Producers planning to use the new Xtend soybean or cotton technologies, with either XtendiMax or Engenia low-volatile dicamba formulations, will need to take special care to avoid conditions that could result in herbicide drift onto sensitive crops. Factors affecting drift of dicamba (and 2,4-D) molecules include:
- Temperature inversions
- Wind speed
- Air temperatures (volatility increases up to 86 degrees F)
- Herbicide rate
- Surface type (volatility from plant material is higher than from bare soil)
- Humidity (dicamba is more volatile at low humidity, less than 70 percent)
- Droplet size (finer droplets of the carrier will evaporate quicker, leaving the remaining herbicide in the air susceptible to drift)
Most of these factors affecting drift are readily understood. But what about the caution on the product labels regarding temperature inversions: “Do not apply during temperature inversions (most likely in the evening hours with calm conditions).”?
What is a temperature inversion, how often does it occur, and why is it important in this situation?
Normally during the day, the sun warms the surface of the earth and coinciding air near it, while cooler air resides higher in the atmosphere above the surface. Once the sun sets, however, the reverse will happen. Upon the loss of solar heating, temperatures cool at the earth’s surface. Warm air that lifted during the day will remain in the atmosphere above this cooling layer.
This condition, called a “temperature inversion,” is a near-daily occurrence and can be amplified under clear skies and light winds. Large-scale factors such as weather fronts or cloud cover can alter this pattern. Smaller-scale influences such as elevation (river bottoms, higher terrain, etc.) can also impact the strength or presence of an inversion locally.
Upon sunrise when solar radiation begins to reach the surface again, the air above the surface gradually warms and the pattern repeats. It takes several hours for the air to become warm enough to degrade the overnight inversion, often diminishing it by late morning.
Figure 1. On a typical day, the air temperature at the surface is warmer than the temperature in the atmosphere during daylight hours after the air temperature warms up in the morning, allowing volatile compounds to dissipate into the upper air levels. In a temperature inversion (right), the air temperature at the surface is cooler than the temperature in the atmosphere. This can trap volatile compounds near the surface, increasing the likelihood that the compounds can drift onto nearby plants and structures. Graphic by Doug Shoup, K-State Research and Extension.
The labels of XtendiMax and Engenia state:
- “Temperature inversions are characterized by increasing temperatures with altitude and are common on evenings and nights with limited cloud cover and light to no wind.”
- “Their presence can be indicated by ground fog; however if fog is not present, inversions can also be identified by movement of smoke from a ground source or aircraft smoke generator. Smoke that layers and moves laterally in a concentrated cloud (under low wind conditions) indicates an inversion, while smoke that moves upward and rapidly dissipates indicates good vertical air mixing.”
- “The inversion will often dissipate with increased winds (above 3 mph) or at sunrise when the surface air begins to warm (generally 3oF from morning low).”
Measurements taken by researchers at the University of Missouri indicate the time of day when inversions usually start from March to July in the Bootheel of Missouri (Bish, M. and K. Bradley, 2016. What is a temperature inversion? Proc. NCWSS. 71:204). Inversions can last for up to 10 hours or more after they begin, the authors noted.
|
Bootheel of Missouri |
2015 |
2016 |
|
|
Approximate time the inversion began |
|
|
March |
4:00 to 5:00 p.m. |
4:00 to 5:00 p.m. |
|
April |
4:00 to 5:00 p.m. |
4:00 to 5:00 p.m. |
|
May |
4:00 to 6:00 p.m. |
4:00 to 6:00 p.m. |
|
June |
5:00 to 6:00 p.m. |
5:00 to 6:00 p.m. |
|
July |
6:00 to 7:00 p.m. |
6:00 to 7:00 p.m. |
In 2015, the University of Missouri researchers noted 17-23 days per month with temperature inversions. In 2016, inversions occurred on 20-27 days per month.
Inversions typically persist overnight and can last late into the morning depending on the weather conditions (as late as 10 a.m. to noon). Inversions don’t just develop instantly, but take time to develop over several hours. Inversions are very typical in all seasons, not just the months involved in the Missouri study. Inversions are largely influenced by terrain. Therefore, they will be extremely localized in some places, and the strength and impact of these will differ widely.
Doug Shoup, Southeast Area Crops and Soils Specialist
dshoup@ksu.edu
Christopher Redmond, Assistant Scientist KSU Weather Data Library/Mesonet
christopherredmond@ksu.edu
Xiaomao Lin, State Climatologist
xlin@ksu.edu
Mary Knapp, Weather Data Library
mknapp@ksu.edu
Dallas Peterson, Weed Management Specialist
dpeterso@ksu.edu
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