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Be on the watch for temperature inversions

AMES, IA - As postemerge herbicide applications continue for corn and begin for soybeans, it’s important to be on the lookout for temperature inversions and know at what temperature inversions occur, why they can be dangerous for pesticide applications, and how to identify when they’re happening. It is never recommended to spray a pesticide when a temperature inversion exists, and many labels provide guidelines to follow in order to avoid applications during one.

For example, dicamba products labeled for over-the-top applications in soybeans include some of the most specific guidelines. These include making sure to apply between one hour after sunrise and two hours before sunset, and to cease applications when wind speed is less than 3 mph at boom height.

In normal conditions, the sun warms the earth’s surface. As the surface air temperature increases, it rises and the wind mixes it with the cooler air above (allowing for dispersion of spray droplets in the atmosphere). A temperature inversion is the opposite of that: A layer of warm air is trapped between cooler air higher in the atmosphere and dense cooler air close to the earth’s surface as a result of less solar intensity at the end of the day. This warm air layer (inversion layer) prevents upward movement of the cool surface layer, creating very stable conditions. If an application occurs during a temperature inversion, small droplets are trapped in the air above the ground and can move across the landscape.

Temperature inversions usually begin to form at the end of the day around dusk (three to five hours before sunset), and they last through the evening (intensifying throughout the night) up to sunrise. As the sun warms the earth’s surface in the morning and the air mixes, the inversion will dissipate thereafter (usually two to three hours after sunrise).

Conditions that Most Likely Favor Temperature Inversions

  • 25% or less cloud cover
  • Light and variable winds (especially below 3 mph)
  • Dry soil surface
  • Low-elevation areas such as valleys and basins where cool air can sink and collect – inversions will begin sooner, last longer, and be more intense in these areas.

Clues a Temperature Inversion Exists

  • Mist, fog, dew, or frost is present
  • Smoke or dust hang in the air and may move horizontally just above the surface
  • Cumulous clouds disperse as evening approaches/skies clear
  • Distant sounds become easier to hear
  • Distant smells are more distinct during the evening than during the day

To help minimize off-target movement of pesticides, it’s very important to monitor the development of temperature inversions and cease application during the existence of one. Even when selecting a spray nozzle that produces the coarsest drop allowed by the pesticide label, there is still a percentage of spray droplets smaller than 200 microns in diameter. In a temperature inversion, these very small droplets will remain suspended for a long period of time, which significantly increases the potential of off-target movement.

Temperature Inversion Resources

Temperature Inversions and Herbicide Drift: Take Action's “ Inside Weed Management" Series – University of Missouri

Synthetic Auxin Applications 2021: How does the air flow in your fields? – University of Missouri

What Have We Learned from Four Years of Studying Temperature Inversions? – University of Missouri

Temperature inversions: Something to consider before spraying – University of Minnesota

Air Temperature Inversions Causes, Characteristics and Potential Effects on Pesticide Spray Drift – North Dakota State University

Understanding Air Temperature Inversions Relating to Pesticide Drift – North Dakota State University

Are Inversions Really That Common? – University of Minnesota

Author: Terry Basol is an agronomist in north-central Iowa and field specialist for Iowa State University Extension and Outreach.

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