Knowing inversion conditions is the first step to avoiding pesticide misapplication.
“The bad news: we have a thermal inversion to some degree just about every day during spray season,” said Jason Deveau of Ontario’s agriculture department.
“When we have that very still air, or stagnation as they call it in the States, that’s a thermal inversion.”
Deveau said a thermal inversion reaches its climax in late evening, when farmers like to spray.
The layer of warm air just above that layer of cool air at ground level serves as a ceiling. The spray can either sit under that hot air ceiling, or it can move.
“It’s just biding its time, waiting for a slight wind or looking for a slight downhill grade to draw that slab of nastiness away,” he said.
“If you’re spraying a volatile chemical, that product will continue to off-gas and feed this layer of nastiness from below. What makes it so devastating is the fact that it’s so unpredictable.”
Deveau said the new chemistries coming to market have been engineered to make them less volatile than previous generations of pesticides.
However, he thinks mitigating circumstances, such as the water treatment that growers add to their tank, may have a negative impact and return these new chemicals to their original volatile state.
He said this creates the potential for a spray drift nightmare, and nobody will even realize what has happened because thermal inversion can carry small particles and volatile chemicals many kilometres.
Deveau said Washington State University recently released research on inversions and other means by which volatile pesticides move off-target.
A vineyard in an isolated location had long been experiencing mysterious crop damage. One of the researchers happened to be visiting the vineyard for other reasons and noticed what appeared to be glyphosate damage.
“Glyphosate damage seemed unlikely because there were no farms or other agricultural enterprises for miles around,” he said.
“They finally determined that glyphosate was caught in the dew on farms located miles away. Trace amounts evaporated with the dew and was caught up in the precipitation cycle, and rained down in a more diluted state on the distant grape crop.
“It’s something nobody likes to talk about because there’s just no way to control it. It’s the precipitation cycle of nature, and this is what happens when you introduce a new product into that cycle. It’s an unpopular subject. There may be ways to mitigate the damage, but no way to predict it.
“There’s an economic side to this also. When I go out to teach stewardship and best practices, I figure that every drop I can keep out of the environment is an extra drop working for the applicator or the grower.”
Deveau thinks people have relied too heavily on nozzle technology to solve their problems.
He said it is not about the nozzles themselves but how they are used. The best nozzle can be used incorrectly, and in the right hands the cheapest and simplest hollow cone in a 100 degree flat fan can handle a wide range of tasks.
“It’s all about understanding how droplets behave,” he said.
“That’s what growers and applicators need to think about. Physical clues will tell you whether or not it’s safe to spray. If it’s clear, quiet and dry the night before, you’re probably going to be in for a strong inversion.
“If you get up at 5 a.m. and there’s a light fog hanging over the ground, you are now in an inversion. That fog is showing you the layers. Air above the fog is warmer. It’s showing you where it’s cold enough for water to condense at a certain height.
“If you see dust hanging in the air, your spray is going to do the same thing. If the weatherman says you’re in a high pressure zone, listen to him. High barometric pressure is another good indicator.”
He said spraying in the evening and morning is still a good idea because conditions are generally good. The exception is a thermal inversion.
“(Prairie spray drift expert) Tom Wolf has a fool proof method of determining for absolute certain whether or not you’re in a thermal inversion,” he said.
“Get a 50 foot pole and mount a very sensitive thermometer on top and put it up in your yard. Have a similar thermometer at ground level at the foot of the pole. Compare the two readings.”
Deveau and Wolf recently published a new fact sheet, Thermal Inversions and Spray Drift, which can be found at www.sprayers101.com.
It is based on a more detailed 16 page publication published by North Dakota State University.
For more information, contact Deveau at Jason.Deveau@ontario.ca.