The goals of applying any crop protection products include increasing effectiveness, mitigating drift, and maximizing profits. We will focus on mitigating drift, even though all three interact with each other. Mitigating (decreasing) drift will increase spray effectiveness and in turn maximize profits.
Why should we be interested in drift?
Drift may cause spotty pest control, wasted chemicals, off-target damage to high value specialty crops, higher production costs, and negatively affect the environment (water and air quality). Drift also increases the occurrences of problems arising with neighbors and the public’s negative perceptions of pesticides.
So what is drift?
Drift is movement of spray particles and vapors off-target causing less effective control and possible injury to susceptible vegetation, wildlife and people. Vapor drift is associated with volatilization (gas, fumes). Particle drift is movement of spray particles during or after the spray application.
Factors affecting drift
Factors affecting drift are the spray characteristics of the actual chemical, chemical formulation, droplet size, and evaporation. Application equipment such as nozzle type, nozzle size, nozzle pressure, height of release chosen by the applicator, and sprayer calibration can impact drift. Weather factors affecting drift include air movement (wind direction and speed), temperature and humidity, air stability/inversions and topography.
Wind direction is very important. Applicators should know the location of sensitive crop areas and consider safe buffer zones. Drift potential is lowest at wind speeds between 3 and 10 mph (gentle but steady breeze) blowing in a safe direction.
“Dead calm” conditions are not recommended, because drift potential may be high. This is because light winds (0-2 mph) tend to be unpredictable and variable in direction. Calm and low wind conditions may indicate presence of a temperature inversion.
Wind speeds may be different when moving from within the crop canopy to above the crop canopy. Wind speed and direction can drastically affect spray droplet displacement, as structures can affect the wind currents around windbreaks, tree lines, houses, barns, hills and valleys.
Under normal weather, air tends to rise and mix with air above. Droplets will disperse and will usually not cause problems. Temperature inversions are caused when the temperature increases as you move upward in the atmosphere. This prevents air near the surface from mixing with the air above it. Therefore, inversions cause small-suspended droplets to form a concentrated cloud, which can move in unpredictable directions.
Temperature inversions often occur under clear to partly cloudy skies and light winds during the overnight hours; a surface inversion can form as the sun sets. Under these conditions, a surface inversion will continue into the morning until the sun begins to heat the ground. Be careful near sunset and an hour or so after sunrise, unless there is low heavy cloud cover, if the wind speed is greater the 5-6 mph at ground level or there is a 5-degree temperature rise after sun-up.
Originally posted by South Dakota State University Extension.