When planning your Engenia® herbicide application this spring, it’s important to be able to identify if a temperature inversion exists and understand why they must be avoided when making an application.
How temperature inversions form
During daytime hours, dry air naturally cools with higher altitude. Solar radiation warms the earth’s surface, and during days with little cloud cover, convection creates winds and gusts that transport air vertically. As sunset nears, the earth’s surface is no longer heated by the sun. As a result, the ground and the air adjacent to the ground begins to cool more rapidly than parts of the overlying atmosphere. Heat from the warmer air is transferred back to the soil, creating a layer of cooler, denser air near the soil surface. This process creates a temperature inversion, where the cool air at ground level has warmer air above it through the very lowest levels of the atmosphere.
Spraying herbicides during an inversion can result in the off-target movement of small droplets as physical drift, which never reach their intended target. This is not to be confused with volatility, which is when a liquid droplet converts to a gas after it has reached its intended target.
- There are several weather conditions where temperature inversions are likely to occur:
Clear skies during late afternoon and during the night
- Dry soil surface
- Wind speeds < 4 MPH that result in no air mixing
- Low areas, valleys or basins where cool air will sink and collect
- Inversions will form in these areas sooner, persist longer and be more intense
How to identify if an inversion exists:
- Measure air temperature at 6–12 inches above the soil and at 8–10 feet above the soil. An inversion exists if measured air temperature at 8–10 feet above the soil is higher than the measured air temperature at 6–12 inches above the soil. Be sure the instrument is shaded and not influenced by solar heating
- Morning dew
- Morning fog (indicates that an inversion existed prior to fog formation)
- Smoke or dust hanging in the air or moving laterally
- Overnight cloud cover is 25 percent or less
- Inversions can begin forming three to four hours before sunset and can persist until one to two hours after sunrise
The impact of temperature inversions on Engenia herbicide applications
Temperature inversions can negatively impact Engenia herbicide applications by trapping small droplets in the cool air of the inversion layer. These small droplets can then travel long distances, either downslope to low-lying areas or in an unpredictable manner with the light and variable winds. To avoid off-target movement of Engenia due to inversions, be mindful of inversions during the following spray timings:
- Mornings. Very early mornings around sunrise are when inversions can be at their most extreme. One of the worst times to spray is when overnight skies were clear and wind speeds are low. Inversions can persist for one to two hours after sunrise on a calm day.
- Late afternoon/early evening. The lowest five feet closest to the ground can sometimes begin to form an inversion three to four hours before sunset. Evening inversions pose a greater risk for off-target movement because they are very persistent and will intensify until after sunrise.
- Nighttime. Inversions may have already been established and continue to intensify until after dawn.
For more tools and resources, visit EngeniaStewardship.com.
Engenia Herbicide is a U.S. EPA Restricted Use Pesticide. Additional state restrictions may apply.
Always read and follow label directions.
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