Ground level temperature inversions are a natural phenomenon in which cold air masses are formed at the earth’s surface due to cooling of the surface (usually due to the sun setting beyond the horizon) and becoming trapped underneath warm air in the atmosphere. Temperature inversion form at the ground level during the evening hours just prior to sunset on most days during the summer and persist through the morning when the warming earth and wind currents disrupt and disperse the inversions. The cold air masses are generally stable and do not have vertical mixing like what occurs during the day, thus any particles released into the air mass are not allowed to be dispersed into out of the atmosphere.

Spray applications of herbicides that occur during a ground level temperature inversion can result in small droplets becoming suspended within the air mass. In contrast an application made during a non-inversion that would result in any suspended droplets being moved up and out of the atmosphere due to continual vertical mixing or rising air warm air. Droplets that become suspended within a temperature inversion can move over wide expanses of areas that are unpredictable and lead to large scale off-site movement of herbicides. Thus, it is recommended that herbicide spray applications, especially applications of dicamba are not applied during ground level temperature inversions.

Since the introduction of dicamba-tolerant soybean has occurred with a dramatic number of off-target injury events across the soybean growing regions of the United States. While there are numerous events that can lead to off-target movement of dicamba, the one that has been least understood is the role of temperature inversions. Large scale and landscape level injury events that have occurred in regions such as the boothill of Missouri, northeast Arkansas, western Tennessee, and Fulton county Kentucky has raised many questions of what is unique about this geography to lead to such dramatic dicamba injury events. The role of temperature inversions in these areas is still unknown although it is hypothesized that these lower elevation regions may be more prone to temperature inversions.

Current restrictions do not allow dicamba applications to be made from 2 hrs prior to sunset until 1 hr after sunrise. These restrictions were put into place with knowledge that most temperature inversions begin setting up prior to sunset and do not disperse until after sunrise. Although the influence of elevation of the application site on when temperature inversions occur is still in question as wide spread dicamba damage is most significant in lower elevations or river bottoms, especially when considering locations such as Fulton County Kentucky.

RESEARCH DESIGN AND METHODS:

Temperature inversion monitoring stations with temperature probes at 120, 66, and 18” heights were setup at two locations in Fulton County Kentucky. The stations were placed at locations within 1.5 miles of each other, one at a lower elevation of 300ft in the river bottom and other at an elevation of 360ft in an upland location. This placement complements the already existing stations in Indiana, Tennessee, and Arkansas that are setup in a similar fashion. Temperatures at the three levels and wind data was collected by each station on an hourly basis. Data Collected was sent to the University of Missouri for analysis of occurrences, durations, and start times of temperature inversions.

RESULTS:

Using the strictest parameters to determine a temperature inversion based on temperatures at the three probe heights and wind speeds the occurrence of temperature inversions in both locations in June, July, and August were determined. In all three months the river bottom location experienced more evenings with temperature inversions than the upland location. The biggest differences in the number of occurrences occurred in the months of July and August. This data further emphasizes that topography does influence the formation or rather frequency of temperature inversions with inversions occurring more often at lower elevations as compared to higher elevations.

The data from the two locations was also analyzed to determine the time of temperature inversion formations. In the months of May and June inversions had an average formation start time approximately 40 minutes earlier at the upland location. Although the inverse occurred in July and August with average inversion formation being 1.5 to 5 hours earlier in the bottom than in the upland location. Temperature inversion average formation time coincided within 30 minutes of sunset at the river bottom location, further emphasizing the implications of not following cutoff times implemented by current dicamba labels.

It should be noted that 2019 had more precipitation and cloud cover than the average year and thus temperature inversion formations were likely inhibited due to these conditions. Thus, it would be expected that in a more typical year temperature inversion may be more frequent and formation times even earlier.

CONCLUSIONS:

Overall, this small preliminary data set from one year further shows that topography does influence temperature inversion, specifically elevation. Lower elevation or river bottom locations are likely to experience more temperature inversion than upland location. While precautions should always be taken when applying herbicides, the known frequency of temperature inversions in river bottoms and their influence on off-target movement of dicamba should be considered by applicator and farmers prior to making dicamba applications. Further data will be collected in 2020 and compiled with data being collected in surrounding states to further understand the influence of topography on temperature inversions and agriculture spray applications.