3 days after you made the application?”

On the other hand, data from the study indicates the potential for volatilization “peaks in the first 24 hours after an application and goes down considerably after that,” Bradley added.

Another dilemma for applicators is that the best time to spray to avoid physical drift is when the wind dies down. Still, low wind is also a signal that a temperature inversion could form, especially if it’s late in the afternoon.

To Spray Or Not To Spray?

Bradley said that studies haven’t nailed down an optimum time to spray dicamba, especially when inversions are likely.

“I would avoid spraying between 3 p.m. to 4 p.m.,” he specified. “More times than not, between 9 a.m. and 2 p.m. is probably the safest time. Make sure the dew is off the leaves. One of the other problems is that the particular time of the day also is when we expect some of our windiest conditions, which could cause physical drift.”

More weather stations dedicated to tracking temperature inversions could help applicators make better decisions, Bradley said. “One disadvantage of the online tool is that we don’t have enough of them (weather stations). If you’re 30 to 50 miles away from the weather station, you can’t know if what’s happening at the station is also happening where you are.”

“I think you’re going to see more weather stations installed in the future in a lot more states,” Bradley predicted. “We’ve added some each year and hope to add more depending on funding.”

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