How to Spot a Temperature Inversion

Just because it’s calm and clear doesn’t mean it’s a safe time to spray the new dicamba formulations. Here are tips on how (and why) to recognize temperature inversions.

By Marilyn Cummins

Indicators of a Temperature Inversion

  • Clear skies overnight (no clouds)
  • Calm (wind < 3 mph)
  • Closer to sunrise or sunset
  • Dew present
  • Horizontal smoke patterns
  • Dust hanging over a road
  • Ground fog in low-lying areas

When Kevin Bradley investigated what the weather was like on days when dicamba drift damaged soybeans, orchards and vegetable crops in Missouri last June and July, he discovered a big connection with calm days. Just based on wind speed, those days would have been within the label requirements of the new dicamba formulations now registered for use in the 2017 season). So why the drift damage with no wind?

Bradley, the state Extension weed specialist, and fellow scientist Mandy Bish had set out special sensors two years before to monitor temperature inversions—those instances on calm, still nights and early mornings when the air at ground level is colder and denser than the air immediately above it.

Turns out the inversions on those non-windy days were a huge factor during the 2016 season, he says. The dates matched up with dicamba drift damage incidents on mostly windless days. That meant that fine droplets of deposited dicamba were suspended just above the ground in the cool air and then were transported horizontally (sometimes for miles) before dropping on and causing damage to sensitive targets.

“In the morning before temperatures rise 3-4 degrees, those small droplets—maybe less than100 microns in size —they just kind of hang in the air in the absence of a breeze,” says Mark Hanna, an Iowa State University Extension ag engineer. That’s an important factor with growth-regulator type herbicides because those droplets “kind of move in a cloud or a fog and can give off-site symptoms,” such as the cupping (curling) of soybean leaves or twisting, elongated stems and leaf petioles in tomato plants.

So, how can you tell if an inversion is happening?

Under clear to partly cloudy skies and light to no wind, a surface inversion can form as the sun sets and the earth’s surface cools. Ground fog may form in low-lying areas. According to Hanna, sometimes you can tell the conditions are right for an inversion—and not optimal for spraying some pesticides—if the dust on a gravel road that gets kicked up just hangs in the air above where it was disturbed.

To test for sure, Hanna says, an applicator can use smoke by burning a small amount of dry vegetation to see if the smoke dissipates or hangs low to the ground in a defined trail. If the smoke hangs in the air together then moves off slowly without dissipating—that’s what you don’t want, and you should not spray. That goes for the new, less-volatile dicamba formulations, too—they are not immune to the phenomenon.

According to a publication from North Dakota State University, to confirm the presence of an inversion, air temperature should be measured carefully at two heights out of the direct sun: 6 to 12 inches above the ground or 6 to 12 inches above the top of a nearly closed-crop canopy, and at a height of 8 to10 feet above the surface to be sprayed. When the temperature at the higher level is greater than the temperature at the lower level, an inversion exists. The greater the temperature difference between the two levels, the more intense the inversion, and the more stable the lower atmosphere.

As long as the skies remain clear and winds low, a surface inversion that forms in the evening will usually persist some 8 to 10 hours or more, often lasting into the morning, until the sun begins to heat the ground. If the surface temperature rises 3 degrees or more above the morning low, or the wind speed increases to more than 3 mph, spraying likely can begin again.

Bradley has proposed placing many more sensors around the state so that an online or phone app warning system can be set up to let growers know when inversions are happening in their area. Until then, farmers in Missouri and all soybean- and cotton-growing states need to learn the signs—including reading the smoke signals—before spraying the new formulations.

For a detailed explanation of the inversion phenomenon, see this bulletin from North Dakota State University: Air Temperature Inversions–Causes, Characteristics and Potential Effects on Pesticide Spray Drift, and this helpful slide presentation based on the NDSU information: Temperature Inversions and Pesticide Drift.