Temperature Inversion and Pesticide Drift
Pesticide drift can occur when an application is made during an temperature inversion episode (also called thermal inversion), that is, when a mass of cold air forms near the ground surface beneath a mass of warmer air. Although this phenomenon can occur at any time of day, temperature inversions are much more common in the late afternoon or early evening and often persist until the next morning. A temperature inversion creates an environment conducive to the drift of fine spray droplets toward non-target areas. For this reason, in some countries, it is prohibited to apply certain pesticides during a thermal inversion episode.
Effects of an inversion on spray droplets
In a field, air is normally warmer at the ground surface and cools with altitude. During an inversion, the air near the ground or crop is colder than the air above the crop. This creates a very stable air layer that prevents vertical air movement and causes very fine spray droplets (≤ 200 microns) to remain suspended in an inversion layer for several hours, often until morning. They can then be carried by winds as light as 1.6 km/h outside the targeted area where they can cause damage to sensitive crops or vegetation[1]. Farmers should not confuse herbicide drift during an inversion with herbicide movement due to volatilization, which is the transition of liquid droplets to a gaseous state. Spray droplets of any herbicide can drift to non-target areas during a temperature inversion.
Choosing the right time to spray
One might think it is appropriate to apply treatments early in the morning when wind is light (<5 km/h). However, calm winds and a clear night sky favor the formation of a temperature inversion, making this the worst time of day to apply treatments. In calm weather, an inversion usually persists for more than an hour after sunrise, depending on surface conditions. Ground fog and water condensation on leaf surfaces are good indicators of an inversion. On clear mornings, it is better to wait until the surface air warms by at least 1.6 °C above the nighttime minimum temperature or until the wind picks up slightly (>3 km/h) to ensure the inversion has dissipated before applying treatments.
A temperature inversion can begin to form late in the day when air and soil temperatures start to drop. Inversions forming in the evening often pose a greater risk of pesticide drift than morning inversions since they can persist as long as the sky remains clear. Once formed, such an inversion continues to intensify until after sunrise. Clouds or wind will eventually destabilize the inversion. The worst pesticide drift conditions occur when there is both an inversion and light wind; in such cases, very fine spray particles can be transported over long distances[1].
In mountainous regions, cold air drainage into protected valleys, low areas, and shaded slopes can cause intense inversions[1].
Recognizing an inversion
The presence of an inversion can be checked by measuring air temperature at two heights: from 15 cm to 30 cm above the surface to be treated (which is the vegetation cover or, if the crop has not yet emerged, the soil) and from 2.5 m to 3 m above the surface to be treated. An inversion exists if the temperature at the upper level is higher than at the lower level. The greater the temperature difference, the more intense the inversion and the more stable the lower atmosphere[1].
It is not always practical to precisely measure temperature near the ground and 3 m above the vegetation cover. The following conditions indicate that an inversion has likely formed[2]:
- Mist, fog, dew, or frost.
- Dust or smoke suspended in the air moving laterally above the surface. A pest control operator can generate smoke to check for an inversion. Smoke forming a layer moving laterally in a cloud (with light winds) indicates an inversion, whereas smoke rising and dissipating quickly indicates good vertical air mixing.
- Dispersion of cumulus clouds late in the day.
- Steady winds below 11 km/h in the evening and overnight.
- Cool downslope breeze developing in the evening or overnight.
- Increased ease of hearing distant sounds.
- Distant odors more easily perceived in the evening than during the day.
Sources
- ↑ 1.0 1.1 1.2 1.3 Enz, J., Hofman, V. and Thostenson, A. 2019. Air temperature inversions: causes, characteristics and potential effects on pesticide spray drift. North Dakota State University Extension publication AE1705. https://www.ndsu.edu/agriculture/sites/default/files/2024-07/ae1705.pdf
- ↑ Townsend, L. 2017. Surface temperature inversions and spray drift. University of Kentucky Pest News. https://kentuckypestnews.wordpress.com/2017/03/28/surface-temperature-inversions-and-spray-drift/
