That’s another fine mess you’ve gotten us into!

A reading of Graeme Tepper’s extremely valuable GRDC pamphlet ‘Weather Essentials for Pesticide Application’ reveals that fine and wind – strong, weak, upwards, sideways, or non-existent – really should not be mixed, says FMC’s northern region technical extension specialist, Dr David Johnson.

“Eliminating them, or at least reducing them as much as possible, is why On Coarse DRA, the ‘fines terminator’ from FMC, was developed in the first place,” he said.

Inspired by Graeme Tepper’s pamphlet, a few ‘fine facts’ and spraying tips are highlighted below.

“Firstly, a fine is a fine is a fine. No matter what is in the tank mix, a fine droplet will act the same way, no matter what is in it. A fine is simply a spray droplet that has a diameter of 150 microns or less.

“Secondly, all fines are special droplets because they will not fall to ground under their own weight. They will do what the wind they encounter tells them to do.

“Thirdly, all spray qualities produce at least some percentage of fines. For Ultra Coarse it is less than 1%. For eXtremely Coarse 1–3%; Very Coarse 3–6%, Coarse 6–10%; Medium 10–24%; and fine 24–60%.”

Fine tips and facts:

“You should always spray with as few fines as possible no matter what the time of day, the weather conditions or what the tank mix contains,” Dr Johnson said. “General rule of thumb – if the product still works with bowling ball size droplets, then use bowling ball size droplets.

“Fines will hang in the air to either get blown around by the wind sideways, carried upwards by thermals or just hang still in an inversion.

“Fast application speeds create turbulence behind the spray boom and rig. The travel speed can be enough to ‘detrain’ fine droplets from the spray plume which takes them from their initial downward trajectory and puts them into the turbulence created by the boom which projects them upwards and their potential to drift is magnified greatly.

“For every 20cm higher than 50cm in boom height, the drift potential increases fourfold. Therefore, a 1m boom height creates TEN times the drift risk of a 50cm boom height. The outside nozzles of a long boom travelling fast across rough country can easily bounce to 2m above the target.

“Creating fines, even in the daytime, should be avoided because their direction cannot be controlled but at least vertical movement of air will dilute them. But they can still cause problems by being deposited a long distance from the point of application when the thermal collapses.

“Fines have much greater surface area and therefore will get even finer, very fast. Their high surface area (relative to volume) promotes evaporation of what they are carrying.

“Creating fines in an inversion means they will hang in the air where they were created and not be diluted vertically by thermals and then they can concentrate together as drainage winds gather them all together in low points of the landscape.

“The drainage winds can drag them for kilometres from their original point.

“Fines can rise as the inversion base rises while the ceiling doesn’t change following sunrise when lateral winds can then transport them sideways only for them to later descend far away from the original point.

“Fines can travel upslope in ‘Anabatic’ winds (warm winds created when upslope hill-faces warm in the morning sun before the flatter areas beneath them).

“If the tank mix must be sprayed with more fines, then spraying should be done as slowly as possible, with the lowest possible boom height and in ideal spraying conditions.”


“Here’s a couple of great illustrative images from Graeme’s pamphlet. The first image shows what community drift can look like in an inversion. That is not a fog, but simply the accumulation of smoke from all the houses in this cold town in NZ.

“The inversion has trapped the air and the individual contributions of herbicide/smoke from each boom sprayer/chimney is concentrated over the crops/town with no vertical mixing and dilution occurring.

“The second image shows what an inversion layer can do when laminar flow has set up under the inversion. The smoke travels a very long way laterally even though the sun is up, and it is a fine day when one might not expect an inversion to be in place.

The behaviour of wind is very complicated, and so spraying is a tricky business. The best way to avoid the wind’s effects is to not produce the kind of droplets – fines – that will be subject to all its vagaries,” Dr Johnson said.

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