April 10, 2013
The Department of Primary Industries (DPI) is reminding landowners surface temperature inversions can pose a significant risk of spray drift. DPI Chemical Standards Officer Neil Harrison said fine droplets of pesticides can concentrate at relatively high levels under an inversion layer and stay suspended for several hours in the atmosphere, not being influenced by gravity. “Inversions can slowly follow the topography of the landscape down slope due to the cold air drainage effect creating a risk that any suspended particles will be carried off target,” he said. “They can settle out potentially kilometres from the target and may impact upon susceptible non target species causing significant damage. “An inversion can be formed when the overnight cooling effect of the earth’s surface causes the air immediately adjacent to the ground to also cool and subsequently the air is cooler near the ground and then warms up as height increases. “The Australian Pesticides and Veterinary Medicines Authority suggests the potential for inversions to occur and hold high concentrations of airborne pesticides near the surface, should always be anticipated between sunset and up to two hours after sunrise. “That is unless there is continuous overcast, low and heavy cloud, continuous rain or the wind speed remains above 11km/h for the whole period between sunset and sunrise. “The occurrence of any of these three conditions doesn’t wholly exclude surface inversion existence but they indicate conditions not normally conducive to the drift of high concentrations of airborne pesticides,” he said. Mr Harrison said a typical indicator of an inversion layer was the absence of wind or very slight winds. He said almost all chemical sprayers recognise the spray drift risks associated with spraying during strong winds, however fewer consider the risks associated in conditions with no wind or very slight winds. When an inversion layer is not present, a minimum wind speed of more than three km/hr is recommended as a suitable time for spraying. The turbulence of wind over the surface of the ground can assist in transporting spray droplets or combing the droplets to the ground.