Understanding Temperature Inversions: Protecting Against Pesticide Drift During Aerial Drone Spray Applications

Introduction

Spray drift is the movement of pesticide droplets or vapors away from the intended target during or after application (Fig. 1). Drift can reduce the effectiveness of pest control while increasing the risk of injury to neighboring crops, sensitive vegetation, water resources, wildlife, and human health. Although spray drift cannot be eliminated completely, it can be minimized by understanding the factors that influence droplet movement and by following proper application practices (Al Heidary et al., 2014).

Several operational and environmental factors affect spray drift, including droplet size, wind speed and direction, application height, nozzle selection, spray pressure, temperature, relative humidity, and atmospheric stability. These factors influence how long spray droplets remain airborne and how far they travel before reaching the target or settling outside the treated area. Among them, temperature inversion is often overlooked but can greatly increase the risk of long-distance spray drift. During a temperature inversion, cool, stable air becomes trapped beneath a layer of warmer air, suppressing vertical air mixing. As a result, fine spray droplets or pesticide vapors can remain suspended and move horizontally over long distances before settling (Elwakil, 2024).

What Is a Temperature Inversion and Why Does It Increase Spray Drift?

Under normal atmospheric conditions, air temperature decreases with increasing altitude, allowing warm air near the ground to rise and mix with the cooler air above. This vertical air movement helps disperse small spray droplets and reduces the likelihood of long-distance spray drift.

During a surface temperature inversion, however, this normal pattern is reversed. Air near the ground becomes cooler and denser than the warmer air above it, creating a stable atmospheric layer that suppresses vertical air movement. Because atmospheric turbulence is minimal, air within the inversion layer moves primarily horizontally rather than vertically (Nowatzki, 2022).

These conditions create a major risk for off-target pesticide movement. Instead of rising and dispersing into the atmosphere, fine spray droplets become trapped within the cool, stable air layer near the ground. Even under light winds, these suspended droplets can travel hundreds or even thousands of feet before settling, increasing the potential for off- target deposition on neighboring crops, sensitive vegetation, water bodies, and residential areas (Fig. 2).

As inversion-related spray drift often affects sensitive broadleaf crops, the first signs of injury are usually visible on the plants. In soybean, the most common symptoms are cupping and puckering of new leaves, while high-value specialty crops such as grapes and tomatoes may develop leaf malformation and stunting even at very low levels of exposure.

Field reports have shown how far this damage can extend. Fine droplets trapped in a stable inversion layer can travel up to three miles before settling (McCoy, 2021). During the widespread 2017 dicamba drift incidents, an estimated 3.6 million acres of non-tolerant soybean were injured nationwide, highlighting how extensive off-target movement can be under favorable drift conditions (Bradley, 2017). These examples show that a single poorly timed pesticide application can affect crops well beyond the treated field.

How to Identify a Temperature Inversion

The most reliable way to confirm a temperature inversion is to measure air temperature at two heights: approximately 6 to 12 inches above the soil surface (or above the crop canopy) and 8 to 10 feet above the ground. An inversion is present when the air temperature is warmer at the higher measurement point than near the surface. A larger temperature difference indicates a stronger, more stable inversion.

However, applicators can often identify inversion conditions using simple field observations. Common indicators include:

• Mist, fog, dew, or frost near the ground
• Dust or smoke that remains suspended and moves horizontally instead of rising
• The disappearance of cumulus clouds near sunset
• Light, steady winds (generally less than 6.8 mph) during the evening or overnight
• Cool downhill (off slope) breezes
• Sounds or odors that travel unusually long distances

If several of these conditions are observed, pesticide applications should be postponed until the inversion dissipates and normal atmospheric mixing resumes (Enz et al., 2017).

Minimizing Spray Drift During Temperature Inversions

The best way to minimize inversion-related spray drift is to recognize the weather conditions that favor temperature inversions and avoid spraying when they are present. Applicators should monitor local weather using on-farm weather stations whenever possible, as conditions can vary considerably over short distances and may not be accurately represented by nearby weather stations. If a temperature inversion is present or likely to develop, pesticide applications should be postponed until after sunrise, when solar heating warms the ground, restores normal atmospheric mixing, and dissipates the inversion layer. In addition, all pesticide applicators and equipment operators should be trained to recognize the signs of temperature inversions, understand the associated drift risks, and follow label requirements and best management practices to ensure safe and effective pesticide applications (Elwakil, 2024).

Conclusion

Temperature inversions are one of the most overlooked causes of off target pesticide movement. During an inversion, a stable layer of cool air near the ground can keep fine spray droplets suspended for an extended period, allowing them to travel long distances before settling and increasing the risk of injury to neighboring crops, sensitive vegetation, water resources, and other non-target areas. These risks are particularly important for aerial drone (Unmanned Aerial Systems; UAS) applications, as they have lower downwash thrust than manned aircraft and aerial drones are therefore more susceptible to off-target movement under certain conditions. In addition, drone applications often use relatively small droplets due to low-height applications and spray released above the crop canopy during an inversion can remain suspended and move well beyond the intended target area. The best way to minimize inversion-related spray drift is to recognize the weather conditions that favor inversions, such as calm winds, clear skies, dew, fog, or lingering smoke, and postpone pesticide applications until normal atmospheric mixing resumes after sunrise. -When combined with on- farm weather monitoring, appropriate nozzle and droplet size selection, adherence to pesticide label requirements, and proper operator training, avoiding temperature inversions helps ensure safe, effective, and responsible pesticide applications.

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