Spray drift can occur either as spray droplets or vapors—as happens when a volatile liquid changes to a gas. Factors affecting drift include weather, application method, equipment settings, and spray formulation. Weather-related drift increases with temperature, wind velocity, and convection air currents, as well as during temperature inversions.
Wind-related drift can be minimized by spraying during early morning or in the evening when wind velocity is often lower. However, even a light wind can cause considerable drift. Pesticide labels will occasionally provide specific guidelines on acceptable wind velocities for spraying a particular product.
Midday spraying is also less desirable because as the ground warms, rising air can lift the spray particles in vertical convection currents. These droplets may remain aloft for some time, and can travel many miles. Similarly, during temperature inversions, spray droplets become trapped in a cool lower air mass and move laterally above the ground. Inversions often occur when cool night temperatures follow high day temperatures, and are usually worse in early morning before the ground warms.
Finally, low humidity and high temperatures also promote drift through the evaporation of spray droplets and the corresponding reduction of particle size.
Optimal spray conditions for reducing drift occur when the air is slightly unstable with a minimal steady wind.
Spray application methods and equipment settings also strongly influence the potential for drift. Since small droplets are most likely to drift long distances, aerial applications and mist blowers should be avoided whenever possible. Standard boom sprayers should be operated at the lowest effective pressure and with the nozzles set as low as possible. For example, drop nozzles can be used to deliver insecticides below the tassels if bees are seen foraging on sweet corn pollen.
Nozzle type also has a great influence on the amount of drift a sprayer produces. Turbo jet, raindrop, and air-induction nozzles produce substantially less drift than conventional nozzles. Standard flat fan or hollow cone nozzles are generally poor choices. Select nozzles capable of operating at low pressures (15 to 30 psi) to produce larger, heavier droplets.
Finally, oil-based chemical carriers produce smaller, lighter, droplets than water carriers and should also be avoided when possible. Consider using thickening agents if they are compatible with your pesticide.