If you’re planning to spray, beware air temperature inversions on calm, cloudless days.
That was the message from Andrew Thostenson during the Canola Council of Canada’s CanoLAB at Vermilion’s Lakeland College this winter. Thostenson is an extension pesticide specialist with North Dakota University.
Avoiding pesticide damage during an inversion comes down to understanding how inversions work and recognizing the environmental conditions that cause them, Thostenson explained to the gathered agronomists and farmers.
How inversion works
To understand air temperature inversions, a person needs to know a little about thermodynamics. While “thermodynamics isn’t for everybody, this is pretty easy,” says Thostenson.
During the day, short-wave radiation from the sun warms the earth, along with any other objects it hits. At night, those objects emit long-wave, or infrared, radiation, reversing the process.
As the objects lose heat, the ground may cool the surrounding air. The warmest air will be highest, and the coldest air closest to the ground, creating an inversion.
“If there’s enough moisture, fog walls will form. And if you have a fog situation that is an inversion situation,” says Thostenson. Fog might appear to form a layer close to the ground, but it’s not really a layer, he says. It’s just the point where the air temperature is cool enough to condense moisture.
But how does an inversion affect pesticide applications?
Larger droplets will have enough mass to fall through dense, cooler air and hit their targets.
“But if you’re dealing with fine droplets — and all spray nozzles produce a certain amount of fine droplets — they start to move laterally. They become suspended in that cooler, denser air.”
Those suspended droplets are not necessarily going to stay in the field where they were applied.
“What you’ve got to understand is that cold, dense air moves like water,” says Thostenson. The cool air will collect in low spots. “And as it collects, it begins to condense the moisture out of the air.”
A light breeze will also push the suspended droplets to an unwanted place, Thostenson adds.
“The take-home message is that droplets still matter in an inversion. But it’s not 100 per cent. Use the right spray drop, with the right nozzle, and you can minimize this problem dramatically.”
On a clear, calm day, inversions will start to form in the mid- to late afternoon in the southern Prairies or in the evening in northern latitudes. The inversion will be most intense shortly after sunrise the next day.
“It’s because even though the sun is up in the horizon, it isn’t high enough to be able to start striking objects, where you’re standing,” Thostenson explains. “So the coldest part of the day, when the maximum part of the inversion occurs, is just a little after sunrise.”
Late afternoon and early evening are the most hazardous times to spray if an inversion is forming, says Thostenson, because the inversion won’t dissipate until after sunrise. In the meantime, the suspended pesticide could move miles downrange, depending on the weather.
Cloud, wind, surface conditions and humidity
Cloud cover will reflect the infrared radiation back to earth, preventing or weakening the inversion. Thostenson says the “best guess” is that 25 per cent or more cloud cover should prevent a strong inversion.
Wind plays into it, too. The wind’s “tumbling action” will mix warm and cool air, clearing up an inversion. The wind must be at least four to five m.p.h. to weaken the inversion, according to a North Dakota State University publication.
When thinking about inversions, it’s important to know that a closed crop canopy acts as the earth’s surface, says Thostenson. That surface is cooler than bare earth because plants respire and dissipate heat.
“So when you’re spraying over top of a closed crop canopy, the temperature inversion will be much more intense than if you were spraying over bare ground or ground that’s only partially covered with crop canopy,” says Thostenson.
High humidity weakens inversions because the moisture in the air acts as a heat sink, buffering the movement of temperature ranges in the atmosphere, Thostenson says.
Inversions will build faster and be more intense in low humidity situations, he adds. “The good news is that in the morning, when that sun comes up, that inversion isn’t going to last very long.”
If humidity is low, the inversion will likely dissipate an hour or two after sunrise, he says. If humidity is high, it will likely take two and a half to four hours to dissipate after sunrise.
Inversions can set in after windy days, as the weather system passes, Thostenson says. That’s a potential pitfall for people in a rush to spray after weather delays.
Thostenson says that significant inversions don’t happen every day, so there’s no need to be paranoid.
“But you need to watch the local environment and the terrain. Understand the topography, the wind breaks. All those sorts of things play into it.”
Sensing an inversion
“If you know what you’re looking for, you can tell if there’s an inversion in place or not,” says Andrew Thostenson, extension pesticide specialist with North Dakota University.
Dust hanging in the air is one sign of an inversion, he says. As well, smoke will move laterally instead of rising during an inversion.
“You can smell an inversion,” he adds. Odours are more intense because the smell stays closer to the ground. In Oklahoma, livestock producers have to check a website on certain days to make sure
there isn’t an inversion before spreading manure, he says.
People an also hear sounds from farther away during an inversion, Thostenson says. Cold air bends sound waves, projecting them farther along the earth’s surface. Anyone who’s camped near a lake during the summer has probably experienced this. The cold air above the lake bends sound waves, amplifying conversations from across the lake.