What Is Delta T and Why Does It Matter for Spraying?
Most growers watch wind speed closely when planning a spray job, but wind is only part of the picture. Delta T is the other half: a single value combining temperature and humidity that indicates how quickly spray droplets are likely to evaporate after leaving the nozzle, and it has become a standard part of spray planning across modern farming operations.
Key takeaways
Delta T combines temperature and humidity into one evaporation indicator, calculated as the difference between dry bulb and wet bulb temperature.
High Delta T means hot, dry, highly evaporative conditions where droplets shrink fast and drift risk rises.
Very low Delta T can bring its own risk, since calm, humid conditions can signal a temperature inversion.
Delta T works best alongside wind speed, wind direction and on-farm local weather data, not on its own.
Why evaporation matters once the droplet leaves the nozzle
The moment a droplet leaves the nozzle, it starts interacting with the surrounding atmosphere. In hot, dry conditions, small droplets can evaporate before properly reaching the crop, becoming lighter, more wind-affected and increasingly prone to off-target drift. Shrinking droplets also tend to give poorer coverage and reduced chemical deposition, which cuts spray effectiveness while raising both chemical loss and environmental risk.
The complication is that evaporation isn't governed by temperature alone. Humidity plays just as big a role, and a moderately warm, humid day can produce far lower evaporation than a drier day at a similar temperature. That's exactly why Delta T is useful: rather than reading temperature and humidity separately, it folds both into one number that reflects actual atmospheric drying potential.
What high and low values actually mean
High Delta T values point to hot, dry, highly evaporative conditions, where droplets shrink rapidly after leaving the nozzle and drift risk climbs, particularly with smaller droplet sizes or more volatile products. Low Delta T values generally mean cooler, more humid conditions with lower evaporation, but very low values bring a different problem: calm, humid air can indicate a temperature inversion, where droplets stay suspended rather than dispersing normally. In practice, both extremes create spraying challenges, which is why Delta T works best read alongside wind speed, wind direction, temperature trends, humidity and plain operational experience rather than treated as a simple pass or fail.
Why on-farm monitoring beats a regional forecast
Delta T conditions can vary meaningfully between paddocks, driven by local temperature, soil moisture, elevation and vegetation, all of which shape humidity and evaporation near ground level. A weather station many kilometres away may bear little resemblance to what's actually happening in the paddock being sprayed. That's why more growers now run on-farm weather stations that calculate Delta T directly from live temperature and humidity data and push it out through a telemetry dashboard or mobile app, letting operators react quickly as conditions shift, which they often do fast through late morning and early afternoon.
Using Delta T to time the spray window
In many regions, the best spraying conditions fall inside a fairly narrow window, often earlier in the day before temperatures climb and humidity drops. Monitoring Delta T in real time helps identify that window and adjust scheduling accordingly, rather than relying on visual judgement or a general forecast. For broadacre operations running spray programs across multiple paddocks over extended periods, that continuous read on conditions becomes especially valuable.
Delta T is one input, not the whole picture
Delta T shouldn't be read in isolation. Wind speed and atmospheric stability matter just as much for drift risk: suitable Delta T can still coincide with excessive wind, and a calm, humid morning with a low Delta T reading can still carry inversion risk. The most effective spray planning combines Delta T monitoring with wind monitoring, operational experience, nozzle selection, spray setup and chemical-specific recommendations, and modern weather stations make tracking several of these variables at once considerably easier.
The bottom line
Delta T gives growers a practical way to gauge evaporation potential during spraying by combining temperature and humidity into one indicator. Read alongside accurate local weather data and sound operational judgement, it supports better-timed spray decisions and more consistent results, and as agriculture keeps getting more data-driven, tools like Delta T are becoming a standard part of precision spraying.