The goals of applying any crop protection product includes increasing effectiveness, mitigating drift and maximizing profits. “Of these three, if an applicator’s primary focus is mitigating drift, they can increase spray effectiveness as well as maximize profits,” said Gared Shaffer, SDSU Extension Weeds Field Specialist.

“Drift may cause spotty pest control, it wastes chemicals and can cause damage to high value specialty crops, as well as increase higher production costs and negatively impact the environment (water and air quality),” Shaffer said. He added that drift can also create negative neighbor relations as well as create a negative perception of pesticides among the general public.

Drift is off target movement of spray particles and vapors, 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,” Shaffer said.

Factors affecting drift include the following:

  1. Spray characteristics of the actual chemical – chemical formulation, droplet size and evaporation.
  2. Application equipment – nozzle type, nozzle size, nozzle pressure and height of release chosen by the applicator and sprayer calibration.
  3. Weather factors – air movement (wind direction and speed), temperature and humidity, air stability/inversions and topography.
  4. Wind direction – 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 miles per hour (gentle but steady breeze) blowing in a safe direction,” Shaffer said.

He added that “dead calm” conditions are not recommended, because drift potential may be high. “This is because light winds (0-2 miles per hour) tend to be unpredictable and variable in direction,” he said. “Calm and low wind conditions may indicate presence of a temperature inversion.”

5. Wind speed – speed may vary 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,” Shaffer said.

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,” said Laura Edwards, SDSU Extension State Climatologist.

Edwards explained that temperature inversions 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,” she said.

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,” she said. “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 miles per hour at ground level or there is a 5-degree temperature rise after sun-up.”

Gared Shaffer is a SDSU Extension Weeds Field Specialist.