Pesticide Drift: Calm, Still Days Are Most Dangerous – DTN

    Every pesticide applicator worth his salt knows not to spray when gusts of wind bend trees and scatter hats. But not everyone knows that the opposite — a sunny afternoon with no wind — could actually be the more dangerous time to spray.

    Mornings and late afternoons with no wind and clear skies can often be signs of a dangerous atmospheric phenomenon called a temperature inversion, experts explained to DTN.

    “We have seen more drift destruction in inversion conditions than on a really windy day,” said Andrew Thostenson, a pesticide program specialist with North Dakota State University.

    During inversions, fine spray droplets become suspended near the ground in dense, cool air for hours. In a particularly intense and long-lived inversion, droplets can drift up to 4 to 5 miles off-target, he said.

    “A bad inversion day is the absolute worst time to spray,” Thostenson said. Avoiding temperature inversions during the spray season is challenging even for professional sprayers, but researchers are slowly gathering more information about how they work and how growers can avoid them.

    Such research couldn’t come at a better time; the risk of herbicide drift damage to sensitive crops will only rise as more herbicide-tolerant crops enter the market, Thostenson noted.


    In a normal day, the sun heats the ground, and the resulting warm air expands and rises. The cooler, dense air that moves in to take its place is likewise warmed and begins to rise, setting in motion a constant shuffling of air near the earth’s surface, explained University of Missouri senior research specialist Mandy Bish.

    Sometimes during the evenings and mornings of clear days, the ground cools faster than usual. Without cloud cover to trap the heat, cool air collects near the surface and sits there, heavy and still, Bish said.

    When spray droplets hit this cooler air mass, the density slows them down and they hang suspended, unable to finish their journey to the ground, Thostenson said.

    Slight breezes can catch these suspended droplets and move them laterally for as long as the inversion lasts. Since most inversions start in the late afternoon and persist overnight, some droplets can move miles before landing on an unintended target in the morning, Thostenson warned.


    Much remains unknown about how often inversions occur, but Mother Nature does give us some hints that they are underway.

    “In the morning, we have some pretty big clues — a frost, heavy dew, some fog, and smoke or odors trapped near the ground — because the inversion has reached maximum intensity at that point,” Thostenson said.

    Don’t necessarily assume you’re in the clear if one or some of these signs don’t appear, warned Scott Bretthauer, a pesticide application specialist from the University of Illinois. For example, fog and dew occur as a result of moisture, so a temperature inversion on a particularly dry morning would not generate those clues, he pointed out.

    Afternoon and evening inversions are both harder to detect and potentially more dangerous. “The afternoon or evening inversion is the worst because it only gets worse,” Thostenson noted. “They are the ones I worry about the most. I tell my applicators that this is the time that they will sneak up on you gradually before you realize what has happened.”

    Signs of an afternoon inversion are subtle, but clear skies and wind speeds from zero to 3 miles per hour are generally prerequisites, Thostenson said. At this point in the day, between 3 p.m. and 6 p.m., applicators have most likely been spraying all day and are finishing up fields and less likely to be monitoring the atmospheric conditions, he noted.

    Aerial applicators have an advantage; most have smoking devices on their planes that can eject a plume of smoke. An educated aerial sprayer can watch the smoke and observe if it hangs in the air without dissipating or rising — a sign that an inversion has begun.

    On the ground, applicators can make a point of watching how dust behaves either in their field or on nearby gravel roads. On clear, still days, they should also make a habit of climbing out of the sprayer in the mid-afternoon and surveying the atmospheric conditions, Thostenson said.


    Temperature can be a good indicator of an inversion. During an intense inversion, the temperature at ground level will vary visibly from the temperature 10 feet above the ground, Thostenson said.

    North Dakota State University researchers are working on a temperature device that would help farmers and applicators quickly assess whether an inversion is underway, but for now, a good handheld device can work, Thostenson said.

    Carefully shielding the handheld device from direct sunlight, take the temperature just 6 inches off the ground and then take a second reading 10 feet in the air. In a severe inversion, the temperatures could differ by 6 to 7 degrees, Thostenson said. A 1 to 2 degree difference is usually safe, but anything at 3 degrees or above should be considered an inversion and applicators should cease spraying, he advised.

    Scientists still don’t have a clear idea of how frequently inversions occur, University of Missouri researcher Bish said. MU researchers have recently set up a multi-year project to assess how often temperature inversions occur in the state of Missouri.

    Seasons appear to have some effect on the frequency of inversions, as does geography, Thostenson said. In North Dakota, inversions appear to worsen in duration and intensity in the late summer and early fall, when humidity levels fall. But in the mountains and valleys of the Pacific Northwest, inversions pick up in the mid-fall and last until spring, with some lasting days at a time, he added.


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