Delta T: A new tool to make every spray drop count
Delta T is a calculation that combines relative humidity and temperature into a practical measure. It can help farmers and their CCAs determine optimal spraying conditions to reduce herbicide drift and evaporation, improving weed control effectiveness. Monitoring Delta T and adjusting spraying practices accordingly can preserve herbicide performance and combat increasing herbicide resistance.
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It’s not just enough to spray herbicides, they need to be sprayed well. Effective herbicide application means using the right products at the right time, and the environment in which we spray is a key factor in determining how well they can do their job. Poor spraying conditions that affect herbicide efficacy will reduce weed control. This contributes to herbicide resistance, the single biggest threat to contemporary weed management. Every season gets more challenging as we see herbicides start to fail on new weeds and already resistant weeds becoming resistant to more products. Without new modes of action coming to our rescue, we need to preserve the herbicides we have by spraying under optimum conditions.
Temperature and humidity are key factors affecting herbicide movement to the target weeds. Once the herbicide solution leaves the tank and becomes airborne, spray droplets are susceptible to being lost through drift or evaporation. Spraying under the right environmental conditions, especially temperature and humidity, will reduce droplet loss. Monitoring these factors is crucial to ensure what’s being sprayed is hitting the mark. Low humidity will cause droplets to evaporate faster, which can reduce uptake and increase drift. But simply looking at relative humidity (RH) doesn’t provide a clear picture of risk—this is because evaporation changes at different air temperatures even though RH is the same.
How Delta T works and how it is measured
Delta T is a calculation that combines RH and temperature into a practical measure (Figure 1). It shows how dry the atmosphere is by measuring the temperature difference between a dry bulb and a wet bulb thermometer. A dry bulb is an air temperature sensor exposed directly to air while a wet bulb is an identical air temperature sensor wrapped in wet material. Water is evaporating from the wet bulb and cooling it, resulting in a lower temperature compared with the dry bulb. The drier the air, the faster the evaporation rate and the larger the temperature difference between the two bulbs. The Delta T value and the potential for spray droplets to evaporate is higher. In humid conditions when evaporation is slow, the wet bulb temperature will be close to the dry bulb temperature, and Delta T is low.
Delta T can be measured with a sling psychrometer (Figure 2) that has identical dry and wet bulb thermometers. The psychrometer is swung in a circle, and each bulb measures temperature as it is exposed to moving air. Subtract the wet bulb temperature from the dry bulb temperature; for example, if the dry bulb temperature is 26oC and wet bulb temperature is 22oC, then Delta T is 4oC. We can get Delta T directly from other sources like handheld devices that measure temperature and humidity and calculate Delta T. Some sprayers and many stationary weather stations also calculate Delta T. Now that Delta T is being widely reported, we can make use of this practical tool through real-time measurement and forecasting.
Using Delta T to improve spraying practices
How and why do we use Delta T? Applicators in Australia started to use this measurement because spraying lower water volumes in hot, dry conditions affects spray quality. Fine droplets can evaporate completely, and larger spray droplets will shrink and become more driftable. Recommendations from Australia suggest conditions are safest to spray when Delta T is between 2 and 8oC. Higher Delta T conditions mean rapid evaporation, and coverage and efficacy could be reduced. Low Delta T conditions occur under high humidity, and there is potential for temperature inversions, which can increase drift. It’s important to monitor Delta T continuously as it will change throughout the day with changing temperature and RH. Delta T can rise significantly in the hottest and driest parts of the day, and spray performance can compromised.
Can we still spray when Delta T is high? Andrew Thostensen, Pesticide Program Specialist (Emeritus) from North Dakota State University, states that when Delta T conditions become marginal for spraying, applicators may want to consider altering their application practices and/or curtail operations when Delta T values become extreme. Using coarser droplet size and higher water volumes can mitigate some risk.
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