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Air temperatures in crop fields are typically warmest near the ground and cooler at higher elevations. When an inversion occurs, the air near the crop or soil surface is cooler than the air above.1 The result is a very stable layer of air that prevents vertical air motion which causes small, suspended droplets to remain in a concentrated cloud of air for hours and potentially through the night and into the morning (Figure 1).
In the time between when they are sprayed and when they settle onto vegetation, these small particles can move to non-target areas, potentially damaging susceptible crops or other vegetation (Figure 2).1 Even sprayer tips designed to produce large droplets also produce some small (200 microns and less) droplets.2
Applicators should not confuse spray drift during an inversion with herbicide movement through volatization (when a liquid droplet converts to a gas). Small droplets of any herbicide formulation can move to non-target areas during an air temperature inversion.
Early in the morning, when wind speeds are low (< 3 mph), may seem to be a good time to apply herbicides. However, when overnight skies were clear and wind speeds are low 13;10;(< 3 mph), temperature inversions are likely to occur, making this one of the worst times to spray.2 Ground fog or moisture on plant leaves are indicators that an inversion exists.
An inversion usually persists for 1 to 2 hours, and sometimes longer, after sunrise on a clear calm day, depending on the surface conditions.2 In the morning when skies are clear, wait until the surface air begins to warm (generally 3 °F from the morning low) or with increased winds (above 3 mph) to ensure the inversion has lifted.
In the late afternoon (1 to 3 hours before sunset, and sometimes earlier), soil temperature and the temperature of the air above the soil begin to cool. These conditions can set up the beginning of an air temperature inversion.2
Evening inversions may pose a greater risk for spray drift compared to morning inversions due to the fact that once formed, the inversions are persistent as long as skies remain clear. Once formed, the inversion will continue to intensify until shortly after sunrise. Windy or cloudy conditions will eventually disrupt the evening inversion. An inversion, plus low wind speed, is the best possible situation for long distance drift of spray droplets.2
In mountainous areas, protected valleys, low areas, and shaded hillsides, cold air drainage can cause intense inversions. Applicators should be aware of the increased potential for inversion development and the increased risk of spray drift in these areas.2
Figure 1. Surface temperature inversion layer and its effect on sprayed herbicide distribution.3
Applicators can confirm the presence of an inversion by measuring the air temperature at two heights: 6 to 12 inches above the sprayed surface (soil if no crop is emerged or above the crop canopy) and 8 to 10 feet above the surface to be sprayed. When the temperature at the higher level is greater than the temperature at the lower level, an inversion exists. The greater the temperature difference between the two levels, the more intense the inversion, and the more stable the lower atmosphere.2
Precisely measuring temperatures at the soil or crop surface as well as 8 to 10 feet above the surface may be impractical. The following conditions indicate that a surface temperature inversion is likely to be present:3