A temperature inversion occurs when a layer of warm air covers a layer of cooler air and acts like a lid, preventing the cooler air from rising and dissipating into the upper atmosphere.
During a temperature inversion, spray particles can become trapped in the warmer layer of air and stay suspended until the wind increases. Gases, then, can move off target into neighboring fields, lawns and gardens and may cause injury to susceptible crops.
Calm winds, clear skies and long nights increases the likelihood of a temperature inversion occurring.
“We want a light wind — 3 to 10 miles per hour — when making a herbicide application,” says Haley Nabors, herbicide trait field specialist. Within a temperature inversion, applied products can move great distances. “Furthermore, the direction the trapped air will move is unpredictable.”
Nabors says farmers applying herbicides and other crop protection products need to watch for common conditions that create temperature inversions. If these conditions occur, do not apply any herbicides until the temperature inversion clears and the environment is favorable for a successful, on-target application.
“We tend to associate temperature inversions with early mornings or late afternoons, dawn or dusk,” Nabors says. “The traditional expectation is that we’ll see a fog hovering over the field during a temperature inversion.”
However, Nabors says in West Texas, New Mexico and other areas with low humidity, the telltale layer of fog may not develop. Therefore, there’s no visual signal of a temperature inversion.
Farmers should monitor temperatures and check field conditions. If the temperature is within 5 degrees of the overnight low, check the wind speed and particle movement in the field. Use an anemometer to measure wind speed. If it’s less than 3 mph, do not apply herbicide.
“If you apply herbicide during a temperature inversion, the spray particles may never hit the intended surface,” Nabors says. This makes the application less effective for the crop. “If it doesn’t reach the weeds, you’re wasting your herbicide dollars.”
In addition, farmers run the risk of damaging susceptible plants in nearby fields, lawns and gardens.
Applicators should continue to monitor conditions in the field throughout the herbicide application. Release smoke or powder to 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. Do not spray.
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.
Haley Nabors is a herbicide trait field specialist and serves as the in-field expert for the Enlist™ weed control system in Texas, Oklahoma, New Mexico and southwestern Kansas. She is a graduate of Oklahoma State University with a bachelor’s degree in agricultural economics. Her area includes a large number of cotton growers as well as some corn and soybean acres. Herbicide trait field specialists are experts in weed management, application technology and crop research.