Extension Ag Update
September/October 2004
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Weather or Not To Spray

Scott Bretthauer, Extension Specialist, Dept of Ag. Engineering

Of all the factors that can influence a pesticide application, one of the most important is entirely outside your realm of control: the weather. When it comes to weather and spraying, the only decision you have is if you can make an application safely and effectively. It is critical to make sure that you avoid spraying during conditions that increase the risk of drift. Understanding how weather affects spraying and how you can monitor the weather will improve your decision making process. There are two kinds of drift. Particle drift is the movement of the liquid spray particles, or droplets. Vapor drift is the movement of vapors, after evaporation (or volatilization) has occurred. Weather conditions strongly influence both types of drift, but in somewhat different manners. Remember to always consult the pesticide label for specific instructions related to weather conditions. There are three main components of weather that directly affect spraying: temperature, humidity, and wind.

Temperature primarily affects the evaporation of a pesticide. The warmer it is, the quicker evaporation occurs and the greater the risk of vapor drift. Humidity also affects the evaporation of a pesticide formulation. Humidity refers to how much water vapor is in the air. Low humidity means there is a low level of water vapor in the air, indicating that conditions are dry. Low humidity increases evaporation, thus increasing the risk of vapor drift. Some pesticides, particularly ester or oil based formulations, are especially volatile, and care must be taken to avoid spraying with these formulations on hot, dry days.

Wind has a strong influence on the risk of particle drift. High wind speeds carry spray particles longer distances, increasing the chance of off-target damage. It is generally recommended not to spray if wind speed is greater than 10 miles per hour (mph). A 100 micron droplet (see the January 2004 issue of Illinois Pesticide Review, available at http://www.pesticidesafety.uiuc.edu/, for a review of droplet measurement) can drift 3 feet in a 2 mph wind when released 18 inches above the ground. In a 10 mph wind, that 100 micron droplet can travel over 17 feet before reaching the ground. Wind speed increases as altitude increases, so the wind is usually blowing faster several feet above the ground. This is why releasing the spray as close to the target as possible is important, but remember to maintain proper nozzle overlap for spray uniformity. Wind direction is also important. Since drift occurs down wind, it is important to know which direction the wind is blowing, and where areas that might be sensitive to the pesticide you are applying are located. Do not spray if the wind is blowing towards a sensitive area. Remember that even nozzles designed to reduce the number of small spray droplets, such air-induction nozzles, still create some small droplets which can drift.

Very calm conditions can also cause problems. Under calm conditions, when the air is very still with little or no wind, small spray droplets can remain suspended in the air in a concentrated group. These droplets can then be blown off-target later in the day when the wind speed increases, or evaporate while suspended and lead to vapor drift. Also problematic are shifting winds, when wind speed, direction, or both are changing during the application. The ideal time to spray is usually when the wind is blowing steadily between 3 and 10 mph and in a safe direction, away from sensitive areas.

Inversions are particular instances of very calm weather. Inversions occur when the temperature profile of the air becomes inverted, or turned upside down. In a normal temperature profile, the warmest air is at the earth’s surface and the air temperature decreases as altitude increases. Since warm air rises, the air at ground level rises and is replaced by cooler air, which in turn is warmed and rises. This creates a continuous vertical mixing of air, which disperses small spray droplets and prevents them from remaining suspended in the air and being blown off-target in a concentrated mass later during the day. During an inversion, the air at ground level is cooler than the air above it. Since it is warm air that rises, the cool air is essentially trapped below the layer of warm air above it. This prevents vertical air mixing, creating very calm conditions and allowing small spray droplets to remain suspended in the cooler air at ground level, like fog early in the morning. Inversions typically occur early in the morning or late in the evening. Spraying should be avoided when an inversion exists.

For small spray droplets, temperature, humidity, and wind speed can all interact to create situations involving both particle and vapor drift. Spray droplets begin to evaporate immediately after being released, and small droplets can be blown off target as particle drift but evaporate completely downwind while still airborne. For instance, when droplets with a diameter of 50 microns are released from a height of 18 inches in a 10 mph wind, they can drift over 50 feet before finally evaporating, never reaching the ground. In high temperatures and low humidity, the distance these particles travel is less because the droplets evaporate quicker. Lower temperatures and high humidity decrease the evaporation rate and allow the particles to drift greater distances. Since neither situation is ideal, it is important to reduce the amount of small droplets created during an application.

How do you determine if weather conditions are suitable for spraying? For temperature, any accurate thermometer will work fine. Measuring wind speed is critical not only for determining if an application should be made, but also for maintaining accurate records of the application. Wind speed can be easily and accurately measured with a handheld wind meter. Wind meters are available that measure a variety of additional weather factors, such as temperature, humidity, heat index, and dew point. More expensive models store data for later recording. A benefit of wind meters is their small size and portability, which allows you to check and record wind speed and other weather factors throughout an application. Use a compass to precisely determine and record wind direction. Complete weather stations are also available, but at a higher cost. For more information about the various types of products available for measuring weather conditions, visit http://www.ambientweather.com. To determine if an inversion exists, you can use a smoke bomb or other similar device to create a cloud of smoke. If the smoke hovers close to the ground and fails to disperse, there is an inversion and you should not spray.

However you choose to measure wind speed and other weather factors, remember that monitoring and recording them periodically throughout an application is important. Weather conditions can change during an application, requiring spraying operations to be halted. A written record of wind speed, direction, and other factors throughout an application, not just at the beginning, can be beneficial during a drift complaint. It also demonstrates your professionalism and dedication to making safe pesticide applications.