Herbicides And Inversion Layers

As herbicide applications are made during the growing season, a main concern is the possibility of herbicide drift onto sensitive crops. The usual culprit is wind, but a clear, still night can result in the same effect. It’s a weather anomaly called “temperature inversion.”

Mandy Bish is a senior research specialist at the University of Missouri. She says temperatures are inverted when air closer to the earth surface is cooler than the air above it, and the result is a stable air mass.

"The problem is that during an inversion, there’s not a lot of wind to mix or disperse herbicide droplets, so droplets applied during inversion conditions into that stable air mass can stay suspended and may not disperse," says Bish. "I like to use pollution or smog in large cities as an example because I think it’s a good visual. The pollution particles can’t disperse because they’re trapped in that stable air mass that is the result of an inversion."

While suspended, the particles are susceptible to horizontal wind gusts capable of blowing them off target. Inversions can be hard to predict, but Bish says calm, clear weather conditions are the best indicator.

"What we like to tell applicators is that when an inversion is forming, they’re going to start setting in a little before sunset on clear evenings. So anytime you see the clouds start dissipating and the wind dying down, it may seem like a great time to spray, but in actuality it could be one of the worst times to spray," she says. "We’ve also been looking at smoke bombs that you can purchase online. If the smoke bomb cloud lingers, it could be an indicator that an inversion is forming."