What Is An Inversion And Why Should You Know?

   As the sun heats the ground during the day, warm air rises and is replaced by cooler air, which in turn warms and rises. This continual motion keeps air moving near the ground, producing wind. Sometimes, on very clear days, the air can become dangerously still. This happens most often as the sun sets and the ground cools and stops rising, resulting in a stable air mass hanging just above the ground, unmoving.
   When growers spray during these conditions, the spray droplets cannot fall to the ground and instead hang, suspended in that air mass, until the inversion ends and wind arrives to move them – possibly off-target. These conditions can be detected by measuring air temperatures at different heights – during a temperature inversion, the temperature near the ground will be cooler than the air above it. For two years, University researchers have been measuring temperature 24 hours a day, year round in three different locations in Missouri at three different heights – every five minutes. The results have been eye-opening.
   University of Missouri weed scientist Mandy Bish found, “Across all growing seasons, from March to July, it looks like inversions occur at least half of the evenings at some point. We estimate that in the early part of season, they’re mostly setting up between 5 and 6 p.m., and later, in June and July, they’re setting up between 6 and 8 p.m.”
   In the past, scientists have pegged fog, dew or frost as signs of a temperature inversion. However, these are characteristics of morning hours, when temperature inversions are usually breaking up for the day. Based on the new data applicators and growers need to be on alert for clear, windless evenings.
   When looking at the data over two years in June and July, evenings when winds fell below 3 miles per hour (mph) predicted the beginning of an inversion 90 percent of the time. “When the wind dies down on a clear night, that’s when it’s time to stop spraying,” said Bish. Other signs include dust or smoke hanging or moving laterally in the air.
   To demonstrate volatility of an herbicide, researchers sprayed bare ground plots with the herbicide dicamba. Thirty minutes after application, they placed potted tomato and pepper plants on plastic film on the sprayed test plot and on an unsprayed plot 40 feet away and downwind of the treated plot. After six hours, they removed the plants and grew them in the greenhouse for nearly two weeks, monitoring the plants for dicamba injury. Within one week, the tomato plant in the untreated plot that was downwind of the dicamba application began showing dicamba injury. By two weeks after application, both the pepper and tomato plants had cupped leaves. Plants in the treated plots had much less injury.
“Herbicide droplets may initially land on target plants and/or soil,” Bish says. “Conditions that favor evaporation instead of absorption can allow herbicide to move back into the air.” Moisture, temperature, ground cover and wind speed influence movement. Herbicide formulations matter as well. Low-volatile herbicide formulations are essential in minimizing injury of nontarget plants due to volatility.
   Temperature inversions can also affect off-target herbicide movement. Inversions occur when the air temperature near the ground is cooler than at higher altitudes. This causes a stable air mass with little wind. When this occurs, spray particles remain suspended instead of reaching the target surface.
   Most temperature inversions occur around sunset to sunrise when the following conditions occur:
   Wind speeds less than 2-3 miles per hour; dew or frost; after reaching their target, spray particles may evaporate before being absorbed; horizontal smoke patterns, which indicate horizontal wind movement; ground fog in low-lying areas and clear evening skies, which indicate a lack of clouds to trap warm air in the atmosphere.
   For more information about how a temperature inversion occurs visit
   The following websites will also help applicators:
   DriftWatch, at, is a communication tool that helps crop producers, beekeepers and pesticide applicators work together to protect specialty crops and apiaries.
   Missouri Mesonet, at, provides real-time weather data from stations across Missouri. ∆
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