A thin layer of fog sits atop a young crop in the early morning hours. It has a peaceful appearance, but these conditions may provoke horizontal movement of surface-level air, causing any trapped particles — such as pesticides — to move into neighboring fields.
What you see may be a temperature inversion. A layer of warm air covers a layer of cooler air and acts like a lid, preventing the cooler air from rising and mixing with the upper atmosphere.
According to the National Oceanic and Atmospheric Administration (NOAA), temperature inversions commonly form when the air near the ground cools at night. Calm winds, clear skies and long nights increase the likelihood of a temperature inversion occurring.
Gases trapped near the surface can’t mix with the warmer air above and dissipate naturally, so they hover near the ground and often drift sideways. Pesticides hanging in this layer of air can move into neighboring fields, lawns and gardens with unintended consequences.
“We want a light wind — 3 to 10 miles per hour — when making a herbicide application,” says Haley Nabors, Enlist™ field specialist. Within a temperature inversion, applied products can move great distances. “Furthermore, the direction the trapped air will move is unpredictable.”
Warn growers about temperature inversions
You can remind customers or anyone applying crop protection products to watch for common conditions that create temperature inversions. If these conditions occur, avoid applying any herbicides until the environment is favorable for a successful, on-target application.
Temperature inversions are most likely to occur when wind speeds are less than 3 mph and/or if the temperature is within five degrees of the nighttime low. That’s why spraying in wind conditions of zero to 3 mph is never recommended.
Technology helps identify conditions
All applicators should plan to check local weather conditions before making any herbicide application.
“If you identify a temperature inversion, do not make an application,” Nabors says. “The spray particles may never hit the intended surface, which makes the application less effective for your crop. If it doesn’t reach the weeds, you’re wasting your herbicide dollars.”
In addition, you run the risk of damaging susceptible plants in nearby fields, lawns and gardens.
Nabors urges you to check conditions before every application and even during applications. Weather apps for smartphones and tablets can be useful tools to monitor changing weather throughout a herbicide application. Always monitor conditions while you are in the field. In addition to weather apps or websites, an inexpensive windsock shows wind direction. An anemometer provides wind speed. A quick check of the temperature also is a good idea.
If you prefer a visual sign, releasing smoke or powder can indicate particle movement. The smoke or powder should drift gently with the wind. If it gathers in a stationary, suspended cloud, that indicates a temperature inversion, which may cause an application to move far and wide.
“With Enlist herbicides, we recommend a minimum wind speed of 3 mph,” Nabors says. “This allows some stirring in the atmosphere to dissipate any potential inversion layer.”
Remember, a complete lack of wind is a warning. Do not apply herbicides. Wait until later in the day and check again for a more favorable application environment.