Home-InternationalThose Pesky Inversions

Those Pesky Inversions

Being an ag pilot is challenging, and sometimes it seems as if Mother Nature is in a bad mood, trying to sabotage all efforts by ag pilots to get the job done. Susceptible crops bordering the area to be treated are always downwind. Whenever there is an unexpected massive outbreak of pests, the winds will stay just above limits for days on end. That line of severe thunderstorms forecast for tomorrow arrives today and grounds the fleet. In these kinds of situations, patience is indeed a virtue.


Some of Mother Nature’s tricks aren’t so obvious and upfront. One of the most insidious has to do with atmospheric inversions. One of my first experiences with these weather demons happened while I was checking a wheat field to assess the amount of volunteer canola present from the previous year’s crop. An AgTruck treating the field next door really caught my attention, given how still and cool the air had become.


There were a few low-lying areas where the fog had already started to form, and I could see from my vantage point less than a quarter of a mile away that the spray was not going down. In fact, it was hanging about 6 feet off the ground as a ghostly white mist. More to the point, the topography was a gentle downslope from the field being treated to a nearby ravine, and you could see the suspended spray droplets slowly but surely flowing downhill toward the ravine.


This is never a good situation, but it can be costly when the pesticide being applied harms susceptible crops. You don’t want any drift damage incidents because they can really eat into the bottom line, not to mention the financial grief that can result for the farmers whose crop has been damaged.


How bad can it get? If a crop is being sold for $20 per bushel, every bushel that doesn’t make it into the storage bin due to drift damage (or other factors) means the farm’s bottom line takes a hit. If the crop could typically produce 40 bushels per acre, every acre where the yield has essentially become zero results in an $800 reduction in returns. Every ten acres result in a reduction of $8000. In short, bad things add up in a hurry.


How could that happen? All too quickly, for one thing. In today’s age of very large equipment and field sizes, you must be aware of the damage neighboring crops could sustain from off-site drift.


Let’s take, for example, an AT-802 applying herbicide on wheat next door to a canola field, with both fields being two miles long. With an 85-foot swath, the AT covers approximately 20 acres per two-mile run in 50 seconds. If an inversion is present along with a one mph wind that causes the suspended spray to drift over and destroy the same acres of canola covered in a single swath, the tab for off-site drift comes to $800/acre x 20 acres = $16,000!


Keep in mind it could even get worse as small spray droplets can be transported horizontally, sometimes for miles, before dropping on and causing damage to sensitive targets.


Indications of a Temperature Inversion


Those who live in mountainous areas or even areas with rolling terrain are quite familiar with temperature inversions, where cold air descends into valleys and other low-lying areas. Just driving down a road into a valley, you can feel the undeniable cool air as you descend downhill. Same with municipalities such as Los Angeles, where the pollutants, car exhaust, and industrial smoke get trapped on top of the cold layer, producing that well-known smog phenomenon.


According to the National Weather Service Glossary, an inversion exists if the temperature at 8–10 feet above the ground is higher than the temperature at 6–12 inches. If you have the capability to measure both temperatures, you will have a direct indicator if an inversion is present or not. Remember that the more significant the temperature difference between the two levels, the more intense and stable the inversion will be.


Some other indicators that an inversion is present or is in the process of forming are:


  • Calm wind
  • Close to sunrise or sunset
  • Dew present
  • Horizontal smoke patterns
  • Dust hanging over a road from a passing vehicle
  • Ground fog in low-lying areas


Any of these symptoms should alert you to the presence of an inversion, particularly the last three.



Causes of Inversions


A short reminder of how inversions develop will help in dealing with them. They are not uncommon in ag applications, and one of the first indications is when you start getting spray droplets from the previous swath on the windscreen reducing visibility, particularly when flying into the sun. Or, in the turn to the next swath, you look back and see the spray from the last swath hanging in the air.


Near sunset, the earth’s surface cools rapidly due to radiation cooling. As a direct result, the air near the surface is also cooled rapidly, resulting in cooler air being below and warmer air aloft, the opposite of the normal lapse rate.


Another way to test for an inversion is by lighting a small fire and observing the smoke. If the smoke hangs in the air and moves off slowly without dissipating, you have an inversion present, and it’s time to hangar the aircraft.


To summarize, the times when you are most likely to get an inversion are:


  • Early mornings when overnight skies were clear, and wind speeds were low.
  • Early evenings when an inversion can begin to form a few hours before sunset.
  • Nighttime when an inversion is already present and may intensify overnight.


Ag aviation is not an easy job. But dealing successfully with inversions is well within our control and one that can go a long way towards minimizing or eliminating drift issues.


If the smoke is hanging, hang up the spurs. While it is very tempting to do “just one last pass,” damaging your crop through off-site drift is one thing. Damaging others is just not worth the risk.





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