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Delta T: A New (Old) Tool for Pesticide Application Decision Making (05/23/19)

In March of 2019, the North Dakota Ag Weather Network (NDAWN) announced they were estimating and reporting a new value for pesticide applicators. It is Delta T.

In March of 2019, the North Dakota Ag Weather Network (NDAWN) announced they were estimating and reporting a new value for pesticide applicators. It is Delta T. This is the temperature difference between a dry bulb (air temperature sensor exposed directly to the air) and a wet bulb (air temperature sensor enclosed in wetted material so that water is constantly evaporating from it and cooling the bulb). The higher the Delta T value, the drier the atmosphere is with greater potential to evaporate spray drops.

The Delta T is the primary method by which applicators in Australia decide when and how to apply pesticides to improve efficacy and reduce spray drift. It originated over 30 years ago. It was designed in the era when the primary spray nozzle was a flat fan. Because these nozzles produce a relatively high proportion of fine spray drops, there was widespread concern, especially in the drier and hotter parts of Australia, that significant evaporation of these fine drops would lead to coverage/efficacy issues as well as increased spray drift. The Australian’s also use relatively low Delta T values as an indication that an air temperature inversion is likely occurring and/or the humidity is so high that fine spray drops would have a tendency to move further down range.

In more recent times, the Delta T concept has gained some adoption in the arid portions of Western Canada. Weather instrumentation manufacturers in North America have also built a Delta T value into their devices. Kestrel Meters and Weather Flow both make instruments that report a Delta T value. In the United States, Delta T has not been widely used. It is not part of standard pesticide application curricula.



Figure 1 is a graphic depicting how the Delta T value changes relative to humidity and temperature. The Delta T is reported on the right side of this graphic. It is expressed in degrees F. On the bottom of the graphic, the legend describes whether it is acceptable or not to spray, what is optimum, and when the conditions are marginal and under what circumstances. You will note that the temperatures reported in the chart are not rounded to customary five or ten degree increments. That is because this chart was originally created in degrees Celsius. In order to maintain the integrity of the curves, Fahrenheit degrees have simply been substituted to replace the Celsius temperatures.

As you examine this chart, think about the influence that progressively warmer and drier conditions will have on the evaporation of a spray drop. A typical flat fan spray nozzle set at 40 psi will produce 30% or more fine spray drops.


All those drops are likely to evaporate before they hit the target with a Delta T value of 18 or more. That will result in coverage and efficacy issues. Further, because ALL the spray drops are shrinking because of evaporation, more and more of the spray will be subjected to wind movement and drift. You will note the impact of evaporation on a spray application can be partially offset by increasing droplet size. Coarse or greater spray quality drops, those often produced by an Air Induction or a Turbot Teejet Induction nozzle, can be used up to a Delta T of 21.6. However, after that, the evaporation rate becomes so problematic that applications are no longer recommended.

INSERT Chart Photo:

Table 1. Sample weather variables from selected NDAWN Stations. The Delta T value is reported in green because the estimated Delta T is less than 14.4 degrees F but more than 3.6 degrees F. This is in the preferred range for spraying.

Just like other weather variables, Delta T will change throughout the day. Generally, in the early morning hours, the value will be low, but as the day warms, the number will rise. As evening and night sets in, the numbers will fall again. In North Dakota, I would also expect Delta T values will generally be higher in the SW and lower in the NE region of the state (relatively warmer and drier climate versus a cooler and higher precipitation one).

Delta T is an excellent way to understand the impact of temperature and humidity on a spray drop. However, it is not a substitute for ignoring other the label instructions. I do not think it will ever be closely followed in the U.S. On the other hand, the good news is, we have a new tool to consider, especially on those relatively hot and dry days in June and July. On those days, applicators would do well to consider some of these issues covered in previous Crop and Pest Report articles:

Andrew A. Thostenson

Pesticide Program Specialist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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