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Weather-Wise Application

Weather-wise application can reduce pesticide hazard to the environment. A good applicator carefully checks the weather conditions before beginning spray procedures. Not only do a few simple precautions protect the environment, but in terms of dollars and cents they aid the applicator. Pesticides which do not reach or remain on the target areas are wasted. More pesticide, time, and money must be used to control the pests in the target area.

Goals of This Chapter

Learn the role that weather conditions can play in both helping and hindering the applicator.
Understand the hazards of windy day application and who is legally responsible for mistakes.
Learn the advantages of early morning or evening application.
Understand the roles of humidity and temperature inversion in regard to pesticide application.

Avoid High Temperature and Low Humidity Conditions

Temperature and humidity affect pesticide drift. High temperature and low humidity increase the rate of evaporation of the pesticide. Small droplets that completely evaporate leave pesticide particles in the air that may be carried several miles away from the treatment area (vapor drift).

Avoid Windy Days

High winds increase drift and result in the loss of pesticide from treated areas. Drifting pesticides increase the possibility of injury to wildlife, pollinators, and domestic animals. They may settle on forage, pasture or wildlife areas or contaminate water. Pesticide application on quiet days reduces the inhalation and contact hazard to the applicator and the bystander. Drift onto sensitive crop areas can also be avoided in this way. The applicator is legally responsible for any injury or money loss due to pesticide drift onto non-target areas. Don't take a chance by spraying in the wind.

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Other Wind and Temperature Considerations

The drift of pesticides is also affected by the air turbulence. The air turbulence is determined by the difference between the temperature at ground level and the temperature of the air above it. Normal weather conditions cause heating of the soil. If the air just above the soil is warmer than the air aloft, upward air currents begin. The larger the temperature difference between air above the soil surface and the air aloft, the stronger the air currents. These air currents could carry spray droplets and pesticide particles a considerable distance away from the treatment area. Do not apply pesticides when such turbulent conditions exist.

An inversion occurs when the air near the soil surface is cooler than the air above it. The warm air forms a cap that blocks upward air movement that would otherwise help disperse the chemicals. Wind can aid in air mixing and reduce inversion conditions. However, low wind conditions during inversion conditions may cause small spray drops to remain suspended in the air. The droplets will eventually move out of the treatment area as a concentrated cloud. Smoke can be used as a good indicator of an inversion condition. Do not apply pesticides when inversion conditions exist.

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Avoid Application Just Before Rains

Spray applications should not be made just before a rain, because the pesticide washes off and the pests are not controlled. Rains cause runoff and tend to wash the pesticide away from the target areas. The runoff can carry the pesticide into sensitive areas where crops or wildlife could be injured. Runoff can also reach surface waters such as farm ponds, streams and waterways. Ultimately, this can cause contamination, fish kills, and injury to domestic animals.

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Consider Early Morning or Evening Application

Wind speed is usually lowest and humidity higher at these times of the day, thus drift hazard is greatly reduced. Children and domestic animals are less likely to be in sprayed areas during these hours. Avoiding full daylight hours lowers the contact danger to wildlife such as birds, mammals, and pollinators, who often visit crop lands during the day.

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