Temperature inversions and spraying

FARGO, N.D. - A new report from North Dakota State University describes in detail how temperature inversions work and how they can cause pesticide spray drift and damage.

John Enz, professor emeritus in the NDSU soils science department, Vern Hofman, professor emeritus in Agricultural and Biosystems Engineering, and Andrew Thostenson, extension pesticide specialist, co-authored the report, released April 30.

Pesticide manufacturers recently have included label warnings on their products that caution applicators not to use them at all during temperature inversions. State laws require applying chemicals according to labels, Thostenson says. Labels often have maximum wind speeds above 10 mph, but the drift risk is actually more intense with the temperature inversion, Thostenson says.

“I don’t think anyone recognizes how common these temperature inversions are,” Enz says. Thostenson says he’d like to see research on their frequency or intensity, especially during spraying season.

What’s an inversion?

The report says an inversion occurs when temperatures near or at the soil surface are lower than temperatures above the ground. The difference can be gradual. An inversion plus low wind speed is the likeliest situation for long-distance damage from spray drift.

“The drift can go a long ways,” Enz says. “There is documentation it can go miles because the relative humidity is high in an inversion, so the falling droplets don’t evaporate. I’ve seen court cases where they can go two miles and cross roads and ditches.”

If road or field dust rises slowly or hangs in the air near the surface, dissipating slowly, it could be a sign of a temperature inversion taking place, the report says. Other indications can be strong odors or sounds (conversations, or traffic for example) that normally can’t be sensed, because the source seems too far away.

Ground fog always confirms that an inversion exists, but the absence of ground fog doesn’t mean no inversion exists.

“All it means is the air temperature hasn’t cooled to the dew point temperature,” Enz says.

The inversion can only be confirmed by measuring the temperature at two heights - 8 to 10 inches above the crop canopy, and 8 to 10 feet above the ground. The greater the difference in temperature, the more intense the inversion.

Another indicator is when there is dew on the upper exposed leaves of a crop canopy.

To confirm an inversion, it’s best to measure air temperature in an area where the soil and crop conditions are the same as the area to be sprayed. Avoid packed soil near field entrances. Remember that thermometers need time to adjust after being stored in a cold or warm vehicle or in a pocket. The instrument’s temperature sensor must be shaded to be accurate. If two instruments are used and compared, they should be calibrated with each other.

Spray drift risk

Inversions are most likely when evening skies are clear and wind speeds low. Spraying in the early morning shortly after sunrise can cause drift problems, as that is when inversions are deepest and most intense. Inversions tend to form more rapidly over mulch or porous surfaces. Inversions over closed-canopy crops will form sooner in the evening and probably will be more intense than with bare-surface soils.

Evening inversions tend to pose greater risks for spray drift than morning inversions. They can start three to five hours before sunset and intensify until shortly after sunrise. Windy or cloudy conditions will weaken or disrupt an inversion. In protected areas, inversions persist for several hours, Thostenson says.

“The longer you wait after sunrise to spray on a clear day, the less likely there is to be an inversion,” Enz says. “It takes one to two hours or more for an inversion to be gone a 10- to 12-foot level above the ground.”

To avoid drift, farmers should use spray nozzles that produce the coarsest drop size recommended for a pesticide. Nozzle manufacturers provide drop-size distribution recommendations at typical operating pressures for either ground or aerial application. Most pesticides provide recommendations for drop size for a particular pest target. It’s important to realize nozzles produce at least some drops smaller than 200 microns, which are highly driftable, Thostenson says.

Thostenson says many pesticide labels now suggest a wind speed of 3 to 10 mph, with the most ideal at 4 to 5 mph.