Recent rains over much of the state are encouraging crop and weed growth and pesticide applications will be a primary focus as soon as fields are suitable. In a May 15, 2009 ICM news article, management factors to reduce drift potential were reviewed, with special focus on the effect of droplet size and wind speed on physical spray drift. As daily high temperatures increase into the 70-80° F range, mid-day relative humidity frequently drops below 50%. This article takes a look at the effect of temperature and humidity on pesticide drift.

Temperature and humidity affect the evaporation of liquid droplets. The higher the temperature and lower the humidity, the faster evaporation occurs. As a result of evaporation, spray droplet size decreases. The smaller droplets are then more susceptible to moving off target with prevailing winds as they become entrained in ambient air currents.

The adverse effects of high temperature, low relative humidity and increasing wind speeds on the distance liquid spray solutions drift are illustrated here. Values in the figure are an indication of what might be expected when using a number four tip at 40 psi with a sensitive target 30 ft. downwind. Such conditions can be common for a mid-season insecticide or fungicide application using a medium spray droplet quality.

At 50° temperatures with 80% relative humidity and an average wind speed of 10 mph, about 1% of spray volume will drift 30 ft. away from the nozzle. If the temperature increased to 86° with a lower relative humidity of 40%, and the wind speed remains unchanged, twice as much material – about 2% – will drift 30 ft. away from the nozzle. With average wind speeds increased to 15 mph under the 86° temperatures, 40% relative humidity conditions, the amount of spray drift moving 30 ft. away from the nozzle is nearly quadrupled to 3.5%.

In addition to making nozzle size and type, boom height, sprayer pressure and speed adjustments, drift-retardant additives may also be considered. The addition of deposition aids or drift-reducing adjuvants to a spray application can help in some situations to reduce off site movement and help to deposit and retain more of the pesticide product on target. There are many drift reducing products available. Some are formulated with AMS and other surfactants. Careful considerations of the pesticide products being applied, the label requirements in terms of additives, the composition and qualities of the drift retardant product and the type of nozzles and sprayer setup are important before selecting and using a drift retardant.