How Drift Happens

Drift is the movement of pesticide from the target field onto nearby sensitive plants. Most drift consists of spray droplets moving downwind during an application. This type of drift (referred to as droplet, physical, primary, or particle drift) can be minimized by following the label recommendations for spray equipment, procedures, and weather conditions.

In addition to wind, temperature inversions have a critical effect on drift. Temperature inversions occur commonly during Midwestern summers (Bish 2019), typically forming in the late afternoon or evening and persisting through 1-2 hours after sunrise. Temperature inversions occur most often under light or calm winds, due to a lack of vertical air movement (See Figure 1 below). Since inversions are marked by very little wind, it may seem like a good time to spray. It is not. The lack of vertical air movement means spray droplets can become suspended near the ground surface in colder, denser air. Air can move horizontally, like a fog, onto neighboring fields, carrying these spray droplets along. Morning sunlight or stronger winds usually mix the air layers and bring an end to inversion conditions. But applications made during an inversion late in the day may have several hours to move off target.  

Dicamba and 2,4-D can also move off-target as a gas. They are among the relatively small number of pesticides that are highly prone to volatilize (turn into a gas). A spike in air temperature can cause these herbicides to turn into a gas even after they have been successfully applied to target surfaces. As temperature inversions form, pesticide vapors lingering in the atmosphere can be pulled back down to the surface where they can cause damage.

The applicator is legally responsible for checking both current and predicted weather conditions, following all label restrictions, and knowing where neighboring sensitive crops are located. 

Off-target plant damage may also result from inadequate cleaning of sprayer equipment, contaminated runoff water, and sometimes from contaminated grass clippings or compost.

Normal Conditions vs. Temperature Inversion

Figure 1. In normal daytime conditions (left), warmer air rises from the earth’s surface, while cooler air from higher in the atmosphere sinks, causing a vertical mixing of air. In the absence of substantial horizontal winds, herbicide droplets will hit their target on the ground or vaporize and dissipate into upper air levels. 
But during a temperature inversion (right), the air temperature close to the ground is cooler than the air column above it and vertical mixing of air does not occur. In these conditions, small pesticide droplets stay suspended in the denser cold air, settling close to the surface. But the air can still move horizontally, moving these droplets onto neighboring crops.


This publication is a product of the North Central IPM Center working group on Herbicide-Drift Risk Management, with support from the USDA National Institute of Food and Agriculture through agreement 2018-70006-28884. 

Reference to any commercial products or trade names implies no discrimination or endorsement by the North Central IPM Center or any of the contributing authors or their universities. Nor does this document constitute legal advice. Always seek legal advice from a professional who is knowledgeable in current agricultural law in your state.

Cassandra Brown, Horticulture and Crop Science, The Ohio State University
Stephen Meyers, Horticulture & Landscape Architecture, Purdue University
Douglas J. Doohan, Horticulture and Crop Science, The Ohio State University
Mary Ann Rose, Pesticide Safety Education Program, The Ohio State University

The following individuals reviewed part or all of this fact sheet: Bill Johnson, Purdue University; Bryan Young, Purdue University; Aaron B. Wilson, Byrd Polar and Climate Research Center; Cathy Herms, The Ohio State University


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