What is a temperature inversion, and how does it affect drift? There are many misconceptions and erroneous opinions. Without an inversion, or under normal conditions, the temperature is highest near the ground and decreases as the elevation or altitude increases. With a temperature inversion, a warm layer of air is formed at some height above cooler temperatures, thus an inversion layer. This may occur at different altitudes from near the ground to 300 feet above, or maybe more.
Under normal conditions, without a temperature inversion, convection currents disperse the smaller spray particles into the upper atmosphere. If they ever come down, they are so diluted that they have no effect. Under inversion conditions, these smaller droplets are held under the inversion layer and move greater distances than would otherwise occur. In either case, larger droplets fall faster and are deposited closer to the spray release.
Smaller droplets travel farther creating a drift pattern. A temperature inversion’s height has an effect on the drift problem. The spray material does not rise up, but instead moves over and comes down again. It keeps drifting with the wind or air movement until dissipated. Droplets do not go through the inversion layer. With or without an inversion, larger droplets fall first and smaller droplets travel further, according to the laws of physics. We can’t change the laws of physics, but need to understand and work with them. A temperature inversion simply extends the drift pattern.
Please resurrect and use the research done by or supported by the National Agricultural Aviation Association a few years back. One of the things this study showed was that, if a material drifted over a certain distance, a temperature inversion existed at the time and the height of spray release made no difference.
A misconception is there are no temperature inversions with wind. Temperature inversions can occur at velocities much greater than when we would normally spray. Once, when I was investigating a claim, the pilot told me, “I know temperature inversions and I did not have one. I had a steady wind of five miles per hour, no inversion”. He was wrong.
I worked for the State Plant Board of Mississippi from 1955 until about 1965 and was stationed in the Mississippi Delta. Back then, rice growers sprayed amine 2,4-D on rice fields with sprayers mounted on tractors. One of my duties was to trace 2,4-D drift from rice to other crops, primarily cotton. My duty was to find out where a drift came from and where it went. This is where I learned the trade of tracing drift and the effects on non-target crops. The State Plant Board had an office in the Delta Branch Experiment Station office building and I had access to the experts there.
As aerial application came into use, I would look for tractor tracks in rice fields to determine if the application was made by ground or air.
At the time, a pilot could buy a used J-3 Cub for about $500.00, attach a wind driven spray pump to the landing gear and booms to the wings, place a 55-gallon drum in the back seat and he was in the spray business. Flying at 60 miles per hour with the nozzles pointed straight back, this made a great 2,4-D airplane.
There was one occasion when I traced 2,4-D drift for as far as six miles, then lost the pattern in another drift pattern coming from a different rice field. There were tractor tracks in the rice fields, so 2,4-D was not applied by aircraft.
Spray materials will drift from ground applications as well as air. A couple of years ago I investigated a case where glyphosate was applied to Roundup-ready cotton by ground with a hooded sprayer. The drift killed milo for nearly a mile, then damaged rice for another 1.5 miles.
Another misconception about temperature inversions is wind direction, which can vary greatly within short distances. Pilots should not rely on wind conditions at the home base when spraying three or four miles away. Once I was traveling west from Indianola, Mississippi on US Highway 82. There was smoke from a cotton gin blowing directly north across the highway. A little further west, smoke from another cotton gin was coming straight south across the highway. A little further west there was a large log pile burning, with the smoke going directly west. All of this was within a three to four-mile radius. Before we had stringent EPA regulations, operators in the Mississippi Delta would burn a used rubber tire at the application site. The pilot could observe the wind direction continuously during the application. However, winds can change drastically during an application. Also, a very hot fire may burn through a low temperature inversion layer making the smoke column inaccurate.
Once droplets leave the nozzle, the applicator has absolutely no control of where they go. Their destination is completely dependent on the laws of physics and gravity. That is why it is imperative to observe all environmental conditions and use this information.
I witnessed firsthand a good example of the effect of a temperature inversion when I worked for the Delta Branch Experiment Station. The station owned an ag aircraft and hired a pilot for it. This was before GPS, so the application had to be flagged manually.
One morning our flag man did not show up and I had to flag the airplane. As the aircraft was headed toward me, I noticed the spray from it was not reaching the cotton. It would come to about three feet above the cotton, then rise up. A temperature inversion had formed three feet above the cotton and would not let the application reach the cotton. I flagged off the pilot and we waited a couple of hours before going back to work. Then, everything worked well.
Above all else, use common sense. If you think the job is risky for a temperature inversion at the time, put it off, or let your competitor get in trouble.
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