Fig.11. Buffer strips can reduce drift famages to sensitive crops.
Weather can critically influence the off-target movement of pesticides. Several factors associated with the microclimate at the application site can contribute to drift. These factors include:
The impact that various weather factors have on drift can be related to the volume of droplets 150 microns or less in diameter. If these small droplets are eliminated, weather's effect on drift is reduced considerably.
You cannot control the weather, but you can control its effect on drift. Here is a brief discussion on how weather-related factors influence drift and how you can minimize their effects
Factors such as droplet size and downward velocity, air turbulence and spray boom height affect the distance a droplet travels before depositing on an object. However, wind velocity is usually the most critical factor of all meteorological conditions affecting drift. The greater the wind speed, the farther off-target a droplet of a given size will be carried. The larger the droplet, the less it is affected by the wind and the faster it falls. High winds, however, can cause even larger droplets to move off-target. Therefore, spraying operations should be stopped if wind speeds are excessively high.
Wind direction is as important as wind velocity in reducing the damage caused by drift. The presence of sensitive vegetation near the spray site, particularly downwind, is one of the first things that should be evaluated, but is often overlooked when beginning a spray application. Check in all directions from a spray site for sensitive vegetation, and be alert to changes in wind direction during application. If there are sensitive crops downwind, leave a buffer strip of at least 100 feet or whatever the label recommends (Fig. 11). Spray the buffer strip later, when the wind has shifted away from sensitive crops.
Relative humidity and temperature go hand-in-hand in affecting spray drift. While they generally are not as critical as wind velocity, they are a strong influence in some geographic regions or under certain meteorological conditions. As a particle falls through the air, surface molecules of water evaporate into the atmosphere. This evaporation reduces the size and mass of the particle, enabling it to remain airborne longer and, under the right conditions, to drift farther from the application site (Fig.12). The rate at which water evaporates from the spray particles depends primarily on the ambient air temperature and relative humidity.
It has been shown that a 100-micron droplet requires a little over 5 seconds to free-fall 5 feet if no evaporation occurs. However, in relatively dry, warm air (e.g., 30 percent relative humidity and 78°F), the same 100-micron droplet quickly loses water by evaporation and becomes less than half its original diameter (one-eighth of its original volume) while falling only 2.5 feet. By comparison, in relatively moist air (e.g., 70 percent relative humidity and 78°F), a 100-micron particle will fall 5 feet and hit the ground before evaporating to half its original diameter. While evaporative loss of spray materials occurs under almost all atmospheric conditions, these losses are less pronounced under the environmental conditions that occur in the cooler parts of the day - early morning and late afternoon. The relative humidity is usually highest during these cool periods.
Temperature influences drift in other ways beyond its effect on evaporative loss of droplets. Pesticide volatility increases with increasing temperature. Temperature also influences atmospheric air turbulence, stability and inversions - all of which, in turn, affect drift.
Atmospheric stability is an important factor influencing drift. Under normal meteorological conditions, the air temperature decreases by 5.4°F per 1,000 feet of height. Cool air tends to sink, displacing lower warm air and causing vertical mixing. As a warm air layer rises, suspended droplets rise with it and dissipate into the upper layers by normal air turbulence and vertical mixing. Under these conditions, the opportunity for crop injury at any specific off target site is very small because the pesticide is dispersed and diluted into the atmosphere.
However, other problems may arise when the atmosphere is very stable. Under stable conditions, a warm air layer at some distance overhead may become a blanket, holding down cooler air underneath. This phenomenon is usually referred to as atmospheric inversion (Fig. 13). Particles suspended in the cool layer cannot move anywhere except laterally, possibly for several miles. Eventually, the suspended cloud may encounter a downdraft, forcing it back to earth and depositing it of-target - possibly over a sensitive crop (Fig. 14).
Inversions are part of a daily atmospheric cycle, occurring in the early morning hours when the ground cools the air layer immediately above it. Inversions tend to dissipate during the middle of the day when wind currents mix the air layers. Applicators concerned about these adverse spray conditions should wait until late afternoon or early evening to spray, when there is less chance of the atmosphere being inverted and conditions are more favorable.
Again, the best way to avoid drift associated with atmospheric inversions is to eliminate the formation of small particles (150 microns or smaller) from the spray effluent. If these particles are not in the spray emission, this weather-related phenomenon can be ignored.